Special Functions

This chapter describes the GSL special function library. The library includes routines for calculating the values of Airy functions, Bessel functions, Clausen functions, Coulomb wave functions, Coupling coefficients, the Dawson function, Debye functions, Dilogarithms, Elliptic integrals, Jacobi elliptic functions, Error functions, Exponential integrals, Fermi-Dirac functions, Gamma functions, Gegenbauer functions, Hermite polynomials and functions, Hypergeometric functions, Laguerre functions, Legendre functions and Spherical Harmonics, the Psi (Digamma) Function, Synchrotron functions, Transport functions, Trigonometric functions and Zeta functions. Each routine also computes an estimate of the numerical error in the calculated value of the function.

The functions in this chapter are declared in individual header files, such as gsl_sf_airy.h, gsl_sf_bessel.h, etc. The complete set of header files can be included using the file gsl_sf.h.

Usage

The special functions are available in two calling conventions, a natural form which returns the numerical value of the function and an error-handling form which returns an error code. The two types of function provide alternative ways of accessing the same underlying code.

The natural form returns only the value of the function and can be used directly in mathematical expressions. For example, the following function call will compute the value of the Bessel function J_0(x):

double y = gsl_sf_bessel_J0 (x);

There is no way to access an error code or to estimate the error using this method. To allow access to this information the alternative error-handling form stores the value and error in a modifiable argument:

gsl_sf_result result;
int status = gsl_sf_bessel_J0_e (x, &result);

The error-handling functions have the suffix _e. The returned status value indicates error conditions such as overflow, underflow or loss of precision. If there are no errors the error-handling functions return GSL_SUCCESS.

The gsl_sf_result struct

The error handling form of the special functions always calculate an error estimate along with the value of the result. Therefore, structures are provided for amalgamating a value and error estimate. These structures are declared in the header file gsl_sf_result.h.

The following struct contains value and error fields.

gsl_sf_result
typedef struct
{
  double val;
  double err;
} gsl_sf_result;

The field val contains the value and the field err contains an estimate of the absolute error in the value.

In some cases, an overflow or underflow can be detected and handled by a function. In this case, it may be possible to return a scaling exponent as well as an error/value pair in order to save the result from exceeding the dynamic range of the built-in types. The following struct contains value and error fields as well as an exponent field such that the actual result is obtained as result * 10^(e10).

gsl_sf_result_e10
typedef struct
{
  double val;
  double err;
  int    e10;
} gsl_sf_result_e10;

Modes

The goal of the library is to achieve double precision accuracy wherever possible. However the cost of evaluating some special functions to double precision can be significant, particularly where very high order terms are required. In these cases a mode argument, of type gsl_mode_t allows the accuracy of the function to be reduced in order to improve performance. The following precision levels are available for the mode argument,

gsl_mode_t
GSL_PREC_DOUBLE

Double-precision, a relative accuracy of approximately 2 * 10^{-16}.

GSL_PREC_SINGLE

Single-precision, a relative accuracy of approximately 10^{-7}.

GSL_PREC_APPROX

Approximate values, a relative accuracy of approximately 5 * 10^{-4}.

The approximate mode provides the fastest evaluation at the lowest accuracy.

Airy Functions and Derivatives

The Airy functions Ai(x) and Bi(x) are defined by the integral representations,

Ai(x) & = {1\over\pi} \int_0^\infty \cos(t^3/3 + xt ) \,dt \\
Bi(x) & = {1\over\pi} \int_0^\infty (e^{-t^3/3 + xt} + \sin(t^3/3 + xt)) \,dt

For further information see Abramowitz & Stegun, Section 10.4. The Airy functions are defined in the header file gsl_sf_airy.h.

Airy Functions

double gsl_sf_airy_Ai(double x, gsl_mode_t mode)
int gsl_sf_airy_Ai_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the Airy function Ai(x) with an accuracy specified by mode.

double gsl_sf_airy_Bi(double x, gsl_mode_t mode)
int gsl_sf_airy_Bi_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the Airy function Bi(x) with an accuracy specified by mode.

double gsl_sf_airy_Ai_scaled(double x, gsl_mode_t mode)
int gsl_sf_airy_Ai_scaled_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute a scaled version of the Airy function S_A(x) Ai(x). For x > 0 the scaling factor S_A(x) is \exp(+(2/3) x^{3/2}), and is 1 for x < 0.

double gsl_sf_airy_Bi_scaled(double x, gsl_mode_t mode)
int gsl_sf_airy_Bi_scaled_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute a scaled version of the Airy function S_B(x) Bi(x). For x > 0 the scaling factor S_B(x) is exp(-(2/3) x^{3/2}), and is 1 for x < 0.

Derivatives of Airy Functions

double gsl_sf_airy_Ai_deriv(double x, gsl_mode_t mode)
int gsl_sf_airy_Ai_deriv_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the Airy function derivative Ai'(x) with an accuracy specified by mode.

double gsl_sf_airy_Bi_deriv(double x, gsl_mode_t mode)
int gsl_sf_airy_Bi_deriv_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the Airy function derivative Bi'(x) with an accuracy specified by mode.

double gsl_sf_airy_Ai_deriv_scaled(double x, gsl_mode_t mode)
int gsl_sf_airy_Ai_deriv_scaled_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the scaled Airy function derivative S_A(x) Ai'(x). For x > 0 the scaling factor S_A(x) is \exp(+(2/3) x^{3/2}), and is 1 for x < 0.

double gsl_sf_airy_Bi_deriv_scaled(double x, gsl_mode_t mode)
int gsl_sf_airy_Bi_deriv_scaled_e(double x, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the scaled Airy function derivative S_B(x) Bi'(x). For x > 0 the scaling factor S_B(x) is exp(-(2/3) x^{3/2}), and is 1 for x < 0.

Zeros of Airy Functions

double gsl_sf_airy_zero_Ai(unsigned int s)
int gsl_sf_airy_zero_Ai_e(unsigned int s, gsl_sf_result * result)

These routines compute the location of the s-th zero of the Airy function Ai(x).

double gsl_sf_airy_zero_Bi(unsigned int s)
int gsl_sf_airy_zero_Bi_e(unsigned int s, gsl_sf_result * result)

These routines compute the location of the s-th zero of the Airy function Bi(x).

Zeros of Derivatives of Airy Functions

double gsl_sf_airy_zero_Ai_deriv(unsigned int s)
int gsl_sf_airy_zero_Ai_deriv_e(unsigned int s, gsl_sf_result * result)

These routines compute the location of the s-th zero of the Airy function derivative Ai'(x).

double gsl_sf_airy_zero_Bi_deriv(unsigned int s)
int gsl_sf_airy_zero_Bi_deriv_e(unsigned int s, gsl_sf_result * result)

These routines compute the location of the s-th zero of the Airy function derivative Bi'(x).

Bessel Functions

The routines described in this section compute the Cylindrical Bessel functions J_n(x), Y_n(x), Modified cylindrical Bessel functions I_n(x), K_n(x), Spherical Bessel functions j_l(x), y_l(x), and Modified Spherical Bessel functions i_l(x), k_l(x). For more information see Abramowitz & Stegun, Chapters 9 and 10. The Bessel functions are defined in the header file gsl_sf_bessel.h.

Regular Cylindrical Bessel Functions

double gsl_sf_bessel_J0(double x)
int gsl_sf_bessel_J0_e(double x, gsl_sf_result * result)

These routines compute the regular cylindrical Bessel function of zeroth order, J_0(x).

double gsl_sf_bessel_J1(double x)
int gsl_sf_bessel_J1_e(double x, gsl_sf_result * result)

These routines compute the regular cylindrical Bessel function of first order, J_1(x).

double gsl_sf_bessel_Jn(int n, double x)
int gsl_sf_bessel_Jn_e(int n, double x, gsl_sf_result * result)

These routines compute the regular cylindrical Bessel function of order n, J_n(x).

int gsl_sf_bessel_Jn_array(int nmin, int nmax, double x, double result_array[])

This routine computes the values of the regular cylindrical Bessel functions J_n(x) for n from nmin to nmax inclusive, storing the results in the array result_array. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

Irregular Cylindrical Bessel Functions

double gsl_sf_bessel_Y0(double x)
int gsl_sf_bessel_Y0_e(double x, gsl_sf_result * result)

These routines compute the irregular cylindrical Bessel function of zeroth order, Y_0(x), for x>0.

double gsl_sf_bessel_Y1(double x)
int gsl_sf_bessel_Y1_e(double x, gsl_sf_result * result)

These routines compute the irregular cylindrical Bessel function of first order, Y_1(x), for x>0.

double gsl_sf_bessel_Yn(int n, double x)
int gsl_sf_bessel_Yn_e(int n, double x, gsl_sf_result * result)

These routines compute the irregular cylindrical Bessel function of order n, Y_n(x), for x>0.

int gsl_sf_bessel_Yn_array(int nmin, int nmax, double x, double result_array[])

This routine computes the values of the irregular cylindrical Bessel functions Y_n(x) for n from nmin to nmax inclusive, storing the results in the array result_array. The domain of the function is x>0. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

Regular Modified Cylindrical Bessel Functions

double gsl_sf_bessel_I0(double x)
int gsl_sf_bessel_I0_e(double x, gsl_sf_result * result)

These routines compute the regular modified cylindrical Bessel function of zeroth order, I_0(x).

double gsl_sf_bessel_I1(double x)
int gsl_sf_bessel_I1_e(double x, gsl_sf_result * result)

These routines compute the regular modified cylindrical Bessel function of first order, I_1(x).

double gsl_sf_bessel_In(int n, double x)
int gsl_sf_bessel_In_e(int n, double x, gsl_sf_result * result)

These routines compute the regular modified cylindrical Bessel function of order n, I_n(x).

int gsl_sf_bessel_In_array(int nmin, int nmax, double x, double result_array[])

This routine computes the values of the regular modified cylindrical Bessel functions I_n(x) for n from nmin to nmax inclusive, storing the results in the array result_array. The start of the range nmin must be positive or zero. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

double gsl_sf_bessel_I0_scaled(double x)
int gsl_sf_bessel_I0_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled regular modified cylindrical Bessel function of zeroth order \exp(-|x|) I_0(x).

double gsl_sf_bessel_I1_scaled(double x)
int gsl_sf_bessel_I1_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled regular modified cylindrical Bessel function of first order \exp(-|x|) I_1(x).

double gsl_sf_bessel_In_scaled(int n, double x)
int gsl_sf_bessel_In_scaled_e(int n, double x, gsl_sf_result * result)

These routines compute the scaled regular modified cylindrical Bessel function of order n, \exp(-|x|) I_n(x)

int gsl_sf_bessel_In_scaled_array(int nmin, int nmax, double x, double result_array[])

This routine computes the values of the scaled regular cylindrical Bessel functions \exp(-|x|) I_n(x) for n from nmin to nmax inclusive, storing the results in the array result_array. The start of the range nmin must be positive or zero. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

Irregular Modified Cylindrical Bessel Functions

double gsl_sf_bessel_K0(double x)
int gsl_sf_bessel_K0_e(double x, gsl_sf_result * result)

These routines compute the irregular modified cylindrical Bessel function of zeroth order, K_0(x), for x > 0.

double gsl_sf_bessel_K1(double x)
int gsl_sf_bessel_K1_e(double x, gsl_sf_result * result)

These routines compute the irregular modified cylindrical Bessel function of first order, K_1(x), for x > 0.

double gsl_sf_bessel_Kn(int n, double x)
int gsl_sf_bessel_Kn_e(int n, double x, gsl_sf_result * result)

These routines compute the irregular modified cylindrical Bessel function of order n, K_n(x), for x > 0.

int gsl_sf_bessel_Kn_array(int nmin, int nmax, double x, double result_array[])

This routine computes the values of the irregular modified cylindrical Bessel functions K_n(x) for n from nmin to nmax inclusive, storing the results in the array result_array. The start of the range nmin must be positive or zero. The domain of the function is x>0. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

double gsl_sf_bessel_K0_scaled(double x)
int gsl_sf_bessel_K0_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled irregular modified cylindrical Bessel function of zeroth order \exp(x) K_0(x) for x>0.

double gsl_sf_bessel_K1_scaled(double x)
int gsl_sf_bessel_K1_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled irregular modified cylindrical Bessel function of first order \exp(x) K_1(x) for x>0.

double gsl_sf_bessel_Kn_scaled(int n, double x)
int gsl_sf_bessel_Kn_scaled_e(int n, double x, gsl_sf_result * result)

These routines compute the scaled irregular modified cylindrical Bessel function of order n, \exp(x) K_n(x), for x>0.

int gsl_sf_bessel_Kn_scaled_array(int nmin, int nmax, double x, double result_array[])

This routine computes the values of the scaled irregular cylindrical Bessel functions \exp(x) K_n(x) for n from nmin to nmax inclusive, storing the results in the array result_array. The start of the range nmin must be positive or zero. The domain of the function is x>0. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

Regular Spherical Bessel Functions

double gsl_sf_bessel_j0(double x)
int gsl_sf_bessel_j0_e(double x, gsl_sf_result * result)

These routines compute the regular spherical Bessel function of zeroth order, j_0(x) = \sin(x)/x.

double gsl_sf_bessel_j1(double x)
int gsl_sf_bessel_j1_e(double x, gsl_sf_result * result)

These routines compute the regular spherical Bessel function of first order, j_1(x) = (\sin(x)/x - \cos(x))/x.

double gsl_sf_bessel_j2(double x)
int gsl_sf_bessel_j2_e(double x, gsl_sf_result * result)

These routines compute the regular spherical Bessel function of second order, j_2(x) = ((3/x^2 - 1)\sin(x) - 3\cos(x)/x)/x.

double gsl_sf_bessel_jl(int l, double x)
int gsl_sf_bessel_jl_e(int l, double x, gsl_sf_result * result)

These routines compute the regular spherical Bessel function of order l, j_l(x), for l \geq 0 and x \geq 0.

int gsl_sf_bessel_jl_array(int lmax, double x, double result_array[])

This routine computes the values of the regular spherical Bessel functions j_l(x) for l from 0 to lmax inclusive for lmax \geq 0 and x \geq 0, storing the results in the array result_array. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

int gsl_sf_bessel_jl_steed_array(int lmax, double x, double * result_array)

This routine uses Steed’s method to compute the values of the regular spherical Bessel functions j_l(x) for l from 0 to lmax inclusive for lmax \geq 0 and x \geq 0, storing the results in the array result_array. The Steed/Barnett algorithm is described in Comp. Phys. Comm. 21, 297 (1981). Steed’s method is more stable than the recurrence used in the other functions but is also slower.

Irregular Spherical Bessel Functions

double gsl_sf_bessel_y0(double x)
int gsl_sf_bessel_y0_e(double x, gsl_sf_result * result)

These routines compute the irregular spherical Bessel function of zeroth order, y_0(x) = -\cos(x)/x.

double gsl_sf_bessel_y1(double x)
int gsl_sf_bessel_y1_e(double x, gsl_sf_result * result)

These routines compute the irregular spherical Bessel function of first order, y_1(x) = -(\cos(x)/x + \sin(x))/x.

double gsl_sf_bessel_y2(double x)
int gsl_sf_bessel_y2_e(double x, gsl_sf_result * result)

These routines compute the irregular spherical Bessel function of second order, y_2(x) = (-3/x^3 + 1/x)\cos(x) - (3/x^2)\sin(x).

double gsl_sf_bessel_yl(int l, double x)
int gsl_sf_bessel_yl_e(int l, double x, gsl_sf_result * result)

These routines compute the irregular spherical Bessel function of order l, y_l(x), for l \geq 0.

int gsl_sf_bessel_yl_array(int lmax, double x, double result_array[])

This routine computes the values of the irregular spherical Bessel functions y_l(x) for l from 0 to lmax inclusive for lmax \geq 0, storing the results in the array result_array. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

Regular Modified Spherical Bessel Functions

The regular modified spherical Bessel functions i_l(x) are related to the modified Bessel functions of fractional order, i_l(x) = \sqrt{\pi/(2x)} I_{l+1/2}(x)

double gsl_sf_bessel_i0_scaled(double x)
int gsl_sf_bessel_i0_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled regular modified spherical Bessel function of zeroth order, \exp(-|x|) i_0(x).

double gsl_sf_bessel_i1_scaled(double x)
int gsl_sf_bessel_i1_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled regular modified spherical Bessel function of first order, \exp(-|x|) i_1(x).

double gsl_sf_bessel_i2_scaled(double x)
int gsl_sf_bessel_i2_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled regular modified spherical Bessel function of second order, \exp(-|x|) i_2(x)

double gsl_sf_bessel_il_scaled(int l, double x)
int gsl_sf_bessel_il_scaled_e(int l, double x, gsl_sf_result * result)

These routines compute the scaled regular modified spherical Bessel function of order l, \exp(-|x|) i_l(x)

int gsl_sf_bessel_il_scaled_array(int lmax, double x, double result_array[])

This routine computes the values of the scaled regular modified spherical Bessel functions \exp(-|x|) i_l(x) for l from 0 to lmax inclusive for lmax \geq 0, storing the results in the array result_array. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

Irregular Modified Spherical Bessel Functions

The irregular modified spherical Bessel functions k_l(x) are related to the irregular modified Bessel functions of fractional order, k_l(x) = \sqrt{\pi/(2x)} K_{l+1/2}(x).

double gsl_sf_bessel_k0_scaled(double x)
int gsl_sf_bessel_k0_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled irregular modified spherical Bessel function of zeroth order, \exp(x) k_0(x), for x>0.

double gsl_sf_bessel_k1_scaled(double x)
int gsl_sf_bessel_k1_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled irregular modified spherical Bessel function of first order, \exp(x) k_1(x), for x>0.

double gsl_sf_bessel_k2_scaled(double x)
int gsl_sf_bessel_k2_scaled_e(double x, gsl_sf_result * result)

These routines compute the scaled irregular modified spherical Bessel function of second order, \exp(x) k_2(x), for x>0.

double gsl_sf_bessel_kl_scaled(int l, double x)
int gsl_sf_bessel_kl_scaled_e(int l, double x, gsl_sf_result * result)

These routines compute the scaled irregular modified spherical Bessel function of order l, \exp(x) k_l(x), for x>0.

int gsl_sf_bessel_kl_scaled_array(int lmax, double x, double result_array[])

This routine computes the values of the scaled irregular modified spherical Bessel functions \exp(x) k_l(x) for l from 0 to lmax inclusive for lmax \geq 0 and x>0, storing the results in the array result_array. The values are computed using recurrence relations for efficiency, and therefore may differ slightly from the exact values.

Regular Bessel Function—Fractional Order

double gsl_sf_bessel_Jnu(double nu, double x)
int gsl_sf_bessel_Jnu_e(double nu, double x, gsl_sf_result * result)

These routines compute the regular cylindrical Bessel function of fractional order \nu, J_\nu(x).

int gsl_sf_bessel_sequence_Jnu_e(double nu, gsl_mode_t mode, size_t size, double v[])

This function computes the regular cylindrical Bessel function of fractional order \nu, J_\nu(x), evaluated at a series of x values. The array v of length size contains the x values. They are assumed to be strictly ordered and positive. The array is over-written with the values of J_\nu(x_i).

Irregular Bessel Functions—Fractional Order

double gsl_sf_bessel_Ynu(double nu, double x)
int gsl_sf_bessel_Ynu_e(double nu, double x, gsl_sf_result * result)

These routines compute the irregular cylindrical Bessel function of fractional order \nu, Y_\nu(x).

Regular Modified Bessel Functions—Fractional Order

double gsl_sf_bessel_Inu(double nu, double x)
int gsl_sf_bessel_Inu_e(double nu, double x, gsl_sf_result * result)

These routines compute the regular modified Bessel function of fractional order \nu, I_\nu(x) for x>0, \nu>0.

double gsl_sf_bessel_Inu_scaled(double nu, double x)
int gsl_sf_bessel_Inu_scaled_e(double nu, double x, gsl_sf_result * result)

These routines compute the scaled regular modified Bessel function of fractional order \nu, \exp(-|x|)I_\nu(x) for x>0, \nu>0.

Irregular Modified Bessel Functions—Fractional Order

double gsl_sf_bessel_Knu(double nu, double x)
int gsl_sf_bessel_Knu_e(double nu, double x, gsl_sf_result * result)

These routines compute the irregular modified Bessel function of fractional order \nu, K_\nu(x) for x>0, \nu>0.

double gsl_sf_bessel_lnKnu(double nu, double x)
int gsl_sf_bessel_lnKnu_e(double nu, double x, gsl_sf_result * result)

These routines compute the logarithm of the irregular modified Bessel function of fractional order \nu, \ln(K_\nu(x)) for x>0, \nu>0.

double gsl_sf_bessel_Knu_scaled(double nu, double x)
int gsl_sf_bessel_Knu_scaled_e(double nu, double x, gsl_sf_result * result)

These routines compute the scaled irregular modified Bessel function of fractional order \nu, \exp(+|x|) K_\nu(x) for x>0, \nu>0.

Zeros of Regular Bessel Functions

double gsl_sf_bessel_zero_J0(unsigned int s)
int gsl_sf_bessel_zero_J0_e(unsigned int s, gsl_sf_result * result)

These routines compute the location of the s-th positive zero of the Bessel function J_0(x).

double gsl_sf_bessel_zero_J1(unsigned int s)
int gsl_sf_bessel_zero_J1_e(unsigned int s, gsl_sf_result * result)

These routines compute the location of the s-th positive zero of the Bessel function J_1(x).

double gsl_sf_bessel_zero_Jnu(double nu, unsigned int s)
int gsl_sf_bessel_zero_Jnu_e(double nu, unsigned int s, gsl_sf_result * result)

These routines compute the location of the s-th positive zero of the Bessel function J_\nu(x). The current implementation does not support negative values of nu.

Clausen Functions

The Clausen function is defined by the following integral,

Cl_2(x) = - \int_0^x dt \log{\left( 2 \sin{(t/2)} \right)}

It is related to the dilogarithm by Cl_2(\theta) = \Im Li_2(\exp(i\theta)). The Clausen functions are declared in the header file gsl_sf_clausen.h.

double gsl_sf_clausen(double x)
int gsl_sf_clausen_e(double x, gsl_sf_result * result)

These routines compute the Clausen integral Cl_2(x).

Coulomb Functions

The prototypes of the Coulomb functions are declared in the header file gsl_sf_coulomb.h. Both bound state and scattering solutions are available.

Normalized Hydrogenic Bound States

double gsl_sf_hydrogenicR_1(double Z, double r)
int gsl_sf_hydrogenicR_1_e(double Z, double r, gsl_sf_result * result)

These routines compute the lowest-order normalized hydrogenic bound state radial wavefunction R_1 := 2Z \sqrt{Z} \exp(-Z r).

double gsl_sf_hydrogenicR(int n, int l, double Z, double r)
int gsl_sf_hydrogenicR_e(int n, int l, double Z, double r, gsl_sf_result * result)

These routines compute the n-th normalized hydrogenic bound state radial wavefunction,

R_n := {2 Z^{3/2} \over n^2}  \left({2Z r \over n}\right)^l  \sqrt{(n-l-1)! \over (n+l)!} \exp(-Z r/n) L^{2l+1}_{n-l-1}(2Z r / n).

where L^a_b(x) is the generalized Laguerre polynomial. The normalization is chosen such that the wavefunction \psi is given by \psi(n,l,r) = R_n Y_{lm}.

Coulomb Wave Functions

The Coulomb wave functions F_L(\eta,x), G_L(\eta,x) are described in Abramowitz & Stegun, Chapter 14. Because there can be a large dynamic range of values for these functions, overflows are handled gracefully. If an overflow occurs, GSL_EOVRFLW is signalled and exponent(s) are returned through the modifiable parameters exp_F, exp_G. The full solution can be reconstructed from the following relations,

F_L(\eta,x) &= fc[k_L] * \exp(exp_F) \\
G_L(\eta,x) &= gc[k_L] * \exp(exp_G)

F_L'(\eta,x) &= fcp[k_L] * \exp(exp_F) \\
G_L'(\eta,x) &= gcp[k_L] * \exp(exp_G)

int gsl_sf_coulomb_wave_FG_e(double eta, double x, double L_F, int k, gsl_sf_result * F, gsl_sf_result * Fp, gsl_sf_result * G, gsl_sf_result * Gp, double * exp_F, double * exp_G)

This function computes the Coulomb wave functions F_L(\eta,x), G_{L-k}(\eta,x) and their derivatives F'_L(\eta,x), G'_{L-k}(\eta,x) with respect to x. The parameters are restricted to L, L-k > -1/2, x > 0 and integer k. Note that L itself is not restricted to being an integer. The results are stored in the parameters F, G for the function values and Fp, Gp for the derivative values. If an overflow occurs, GSL_EOVRFLW is returned and scaling exponents are stored in the modifiable parameters exp_F, exp_G.

int gsl_sf_coulomb_wave_F_array(double L_min, int kmax, double eta, double x, double fc_array[], double * F_exponent)

This function computes the Coulomb wave function F_L(\eta,x) for L = Lmin \dots Lmin + kmax, storing the results in fc_array. In the case of overflow the exponent is stored in F_exponent.

int gsl_sf_coulomb_wave_FG_array(double L_min, int kmax, double eta, double x, double fc_array[], double gc_array[], double * F_exponent, double * G_exponent)

This function computes the functions F_L(\eta,x), G_L(\eta,x) for L = Lmin \dots Lmin + kmax storing the results in fc_array and gc_array. In the case of overflow the exponents are stored in F_exponent and G_exponent.

int gsl_sf_coulomb_wave_FGp_array(double L_min, int kmax, double eta, double x, double fc_array[], double fcp_array[], double gc_array[], double gcp_array[], double * F_exponent, double * G_exponent)

This function computes the functions F_L(\eta,x), G_L(\eta,x) and their derivatives F'_L(\eta,x), G'_L(\eta,x) for L = Lmin \dots Lmin + kmax storing the results in fc_array, gc_array, fcp_array and gcp_array. In the case of overflow the exponents are stored in F_exponent and G_exponent.

int gsl_sf_coulomb_wave_sphF_array(double L_min, int kmax, double eta, double x, double fc_array[], double F_exponent[])

This function computes the Coulomb wave function divided by the argument F_L(\eta, x)/x for L = Lmin \dots Lmin + kmax, storing the results in fc_array. In the case of overflow the exponent is stored in F_exponent. This function reduces to spherical Bessel functions in the limit \eta \to 0.

Coulomb Wave Function Normalization Constant

The Coulomb wave function normalization constant is defined in Abramowitz 14.1.7.

int gsl_sf_coulomb_CL_e(double L, double eta, gsl_sf_result * result)

This function computes the Coulomb wave function normalization constant C_L(\eta) for L > -1.

int gsl_sf_coulomb_CL_array(double Lmin, int kmax, double eta, double cl[])

This function computes the Coulomb wave function normalization constant C_L(\eta) for L = Lmin \dots Lmin + kmax, Lmin > -1.

Coupling Coefficients

The Wigner 3-j, 6-j and 9-j symbols give the coupling coefficients for combined angular momentum vectors. Since the arguments of the standard coupling coefficient functions are integer or half-integer, the arguments of the following functions are, by convention, integers equal to twice the actual spin value. For information on the 3-j coefficients see Abramowitz & Stegun, Section 27.9. The functions described in this section are declared in the header file gsl_sf_coupling.h.

3-j Symbols

double gsl_sf_coupling_3j(int two_ja, int two_jb, int two_jc, int two_ma, int two_mb, int two_mc)
int gsl_sf_coupling_3j_e(int two_ja, int two_jb, int two_jc, int two_ma, int two_mb, int two_mc, gsl_sf_result * result)

These routines compute the Wigner 3-j coefficient,

\left(
\begin{array}{ccc}
  ja & jb & jc \\
  ma & mb & mc
\end{array}
\right)

where the arguments are given in half-integer units, ja = two_ja/2, ma = two_ma/2, etc.

6-j Symbols

double gsl_sf_coupling_6j(int two_ja, int two_jb, int two_jc, int two_jd, int two_je, int two_jf)
int gsl_sf_coupling_6j_e(int two_ja, int two_jb, int two_jc, int two_jd, int two_je, int two_jf, gsl_sf_result * result)

These routines compute the Wigner 6-j coefficient,

\left\{
\begin{array}{ccc}
  ja & jb & jc \\
  jd & je & jf
\end{array}
\right\}

where the arguments are given in half-integer units, ja = two_ja/2, ma = two_ma/2, etc.

9-j Symbols

double gsl_sf_coupling_9j(int two_ja, int two_jb, int two_jc, int two_jd, int two_je, int two_jf, int two_jg, int two_jh, int two_ji)
int gsl_sf_coupling_9j_e(int two_ja, int two_jb, int two_jc, int two_jd, int two_je, int two_jf, int two_jg, int two_jh, int two_ji, gsl_sf_result * result)

These routines compute the Wigner 9-j coefficient,

\left\{
\begin{array}{ccc}
  ja & jb & jc \\
  jd & je & jf \\
  jg & jh & ji
\end{array}
\right\}

where the arguments are given in half-integer units, ja = two_ja/2, ma = two_ma/2, etc.

Dawson Function

The Dawson integral is defined by

\exp(-x^2) \int_0^x dt \exp(t^2)

A table of Dawson’s integral can be found in Abramowitz & Stegun, Table 7.5. The Dawson functions are declared in the header file gsl_sf_dawson.h.

double gsl_sf_dawson(double x)
int gsl_sf_dawson_e(double x, gsl_sf_result * result)

These routines compute the value of Dawson’s integral for x.

Debye Functions

The Debye functions D_n(x) are defined by the following integral,

D_n(x) = {n \over x^n} \int_0^x dt {t^n \over e^t - 1}

For further information see Abramowitz & Stegun, Section 27.1. The Debye functions are declared in the header file gsl_sf_debye.h.

double gsl_sf_debye_1(double x)
int gsl_sf_debye_1_e(double x, gsl_sf_result * result)

These routines compute the first-order Debye function D_1(x).

double gsl_sf_debye_2(double x)
int gsl_sf_debye_2_e(double x, gsl_sf_result * result)

These routines compute the second-order Debye function D_2(x).

double gsl_sf_debye_3(double x)
int gsl_sf_debye_3_e(double x, gsl_sf_result * result)

These routines compute the third-order Debye function D_3(x).

double gsl_sf_debye_4(double x)
int gsl_sf_debye_4_e(double x, gsl_sf_result * result)

These routines compute the fourth-order Debye function D_4(x).

double gsl_sf_debye_5(double x)
int gsl_sf_debye_5_e(double x, gsl_sf_result * result)

These routines compute the fifth-order Debye function D_5(x).

double gsl_sf_debye_6(double x)
int gsl_sf_debye_6_e(double x, gsl_sf_result * result)

These routines compute the sixth-order Debye function D_6(x).

Dilogarithm

The dilogarithm is defined as

Li_2(z) = - \int_0^z ds {\log{(1-s)} \over s}

The functions described in this section are declared in the header file gsl_sf_dilog.h.

Real Argument

double gsl_sf_dilog(double x)
int gsl_sf_dilog_e(double x, gsl_sf_result * result)

These routines compute the dilogarithm for a real argument. In Lewin’s notation this is Li_2(x), the real part of the dilogarithm of a real x. It is defined by the integral representation

Li_2(x) = - \Re \int_0^x ds \log(1-s) / s

Note that \Im(Li_2(x)) = 0 for x \le 1, and -\pi\log(x) for x > 1.

Note that Abramowitz & Stegun refer to the Spence integral S(x) = Li_2(1 - x) as the dilogarithm rather than Li_2(x).

Complex Argument

int gsl_sf_complex_dilog_e(double r, double theta, gsl_sf_result * result_re, gsl_sf_result * result_im)

This function computes the full complex-valued dilogarithm for the complex argument z = r \exp(i \theta). The real and imaginary parts of the result are returned in result_re, result_im.

Elementary Operations

The following functions allow for the propagation of errors when combining quantities by multiplication. The functions are declared in the header file gsl_sf_elementary.h.

double gsl_sf_multiply(double x, double y)
int gsl_sf_multiply_e(double x, double y, gsl_sf_result * result)

This function multiplies x and y storing the product and its associated error in result.

int gsl_sf_multiply_err_e(double x, double dx, double y, double dy, gsl_sf_result * result)

This function multiplies x and y with associated absolute errors dx and dy. The product xy \pm xy \sqrt{(dx/x)^2 +(dy/y)^2} is stored in result.

Elliptic Integrals

The functions described in this section are declared in the header file gsl_sf_ellint.h. Further information about the elliptic integrals can be found in Abramowitz & Stegun, Chapter 17.

Definition of Legendre Forms

The Legendre forms of elliptic integrals F(\phi,k), E(\phi,k) and \Pi(\phi,k,n) are defined by,

F(\phi,k)   &= \int_0^\phi dt {1 \over \sqrt{(1 - k^2 \sin^2(t))}} \\
E(\phi,k)   &= \int_0^\phi dt   \sqrt{(1 - k^2 \sin^2(t))} \\
\Pi(\phi,k,n) &= \int_0^\phi dt {1 \over (1 + n \sin^2(t)) \sqrt{1 - k^2 \sin^2(t)}}

The complete Legendre forms are denoted by K(k) = F(\pi/2, k) and E(k) = E(\pi/2, k).

The notation used here is based on Carlson, “Numerische Mathematik” 33 (1979) 1 and differs slightly from that used by Abramowitz & Stegun, where the functions are given in terms of the parameter m = k^2 and n is replaced by -n.

Definition of Carlson Forms

The Carlson symmetric forms of elliptical integrals RC(x,y), RD(x,y,z), RF(x,y,z) and RJ(x,y,z,p) are defined by,

RC(x,y)   &= 1/2 \int_0^\infty dt (t+x)^{-1/2} (t+y)^{-1} \\
RD(x,y,z) &= 3/2 \int_0^\infty dt (t+x)^{-1/2} (t+y)^{-1/2} (t+z)^{-3/2} \\
RF(x,y,z) &= 1/2 \int_0^\infty dt (t+x)^{-1/2} (t+y)^{-1/2} (t+z)^{-1/2} \\
RJ(x,y,z,p) &= 3/2 \int_0^\infty dt (t+x)^{-1/2} (t+y)^{-1/2} (t+z)^{-1/2} (t+p)^{-1}

Legendre Form of Complete Elliptic Integrals

double gsl_sf_ellint_Kcomp(double k, gsl_mode_t mode)
int gsl_sf_ellint_Kcomp_e(double k, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the complete elliptic integral K(k) to the accuracy specified by the mode variable mode. Note that Abramowitz & Stegun define this function in terms of the parameter m = k^2.

double gsl_sf_ellint_Ecomp(double k, gsl_mode_t mode)
int gsl_sf_ellint_Ecomp_e(double k, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the complete elliptic integral E(k) to the accuracy specified by the mode variable mode. Note that Abramowitz & Stegun define this function in terms of the parameter m = k^2.

double gsl_sf_ellint_Pcomp(double k, double n, gsl_mode_t mode)
int gsl_sf_ellint_Pcomp_e(double k, double n, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the complete elliptic integral \Pi(k,n) to the accuracy specified by the mode variable mode. Note that Abramowitz & Stegun define this function in terms of the parameters m = k^2 and \sin^2(\alpha) = k^2, with the change of sign n \to -n.

Legendre Form of Incomplete Elliptic Integrals

double gsl_sf_ellint_F(double phi, double k, gsl_mode_t mode)
int gsl_sf_ellint_F_e(double phi, double k, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the incomplete elliptic integral F(\phi,k) to the accuracy specified by the mode variable mode. Note that Abramowitz & Stegun define this function in terms of the parameter m = k^2.

double gsl_sf_ellint_E(double phi, double k, gsl_mode_t mode)
int gsl_sf_ellint_E_e(double phi, double k, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the incomplete elliptic integral E(\phi,k) to the accuracy specified by the mode variable mode. Note that Abramowitz & Stegun define this function in terms of the parameter m = k^2.

double gsl_sf_ellint_P(double phi, double k, double n, gsl_mode_t mode)
int gsl_sf_ellint_P_e(double phi, double k, double n, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the incomplete elliptic integral \Pi(\phi,k,n) to the accuracy specified by the mode variable mode. Note that Abramowitz & Stegun define this function in terms of the parameters m = k^2 and \sin^2(\alpha) = k^2, with the change of sign n \to -n.

double gsl_sf_ellint_D(double phi, double k, gsl_mode_t mode)
int gsl_sf_ellint_D_e(double phi, double k, gsl_mode_t mode, gsl_sf_result * result)

These functions compute the incomplete elliptic integral D(\phi,k) which is defined through the Carlson form RD(x,y,z) by the following relation,

D(\phi,k) = {1 \over 3} (\sin \phi)^3 RD (1-\sin^2(\phi), 1-k^2 \sin^2(\phi), 1)

Carlson Forms

double gsl_sf_ellint_RC(double x, double y, gsl_mode_t mode)
int gsl_sf_ellint_RC_e(double x, double y, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the incomplete elliptic integral RC(x,y) to the accuracy specified by the mode variable mode.

double gsl_sf_ellint_RD(double x, double y, double z, gsl_mode_t mode)
int gsl_sf_ellint_RD_e(double x, double y, double z, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the incomplete elliptic integral RD(x,y,z) to the accuracy specified by the mode variable mode.

double gsl_sf_ellint_RF(double x, double y, double z, gsl_mode_t mode)
int gsl_sf_ellint_RF_e(double x, double y, double z, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the incomplete elliptic integral RF(x,y,z) to the accuracy specified by the mode variable mode.

double gsl_sf_ellint_RJ(double x, double y, double z, double p, gsl_mode_t mode)
int gsl_sf_ellint_RJ_e(double x, double y, double z, double p, gsl_mode_t mode, gsl_sf_result * result)

These routines compute the incomplete elliptic integral RJ(x,y,z,p) to the accuracy specified by the mode variable mode.

Elliptic Functions (Jacobi)

The Jacobian Elliptic functions are defined in Abramowitz & Stegun, Chapter 16. The functions are declared in the header file gsl_sf_elljac.h.

int gsl_sf_elljac_e(double u, double m, double * sn, double * cn, double * dn)

This function computes the Jacobian elliptic functions sn(u|m), cn(u|m), dn(u|m) by descending Landen transformations.

Error Functions

The error function is described in Abramowitz & Stegun, Chapter 7. The functions in this section are declared in the header file gsl_sf_erf.h.

Error Function

double gsl_sf_erf(double x)
int gsl_sf_erf_e(double x, gsl_sf_result * result)

These routines compute the error function \erf(x), where \erf(x) = (2/\sqrt{\pi}) \int_0^x dt \exp(-t^2).

Complementary Error Function

double gsl_sf_erfc(double x)
int gsl_sf_erfc_e(double x, gsl_sf_result * result)

These routines compute the complementary error function \erfc(x) = 1 - \erf(x) = (2/\sqrt{\pi}) \int_x^\infty \exp(-t^2)

Log Complementary Error Function

double gsl_sf_log_erfc(double x)
int gsl_sf_log_erfc_e(double x, gsl_sf_result * result)

These routines compute the logarithm of the complementary error function \log(\erfc(x)).

Probability functions

The probability functions for the Normal or Gaussian distribution are described in Abramowitz & Stegun, Section 26.2.

double gsl_sf_erf_Z(double x)
int gsl_sf_erf_Z_e(double x, gsl_sf_result * result)

These routines compute the Gaussian probability density function Z(x) = (1/\sqrt{2\pi}) \exp(-x^2/2)

double gsl_sf_erf_Q(double x)
int gsl_sf_erf_Q_e(double x, gsl_sf_result * result)

These routines compute the upper tail of the Gaussian probability function Q(x) = (1/\sqrt{2\pi}) \int_x^\infty dt \exp(-t^2/2)

The hazard function for the normal distribution, also known as the inverse Mills’ ratio, is defined as,

h(x) = {Z(x) \over Q(x)} = \sqrt{2 \over \pi} {\exp(-x^2 / 2) \over \erfc(x/\sqrt 2)}

It decreases rapidly as x approaches -\infty and asymptotes to h(x) \sim x as x approaches +\infty.

double gsl_sf_hazard(double x)
int gsl_sf_hazard_e(double x, gsl_sf_result * result)

These routines compute the hazard function for the normal distribution.

Exponential Functions

The functions described in this section are declared in the header file gsl_sf_exp.h.

Exponential Function

double gsl_sf_exp(double x)
int gsl_sf_exp_e(double x, gsl_sf_result * result)

These routines provide an exponential function \exp(x) using GSL semantics and error checking.

int gsl_sf_exp_e10_e(double x, gsl_sf_result_e10 * result)

This function computes the exponential \exp(x) using the gsl_sf_result_e10 type to return a result with extended range. This function may be useful if the value of \exp(x) would overflow the numeric range of double.

double gsl_sf_exp_mult(double x, double y)
int gsl_sf_exp_mult_e(double x, double y, gsl_sf_result * result)

These routines exponentiate x and multiply by the factor y to return the product y \exp(x).

int gsl_sf_exp_mult_e10_e(const double x, const double y, gsl_sf_result_e10 * result)

This function computes the product y \exp(x) using the gsl_sf_result_e10 type to return a result with extended numeric range.

Relative Exponential Functions

double gsl_sf_expm1(double x)
int gsl_sf_expm1_e(double x, gsl_sf_result * result)

These routines compute the quantity \exp(x)-1 using an algorithm that is accurate for small x.

double gsl_sf_exprel(double x)
int gsl_sf_exprel_e(double x, gsl_sf_result * result)

These routines compute the quantity (\exp(x)-1)/x using an algorithm that is accurate for small x. For small x the algorithm is based on the expansion (\exp(x)-1)/x = 1 + x/2 + x^2/(2*3) + x^3/(2*3*4) + \dots.

double gsl_sf_exprel_2(double x)
int gsl_sf_exprel_2_e(double x, gsl_sf_result * result)

These routines compute the quantity 2(\exp(x)-1-x)/x^2 using an algorithm that is accurate for small x. For small x the algorithm is based on the expansion 2(\exp(x)-1-x)/x^2 = 1 + x/3 + x^2/(3*4) + x^3/(3*4*5) + \dots.

double gsl_sf_exprel_n(int n, double x)
int gsl_sf_exprel_n_e(int n, double x, gsl_sf_result * result)

These routines compute the N-relative exponential, which is the n-th generalization of the functions gsl_sf_exprel() and gsl_sf_exprel_2(). The N-relative exponential is given by,

\hbox{exprel}_N(x)
            &= N!/x^N \left(\exp(x) - \sum_{k=0}^{N-1} x^k/k!\right)\cr
            &= 1 + x/(N+1) + x^2/((N+1)(N+2)) + \dots\cr
            &= {}_1F_1(1,1+N,x)\cr

Exponentiation With Error Estimate

int gsl_sf_exp_err_e(double x, double dx, gsl_sf_result * result)

This function exponentiates x with an associated absolute error dx.

int gsl_sf_exp_err_e10_e(double x, double dx, gsl_sf_result_e10 * result)

This function exponentiates a quantity x with an associated absolute error dx using the gsl_sf_result_e10 type to return a result with extended range.

int gsl_sf_exp_mult_err_e(double x, double dx, double y, double dy, gsl_sf_result * result)

This routine computes the product y \exp(x) for the quantities x, y with associated absolute errors dx, dy.

int gsl_sf_exp_mult_err_e10_e(double x, double dx, double y, double dy, gsl_sf_result_e10 * result)

This routine computes the product y \exp(x) for the quantities x, y with associated absolute errors dx, dy using the gsl_sf_result_e10 type to return a result with extended range.

Exponential Integrals

Information on the exponential integrals can be found in Abramowitz & Stegun, Chapter 5. These functions are declared in the header file gsl_sf_expint.h.

Exponential Integral

double gsl_sf_expint_E1(double x)
int gsl_sf_expint_E1_e(double x, gsl_sf_result * result)

These routines compute the exponential integral E_1(x),

E_1(x) := \Re \int_1^\infty dt \exp(-xt)/t.

double gsl_sf_expint_E2(double x)
int gsl_sf_expint_E2_e(double x, gsl_sf_result * result)

These routines compute the second-order exponential integral E_2(x),

E_2(x) := \Re \int_1^\infty dt \exp(-xt)/t^2

double gsl_sf_expint_En(int n, double x)
int gsl_sf_expint_En_e(int n, double x, gsl_sf_result * result)

These routines compute the exponential integral E_n(x) of order n,

E_n(x) := \Re \int_1^\infty dt \exp(-xt)/t^n.

Ei(x)

double gsl_sf_expint_Ei(double x)
int gsl_sf_expint_Ei_e(double x, gsl_sf_result * result)

These routines compute the exponential integral Ei(x),

\hbox{Ei}(x) = - PV \left( \int_{-x}^\infty dt \exp(-t)/t \right)

where PV denotes the principal value of the integral.

Hyperbolic Integrals

double gsl_sf_Shi(double x)
int gsl_sf_Shi_e(double x, gsl_sf_result * result)

These routines compute the integral

\hbox{Shi}(x) = \int_0^x dt \sinh(t)/t

double gsl_sf_Chi(double x)
int gsl_sf_Chi_e(double x, gsl_sf_result * result)

These routines compute the integral

\hbox{Chi}(x) := \Re \left[ \gamma_E + \log(x) + \int_0^x dt (\cosh(t)-1)/t \right]

where \gamma_E is the Euler constant (available as the macro M_EULER).

Ei_3(x)

double gsl_sf_expint_3(double x)
int gsl_sf_expint_3_e(double x, gsl_sf_result * result)

These routines compute the third-order exponential integral

{\rm Ei}_3(x) = \int_0^x dt \exp(-t^3)

for x \ge 0.

Trigonometric Integrals

double gsl_sf_Si(const double x)
int gsl_sf_Si_e(double x, gsl_sf_result * result)

These routines compute the Sine integral

\hbox{Si}(x) = \int_0^x dt \sin(t)/t

double gsl_sf_Ci(const double x)
int gsl_sf_Ci_e(double x, gsl_sf_result * result)

These routines compute the Cosine integral

\hbox{Ci}(x) = -\int_x^\infty dt \cos(t)/t

for x > 0

Arctangent Integral

double gsl_sf_atanint(double x)
int gsl_sf_atanint_e(double x, gsl_sf_result * result)

These routines compute the Arctangent integral, which is defined as

\hbox{AtanInt}(x) = \int_0^x dt \arctan(t)/t

Fermi-Dirac Function

The functions described in this section are declared in the header file gsl_sf_fermi_dirac.h.

Complete Fermi-Dirac Integrals

The complete Fermi-Dirac integral F_j(x) is given by,

F_j(x) := {1\over\Gamma(j+1)} \int_0^\infty dt {t^j  \over (\exp(t-x) + 1)}

Note that the Fermi-Dirac integral is sometimes defined without the normalisation factor in other texts.

double gsl_sf_fermi_dirac_m1(double x)
int gsl_sf_fermi_dirac_m1_e(double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral with an index of -1. This integral is given by F_{-1}(x) = e^x / (1 + e^x).

double gsl_sf_fermi_dirac_0(double x)
int gsl_sf_fermi_dirac_0_e(double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral with an index of 0. This integral is given by F_0(x) = \ln(1 + e^x).

double gsl_sf_fermi_dirac_1(double x)
int gsl_sf_fermi_dirac_1_e(double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral with an index of 1, F_1(x) = \int_0^\infty dt (t /(\exp(t-x)+1)).

double gsl_sf_fermi_dirac_2(double x)
int gsl_sf_fermi_dirac_2_e(double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral with an index of 2, F_2(x) = (1/2) \int_0^\infty dt (t^2 /(\exp(t-x)+1)).

double gsl_sf_fermi_dirac_int(int j, double x)
int gsl_sf_fermi_dirac_int_e(int j, double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral with an integer index of j, F_j(x) = (1/\Gamma(j+1)) \int_0^\infty dt (t^j /(\exp(t-x)+1)).

double gsl_sf_fermi_dirac_mhalf(double x)
int gsl_sf_fermi_dirac_mhalf_e(double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral F_{-1/2}(x).

double gsl_sf_fermi_dirac_half(double x)
int gsl_sf_fermi_dirac_half_e(double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral F_{1/2}(x).

double gsl_sf_fermi_dirac_3half(double x)
int gsl_sf_fermi_dirac_3half_e(double x, gsl_sf_result * result)

These routines compute the complete Fermi-Dirac integral F_{3/2}(x).

Incomplete Fermi-Dirac Integrals

The incomplete Fermi-Dirac integral F_j(x,b) is given by,

F_j(x,b) := {1\over\Gamma(j+1)} \int_b^\infty dt {t^j  \over (\exp(t-x) + 1)}

double gsl_sf_fermi_dirac_inc_0(double x, double b)
int gsl_sf_fermi_dirac_inc_0_e(double x, double b, gsl_sf_result * result)

These routines compute the incomplete Fermi-Dirac integral with an index of zero, F_0(x,b) = \ln(1 + e^{b-x}) - (b-x)

Gamma and Beta Functions

The following routines compute the gamma and beta functions in their full and incomplete forms, as well as various kinds of factorials. The functions described in this section are declared in the header file gsl_sf_gamma.h.

Gamma Functions

The Gamma function is defined by the following integral,

\Gamma(x) = \int_0^{\infty} dt t^{x-1} \exp(-t)

It is related to the factorial function by \Gamma(n) = (n-1)! for positive integer n. Further information on the Gamma function can be found in Abramowitz & Stegun, Chapter 6.

double gsl_sf_gamma(double x)
int gsl_sf_gamma_e(double x, gsl_sf_result * result)

These routines compute the Gamma function \Gamma(x), subject to x not being a negative integer or zero. The function is computed using the real Lanczos method. The maximum value of x such that \Gamma(x) is not considered an overflow is given by the macro GSL_SF_GAMMA_XMAX and is 171.0.

double gsl_sf_lngamma(double x)
int gsl_sf_lngamma_e(double x, gsl_sf_result * result)

These routines compute the logarithm of the Gamma function, \log(\Gamma(x)), subject to x not being a negative integer or zero. For x < 0 the real part of \log(\Gamma(x)) is returned, which is equivalent to \log(|\Gamma(x)|). The function is computed using the real Lanczos method.

int gsl_sf_lngamma_sgn_e(double x, gsl_sf_result * result_lg, double * sgn)

This routine computes the sign of the gamma function and the logarithm of its magnitude, subject to x not being a negative integer or zero. The function is computed using the real Lanczos method. The value of the gamma function and its error can be reconstructed using the relation \Gamma(x) = sgn * \exp(result\_lg), taking into account the two components of result_lg.

double gsl_sf_gammastar(double x)
int gsl_sf_gammastar_e(double x, gsl_sf_result * result)

These routines compute the regulated Gamma Function \Gamma^*(x) for x > 0. The regulated gamma function is given by,

\Gamma^*(x) &= \Gamma(x)/(\sqrt{2\pi} x^{(x-1/2)} \exp(-x))\cr
            &= \left(1 + {1 \over 12x} + ...\right) \quad\hbox{for~} x\to \infty\cr

and is a useful suggestion of Temme.

double gsl_sf_gammainv(double x)
int gsl_sf_gammainv_e(double x, gsl_sf_result * result)

These routines compute the reciprocal of the gamma function, 1/\Gamma(x) using the real Lanczos method.

int gsl_sf_lngamma_complex_e(double zr, double zi, gsl_sf_result * lnr, gsl_sf_result * arg)

This routine computes \log(\Gamma(z)) for complex z = z_r + i z_i and z not a negative integer or zero, using the complex Lanczos method. The returned parameters are lnr = \log|\Gamma(z)| and arg = \arg(\Gamma(z)) in (-\pi,\pi]. Note that the phase part (arg) is not well-determined when |z| is very large, due to inevitable roundoff in restricting to (-\pi,\pi]. This will result in a GSL_ELOSS error when it occurs. The absolute value part (lnr), however, never suffers from loss of precision.

Factorials

Although factorials can be computed from the Gamma function, using the relation n! = \Gamma(n+1) for non-negative integer n, it is usually more efficient to call the functions in this section, particularly for small values of n, whose factorial values are maintained in hardcoded tables.

double gsl_sf_fact(unsigned int n)
int gsl_sf_fact_e(unsigned int n, gsl_sf_result * result)

These routines compute the factorial n!. The factorial is related to the Gamma function by n! = \Gamma(n+1). The maximum value of n such that n! is not considered an overflow is given by the macro GSL_SF_FACT_NMAX and is 170.

double gsl_sf_doublefact(unsigned int n)
int gsl_sf_doublefact_e(unsigned int n, gsl_sf_result * result)

These routines compute the double factorial n!! = n(n-2)(n-4) \dots. The maximum value of n such that n!! is not considered an overflow is given by the macro GSL_SF_DOUBLEFACT_NMAX and is 297.

double gsl_sf_lnfact(unsigned int n)
int gsl_sf_lnfact_e(unsigned int n, gsl_sf_result * result)

These routines compute the logarithm of the factorial of n, \log(n!). The algorithm is faster than computing \ln(\Gamma(n+1)) via gsl_sf_lngamma() for n < 170, but defers for larger n.

double gsl_sf_lndoublefact(unsigned int n)
int gsl_sf_lndoublefact_e(unsigned int n, gsl_sf_result * result)

These routines compute the logarithm of the double factorial of n, \log(n!!).

double gsl_sf_choose(unsigned int n, unsigned int m)
int gsl_sf_choose_e(unsigned int n, unsigned int m, gsl_sf_result * result)

These routines compute the combinatorial factor n choose m = n!/(m!(n-m)!)

double gsl_sf_lnchoose(unsigned int n, unsigned int m)
int gsl_sf_lnchoose_e(unsigned int n, unsigned int m, gsl_sf_result * result)

These routines compute the logarithm of n choose m. This is equivalent to the sum \log(n!) - \log(m!) - \log((n-m)!).

double gsl_sf_taylorcoeff(int n, double x)
int gsl_sf_taylorcoeff_e(int n, double x, gsl_sf_result * result)

These routines compute the Taylor coefficient x^n / n! for x \ge 0, n \ge 0

Pochhammer Symbol

double gsl_sf_poch(double a, double x)
int gsl_sf_poch_e(double a, double x, gsl_sf_result * result)

These routines compute the Pochhammer symbol (a)_x = \Gamma(a + x)/\Gamma(a). The Pochhammer symbol is also known as the Apell symbol and sometimes written as (a,x). When a and a + x are negative integers or zero, the limiting value of the ratio is returned.

double gsl_sf_lnpoch(double a, double x)
int gsl_sf_lnpoch_e(double a, double x, gsl_sf_result * result)

These routines compute the logarithm of the Pochhammer symbol, \log((a)_x) = \log(\Gamma(a + x)/\Gamma(a)).

int gsl_sf_lnpoch_sgn_e(double a, double x, gsl_sf_result * result, double * sgn)

These routines compute the sign of the Pochhammer symbol and the logarithm of its magnitude. The computed parameters are result = \log(|(a)_x|) with a corresponding error term, and sgn = \sgn((a)_x) where (a)_x = \Gamma(a + x)/\Gamma(a).

double gsl_sf_pochrel(double a, double x)
int gsl_sf_pochrel_e(double a, double x, gsl_sf_result * result)

These routines compute the relative Pochhammer symbol ((a)_x - 1)/x where (a)_x = \Gamma(a + x)/\Gamma(a).

Incomplete Gamma Functions

double gsl_sf_gamma_inc(double a, double x)
int gsl_sf_gamma_inc_e(double a, double x, gsl_sf_result * result)

These functions compute the unnormalized incomplete Gamma Function \Gamma(a,x) = \int_x^\infty dt t^{(a-1)} \exp(-t) for a real and x \ge 0.

double gsl_sf_gamma_inc_Q(double a, double x)
int gsl_sf_gamma_inc_Q_e(double a, double x, gsl_sf_result * result)

These routines compute the normalized incomplete Gamma Function Q(a,x) = 1/\Gamma(a) \int_x^\infty dt t^{(a-1)} \exp(-t) for a > 0, x \ge 0.

double gsl_sf_gamma_inc_P(double a, double x)
int gsl_sf_gamma_inc_P_e(double a, double x, gsl_sf_result * result)

These routines compute the complementary normalized incomplete Gamma Function P(a,x) = 1 - Q(a,x) = 1/\Gamma(a) \int_0^x dt t^{(a-1)} \exp(-t) for a > 0, x \ge 0.

Note that Abramowitz & Stegun call P(a,x) the incomplete gamma function (section 6.5).

Beta Functions

double gsl_sf_beta(double a, double b)
int gsl_sf_beta_e(double a, double b, gsl_sf_result * result)

These routines compute the Beta Function, B(a,b) = \Gamma(a)\Gamma(b)/\Gamma(a+b) subject to a and b not being negative integers.

double gsl_sf_lnbeta(double a, double b)
int gsl_sf_lnbeta_e(double a, double b, gsl_sf_result * result)

These routines compute the logarithm of the Beta Function, \log(B(a,b)) subject to a and b not being negative integers.

Incomplete Beta Function

double gsl_sf_beta_inc(double a, double b, double x)
int gsl_sf_beta_inc_e(double a, double b, double x, gsl_sf_result * result)

These routines compute the normalized incomplete Beta function I_x(a,b) = B_x(a,b) / B(a,b) where

B_x(a,b) = \int_0^x t^{a-1} (1-t)^{b-1} dt

for 0 \le x \le 1. For a > 0, b > 0 the value is computed using a continued fraction expansion. For all other values it is computed using the relation

I_x(a,b,x) = (1/a) x^a {}_2F_1(a,1-b,a+1,x)/B(a,b)

Gegenbauer Functions

The Gegenbauer polynomials are defined in Abramowitz & Stegun, Chapter 22, where they are known as Ultraspherical polynomials. The functions described in this section are declared in the header file gsl_sf_gegenbauer.h.

double gsl_sf_gegenpoly_1(double lambda, double x)
double gsl_sf_gegenpoly_2(double lambda, double x)
double gsl_sf_gegenpoly_3(double lambda, double x)
int gsl_sf_gegenpoly_1_e(double lambda, double x, gsl_sf_result * result)
int gsl_sf_gegenpoly_2_e(double lambda, double x, gsl_sf_result * result)
int gsl_sf_gegenpoly_3_e(double lambda, double x, gsl_sf_result * result)

These functions evaluate the Gegenbauer polynomials C^{(\lambda)}_n(x) using explicit representations for n = 1, 2, 3.

double gsl_sf_gegenpoly_n(int n, double lambda, double x)
int gsl_sf_gegenpoly_n_e(int n, double lambda, double x, gsl_sf_result * result)

These functions evaluate the Gegenbauer polynomial C^{(\lambda)}_n(x) for a specific value of n, lambda, x subject to \lambda > -1/2, n \ge 0.

int gsl_sf_gegenpoly_array(int nmax, double lambda, double x, double result_array[])

This function computes an array of Gegenbauer polynomials C^{(\lambda)}_n(x) for n = 0, 1, 2, \dots, nmax, subject to \lambda > -1/2, nmax \ge 0.

Hermite Polynomials and Functions

The Hermite polynomials exist in two variants: the probabilists’ version He_n(x) and the physicists’version H_n(x). The are defined by the derivatives

He_n(x) & = (-1)^n e^{x^2/2} \left({d \over dx}\right)^n e^{-x^2/2} \\
H_n(x) & = (-1)^n e^{x^2} \left({d \over dx}\right)^n e^{-x^2}

They are connected via

He_n(x) & = 2^{-n/2} H_n \left( {x \over \sqrt{2}} \right) \\
H_n(x) & = 2^{n/2} He_n \left( \sqrt{2} x \right)

and satisfy the ordinary differential equations

He_n^{\prime\prime}(x) - x He_n^{\prime}(x) + n He_n(x) & = 0 \\
H_n^{\prime\prime}(x) - 2x H_n^{\prime}(x) + 2n H_n(x) & = 0

The closely related Hermite functions are defined by

\psi_n(x) = \left( n! \sqrt{\pi} \right)^{-1/2} e^{-x^2/2} He_n \left( {\sqrt{2} x} \right)

and satisfy the Schrödinger equation for a quantum mechanical harmonic oscillator

\psi_n^{\prime\prime}(x) + (2n + 1 - x^2) \psi_n(x) = 0

Maybe most importantly, the Hermite functions \psi_n are eigenfunctions of the (continuous) Fourier transform.

For further information see Abramowitz & Stegun, Chapter 22 and Szego, Gabor (1939, 1958, 1967), Orthogonal Polynomials, American Mathematical Society. The Hermite polynomials and functions are defined in the header file gsl_sf_hermite.h.

Hermite Polynomials

double gsl_sf_hermite_prob(const int n, const double x)
int gsl_sf_hermite_prob_e(const int n, const double x, gsl_sf_result * result)

These routines evaluate the probabilists’ Hermite polynomial He_n(x) of order n at position x.

int gsl_sf_hermite_prob_array(const int nmax, const double x, double * result_array)

This routine evaluates all probabilists’ Hermite polynomials He_n(x) up to order nmax at position x. The results are stored in result_array.

double gsl_sf_hermite_prob_series(const int n, const double x, const double * a)
int gsl_sf_hermite_prob_series_e(const int n, const double x, const double * a, gsl_sf_result * result)

These routines evaluate the series \sum_{j=0}^n a_j He_j(x) with He_j being the j-th probabilists’ Hermite polynomial using the Clenshaw algorithm.

double gsl_sf_hermite_phys(const int n, const double x)
int gsl_sf_hermite_phys_e(const int n, const double x, gsl_sf_result * result)

These routines evaluate the physicists’ Hermite polynomial H_n(x) of order n at position x.

int gsl_sf_hermite_phys_array(const int nmax, const double x, double * result_array)

This routine evaluates all physicists’ Hermite polynomials H_n up to order nmax at position x. The results are stored in result_array.

double gsl_sf_hermite_phys_series(const int n, const double x, const double * a)
int gsl_sf_hermite_phys_series_e(const int n, const double x, const double * a, gsl_sf_result * result)

These routines evaluate the series \sum_{j=0}^n a_j H_j(x) with H_j being the j-th physicists’ Hermite polynomial using the Clenshaw algorithm.

Hermite Functions

double gsl_sf_hermite_func(const int n, const double x)
int gsl_sf_hermite_func_e(const int n, const double x, gsl_sf_result * result)

These routines evaluate the Hermite function \psi_n(x) of order n at position x.

int gsl_sf_hermite_func_array(const int nmax, const double x, double * result_array)

This routine evaluates all Hermite functions \psi_n(x) up to order nmax at position x. The results are stored in result_array.

double gsl_sf_hermite_func_series(const int n, const double x, const double * a)
int gsl_sf_hermite_func_series_e(const int n, const double x, const double * a, gsl_sf_result * result)

These routines evaluate the series \sum_{j=0}^n a_j \psi_j(x) with \psi_j being the j-th Hermite function using the Clenshaw algorithm.

Derivatives of Hermite Polynomials

double gsl_sf_hermite_prob_der(const int m, const int n, const double x)
int gsl_sf_hermite_prob_der_e(const int m, const int n, const double x, gsl_sf_result * result)

These routines evaluate the m-th derivative of the probabilists’ Hermite polynomial He_n(x) of order n at position x.

int gsl_sf_hermite_prob_array_der(const int m, const int nmax, const double x, double * result_array)

This routine evaluates the m-th derivative of all probabilists’ Hermite polynomials He_n(x) up to order nmax at position x. The results are stored in result_array.

int gsl_sf_hermite_prob_der_array(const int mmax, const int n, const double x, double * result_array)

This routine evaluates all derivatives (starting from 0) up to the mmax-th derivative of the probabilists’ Hermite polynomial of order n He_n(x) at position x. The results are stored in result_array.

double gsl_sf_hermite_phys_der(const int m, const int n, const double x)
int gsl_sf_hermite_phys_der_e(const int m, const int n, const double x, gsl_sf_result * result)

These routines evaluate the m-th derivative of the physicists’ Hermite polynomial H_n(x) of order n at position x.

int gsl_sf_hermite_phys_array_der(const int m, const int nmax, const double x, double * result_array)

This routine evaluates the m-th derivative of all physicists’ Hermite polynomials H_n up to order nmax at position x. The results are stored in result_array.

int gsl_sf_hermite_phys_der_array(const int mmax, const int n, const double x, double * result_array)

This routine evaluates all derivatives (starting from 0) up to the mmax-th derivative of the physicists’ Hermite polynomial of order n H_n at position x. The results are stored in result_array.

Derivatives of Hermite Functions

double gsl_sf_hermite_func_der(const int m, const int n, const double x)
int gsl_sf_hermite_func_der_e(const int m, const int n, const double x, gsl_sf_result * result)

These routines evaluate the m-th derivative of the Hermite function \psi_n(x) of order n at position x.

Zeros of Hermite Polynomials and Hermite Functions

These routines calculate the s-th zero of the Hermite Polynomial/Function of order n. Since the zeros are symmetrical around zero, only positive zeros are calculated, ordered from smallest to largest, starting from index 1. Only for odd polynomial orders a zeroth zero exists, its value always being zero.

double gsl_sf_hermite_prob_zero(const int n, const int s)
int gsl_sf_hermite_prob_zero_e(const int n, const int s, gsl_sf_result * result)

These routines evaluate the s-th zero of the probabilists’ Hermite polynomial He_n(x) of order n.

double gsl_sf_hermite_phys_zero(const int n, const int s)
int gsl_sf_hermite_phys_zero_e(const int n, const int s, gsl_sf_result * result)

These routines evaluate the s-th zero of the physicists’ Hermite polynomial H_n(x) of order n.

double gsl_sf_hermite_func_zero(const int n, const int s)
int gsl_sf_hermite_func_zero_e(const int n, const int s, gsl_sf_result * result)

These routines evaluate the s-th zero of the Hermite function \psi_n(x) of order n.

Hypergeometric Functions

Hypergeometric functions are described in Abramowitz & Stegun, Chapters 13 and 15. These functions are declared in the header file gsl_sf_hyperg.h.

double gsl_sf_hyperg_0F1(double c, double x)
int gsl_sf_hyperg_0F1_e(double c, double x, gsl_sf_result * result)

These routines compute the hypergeometric function {}_0F_1(c,x).

double gsl_sf_hyperg_1F1_int(int m, int n, double x)
int gsl_sf_hyperg_1F1_int_e(int m, int n, double x, gsl_sf_result * result)

These routines compute the confluent hypergeometric function {}_1F_1(m,n,x) = M(m,n,x) for integer parameters m, n.

double gsl_sf_hyperg_1F1(double a, double b, double x)
int gsl_sf_hyperg_1F1_e(double a, double b, double x, gsl_sf_result * result)

These routines compute the confluent hypergeometric function {}_1F_1(a,b,x) = M(a,b,x) for general parameters a, b.

double gsl_sf_hyperg_U_int(int m, int n, double x)
int gsl_sf_hyperg_U_int_e(int m, int n, double x, gsl_sf_result * result)

These routines compute the confluent hypergeometric function U(m,n,x) for integer parameters m, n.

int gsl_sf_hyperg_U_int_e10_e(int m, int n, double x, gsl_sf_result_e10 * result)

This routine computes the confluent hypergeometric function U(m,n,x) for integer parameters m, n using the gsl_sf_result_e10 type to return a result with extended range.

double gsl_sf_hyperg_U(double a, double b, double x)
int gsl_sf_hyperg_U_e(double a, double b, double x, gsl_sf_result * result)

These routines compute the confluent hypergeometric function U(a,b,x).

int gsl_sf_hyperg_U_e10_e(double a, double b, double x, gsl_sf_result_e10 * result)

This routine computes the confluent hypergeometric function U(a,b,x) using the gsl_sf_result_e10 type to return a result with extended range.

double gsl_sf_hyperg_2F1(double a, double b, double c, double x)
int gsl_sf_hyperg_2F1_e(double a, double b, double c, double x, gsl_sf_result * result)

These routines compute the Gauss hypergeometric function {}_2F_1(a,b,c,x) = F(a,b,c,x) for |x| < 1.

If the arguments (a,b,c,x) are too close to a singularity then the function can return the error code GSL_EMAXITER when the series approximation converges too slowly. This occurs in the region of x = 1, c - a - b = m for integer m.

double gsl_sf_hyperg_2F1_conj(double aR, double aI, double c, double x)
int gsl_sf_hyperg_2F1_conj_e(double aR, double aI, double c, double x, gsl_sf_result * result)

These routines compute the Gauss hypergeometric function {}_2F_1(a_R + i a_I, aR - i aI, c, x) with complex parameters for |x| < 1.

double gsl_sf_hyperg_2F1_renorm(double a, double b, double c, double x)
int gsl_sf_hyperg_2F1_renorm_e(double a, double b, double c, double x, gsl_sf_result * result)

These routines compute the renormalized Gauss hypergeometric function {}_2F_1(a,b,c,x) / \Gamma(c) for |x| < 1.

double gsl_sf_hyperg_2F1_conj_renorm(double aR, double aI, double c, double x)
int gsl_sf_hyperg_2F1_conj_renorm_e(double aR, double aI, double c, double x, gsl_sf_result * result)

These routines compute the renormalized Gauss hypergeometric function {}_2F_1(a_R + i a_I, a_R - i a_I, c, x) / \Gamma(c) for |x| < 1.

double gsl_sf_hyperg_2F0(double a, double b, double x)
int gsl_sf_hyperg_2F0_e(double a, double b, double x, gsl_sf_result * result)

These routines compute the hypergeometric function {}_2F_0(a,b,x) The series representation is a divergent hypergeometric series. However, for x < 0 we have {}_2F_0(a,b,x) = (-1/x)^a U(a,1+a-b,-1/x)

Laguerre Functions

The generalized Laguerre polynomials, sometimes referred to as associated Laguerre polynomials, are defined in terms of confluent hypergeometric functions as

L^a_n(x) = {(a+1)_n \over n!} {}_1F_1(-n,a+1,x)

where (a)_n is the Pochhammer symbol (rising factorial). They are related to the plain Laguerre polynomials L_n(x) by L^0_n(x) = L_n(x) and L^k_n(x) = (-1)^k (d^k/dx^k) L_{(n+k)}(x) For more information see Abramowitz & Stegun, Chapter 22.

The functions described in this section are declared in the header file gsl_sf_laguerre.h.

double gsl_sf_laguerre_1(double a, double x)
double gsl_sf_laguerre_2(double a, double x)
double gsl_sf_laguerre_3(double a, double x)
int gsl_sf_laguerre_1_e(double a, double x, gsl_sf_result * result)
int gsl_sf_laguerre_2_e(double a, double x, gsl_sf_result * result)
int gsl_sf_laguerre_3_e(double a, double x, gsl_sf_result * result)

These routines evaluate the generalized Laguerre polynomials L^a_1(x), L^a_2(x), L^a_3(x) using explicit representations.

double gsl_sf_laguerre_n(const int n, const double a, const double x)
int gsl_sf_laguerre_n_e(int n, double a, double x, gsl_sf_result * result)

These routines evaluate the generalized Laguerre polynomials L^a_n(x) for a > -1, n \ge 0.

Lambert W Functions

Lambert’s W functions, W(x), are defined to be solutions of the equation W(x) \exp(W(x)) = x. This function has multiple branches for x < 0; however, it has only two real-valued branches. We define W_0(x) to be the principal branch, where W > -1 for x < 0, and W_{-1}(x) to be the other real branch, where W < -1 for x < 0. The Lambert functions are declared in the header file gsl_sf_lambert.h.

double gsl_sf_lambert_W0(double x)
int gsl_sf_lambert_W0_e(double x, gsl_sf_result * result)

These compute the principal branch of the Lambert W function, W_0(x).

double gsl_sf_lambert_Wm1(double x)
int gsl_sf_lambert_Wm1_e(double x, gsl_sf_result * result)

These compute the secondary real-valued branch of the Lambert W function, W_{-1}(x).

Legendre Functions and Spherical Harmonics

The Legendre Functions and Legendre Polynomials are described in Abramowitz & Stegun, Chapter 8. These functions are declared in the header file gsl_sf_legendre.h.

Legendre Polynomials

double gsl_sf_legendre_P1(double x)
double gsl_sf_legendre_P2(double x)
double gsl_sf_legendre_P3(double x)
int gsl_sf_legendre_P1_e(double x, gsl_sf_result * result)
int gsl_sf_legendre_P2_e(double x, gsl_sf_result * result)
int gsl_sf_legendre_P3_e(double x, gsl_sf_result * result)

These functions evaluate the Legendre polynomials P_l(x) using explicit representations for l = 1, 2, 3.

double gsl_sf_legendre_Pl(int l, double x)
int gsl_sf_legendre_Pl_e(int l, double x, gsl_sf_result * result)

These functions evaluate the Legendre polynomial P_l(x) for a specific value of l, x subject to l \ge 0 and |x| \le 1.

int gsl_sf_legendre_Pl_array(int lmax, double x, double result_array[])
int gsl_sf_legendre_Pl_deriv_array(int lmax, double x, double result_array[], double result_deriv_array[])

These functions compute arrays of Legendre polynomials P_l(x) and derivatives dP_l(x)/dx for l = 0, \dots, lmax and |x| \le 1.

double gsl_sf_legendre_Q0(double x)
int gsl_sf_legendre_Q0_e(double x, gsl_sf_result * result)

These routines compute the Legendre function Q_0(x) for x > -1 and x \ne 1.

double gsl_sf_legendre_Q1(double x)
int gsl_sf_legendre_Q1_e(double x, gsl_sf_result * result)

These routines compute the Legendre function Q_1(x) for x > -1 and x \ne 1.

double gsl_sf_legendre_Ql(int l, double x)
int gsl_sf_legendre_Ql_e(int l, double x, gsl_sf_result * result)

These routines compute the Legendre function Q_l(x) for x > -1, x \ne 1 and l \ge 0.

Associated Legendre Polynomials and Spherical Harmonics

The following functions compute the associated Legendre polynomials P_l^m(x) which are solutions of the differential equation

(1 - x^2) {d^2 \over dx^2} P_l^m(x) - 2x {d \over dx} P_l^m(x) +
\left( l(l+1) - {m^2 \over 1 - x^2} \right) P_l^m(x) = 0

where the degree l and order m satisfy 0 \le l and 0 \le m \le l. The functions P_l^m(x) grow combinatorially with l and can overflow for l larger than about 150. Alternatively, one may calculate normalized associated Legendre polynomials. There are a number of different normalization conventions, and these functions can be stably computed up to degree and order 2700. The following normalizations are provided:

  • Schmidt semi-normalization

    Schmidt semi-normalized associated Legendre polynomials are often used in the magnetics community and are defined as

    S_l^0(x) &= P_l^0(x) \\
S_l^m(x) &= (-1)^m \sqrt{2 {(l-m)! \over (l+m)!}} P_l^m(x), m > 0

    The factor of (-1)^m is called the Condon-Shortley phase factor and can be excluded if desired by setting the parameter csphase = 1 in the functions below.

  • Spherical Harmonic Normalization

    The associated Legendre polynomials suitable for calculating spherical harmonics are defined as

    Y_l^m(x) = (-1)^m \sqrt{{2l + 1 \over 4 \pi} {(l-m)! \over (l+m)!}} P_l^m(x)

    where again the phase factor (-1)^m can be included or excluded if desired.

  • Full Normalization

    The fully normalized associated Legendre polynomials are defined as

    N_l^m(x) = (-1)^m \sqrt{(l + {1 \over 2}) {(l-m)! \over (l+m)!}} P_l^m(x)

    and have the property

    \int_{-1}^1 N_l^m(x)^2 dx = 1

The normalized associated Legendre routines below use a recurrence relation which is stable up to a degree and order of about 2700. Beyond this, the computed functions could suffer from underflow leading to incorrect results. Routines are provided to compute first and second derivatives dP_l^m(x)/dx and d^2 P_l^m(x)/dx^2 as well as their alternate versions d P_l^m(\cos{\theta})/d\theta and d^2 P_l^m(\cos{\theta})/d\theta^2. While there is a simple scaling relationship between the two forms, the derivatives involving \theta are heavily used in spherical harmonic expansions and so these routines are also provided.

In the functions below, a parameter of type gsl_sf_legendre_t specifies the type of normalization to use. The possible values are

gsl_sf_legendre_t
Value Description
GSL_SF_LEGENDRE_NONE The unnormalized associated Legendre polynomials P_l^m(x)
GSL_SF_LEGENDRE_SCHMIDT The Schmidt semi-normalized associated Legendre polynomials S_l^m(x)
GSL_SF_LEGENDRE_SPHARM The spherical harmonic associated Legendre polynomials Y_l^m(x)
GSL_SF_LEGENDRE_FULL The fully normalized associated Legendre polynomials N_l^m(x)
int gsl_sf_legendre_array(const gsl_sf_legendre_t norm, const size_t lmax, const double x, double result_array[])
int gsl_sf_legendre_array_e(const gsl_sf_legendre_t norm, const size_t lmax, const double x, const double csphase, double result_array[])

These functions calculate all normalized associated Legendre polynomials for 0 \le l \le lmax and 0 \le m \le l for |x| \le 1. The norm parameter specifies which normalization is used. The normalized P_l^m(x) values are stored in result_array, whose minimum size can be obtained from calling gsl_sf_legendre_array_n(). The array index of P_l^m(x) is obtained from calling gsl_sf_legendre_array_index(l, m). To include or exclude the Condon-Shortley phase factor of (-1)^m, set the parameter csphase to either -1 or 1 respectively in the _e function. This factor is included by default.

int gsl_sf_legendre_deriv_array(const gsl_sf_legendre_t norm, const size_t lmax, const double x, double result_array[], double result_deriv_array[])
int gsl_sf_legendre_deriv_array_e(const gsl_sf_legendre_t norm, const size_t lmax, const double x, const double csphase, double result_array[], double result_deriv_array[])

These functions calculate all normalized associated Legendre functions and their first derivatives up to degree lmax for |x| < 1. The parameter norm specifies the normalization used. The normalized P_l^m(x) values and their derivatives dP_l^m(x)/dx are stored in result_array and result_deriv_array respectively. To include or exclude the Condon-Shortley phase factor of (-1)^m, set the parameter csphase to either -1 or 1 respectively in the _e function. This factor is included by default.

int gsl_sf_legendre_deriv_alt_array(const gsl_sf_legendre_t norm, const size_t lmax, const double x, double result_array[], double result_deriv_array[])
int gsl_sf_legendre_deriv_alt_array_e(const gsl_sf_legendre_t norm, const size_t lmax, const double x, const double csphase, double result_array[], double result_deriv_array[])

These functions calculate all normalized associated Legendre functions and their (alternate) first derivatives up to degree lmax for |x| < 1. The normalized P_l^m(x) values and their derivatives dP_l^m(\cos{\theta})/d\theta are stored in result_array and result_deriv_array respectively. To include or exclude the Condon-Shortley phase factor of (-1)^m, set the parameter csphase to either -1 or 1 respectively in the _e function. This factor is included by default.

int gsl_sf_legendre_deriv2_array(const gsl_sf_legendre_t norm, const size_t lmax, const double x, double result_array[], double result_deriv_array[], double result_deriv2_array[])
int gsl_sf_legendre_deriv2_array_e(const gsl_sf_legendre_t norm, const size_t lmax, const double x, const double csphase, double result_array[], double result_deriv_array[], double result_deriv2_array[])

These functions calculate all normalized associated Legendre functions and their first and second derivatives up to degree lmax for |x| < 1. The parameter norm specifies the normalization used. The normalized P_l^m(x), their first derivatives dP_l^m(x)/dx, and their second derivatives d^2 P_l^m(x)/dx^2 are stored in result_array, result_deriv_array, and result_deriv2_array respectively. To include or exclude the Condon-Shortley phase factor of (-1)^m, set the parameter csphase to either -1 or 1 respectively in the _e function. This factor is included by default.

int gsl_sf_legendre_deriv2_alt_array(const gsl_sf_legendre_t norm, const size_t lmax, const double x, double result_array[], double result_deriv_array[], double result_deriv2_array[])
int gsl_sf_legendre_deriv2_alt_array_e(const gsl_sf_legendre_t norm, const size_t lmax, const double x, const double csphase, double result_array[], double result_deriv_array[], double result_deriv2_array[])

These functions calculate all normalized associated Legendre functions and their (alternate) first and second derivatives up to degree lmax for |x| < 1. The parameter norm specifies the normalization used. The normalized P_l^m(x), their first derivatives dP_l^m(\cos{\theta})/d\theta, and their second derivatives d^2 P_l^m(\cos{\theta})/d\theta^2 are stored in result_array, result_deriv_array, and result_deriv2_array respectively. To include or exclude the Condon-Shortley phase factor of (-1)^m, set the parameter csphase to either -1 or 1 respectively in the _e function. This factor is included by default.

size_t gsl_sf_legendre_array_n(const size_t lmax)

This function returns the minimum array size for maximum degree lmax needed for the array versions of the associated Legendre functions. Size is calculated as the total number of P_l^m(x) functions, plus extra space for precomputing multiplicative factors used in the recurrence relations.

size_t gsl_sf_legendre_array_index(const size_t l, const size_t m)

This function returns the index into result_array, result_deriv_array, or result_deriv2_array corresponding to P_l^m(x), P_l^{'m}(x), or P_l^{''m}(x). The index is given by l(l+1)/2 + m.

double gsl_sf_legendre_Plm(int l, int m, double x)
int gsl_sf_legendre_Plm_e(int l, int m, double x, gsl_sf_result * result)

These routines compute the associated Legendre polynomial P_l^m(x) for m \ge 0, l \ge m, and |x| \le 1.

double gsl_sf_legendre_sphPlm(int l, int m, double x)
int gsl_sf_legendre_sphPlm_e(int l, int m, double x, gsl_sf_result * result)

These routines compute the normalized associated Legendre polynomial \sqrt{(2l+1)/(4\pi)} \sqrt{(l-m)!/(l+m)!} P_l^m(x) suitable for use in spherical harmonics. The parameters must satisfy m \ge 0, l \ge m, and |x| \le 1. These routines avoid the overflows that occur for the standard normalization of P_l^m(x).

int gsl_sf_legendre_Plm_array(int lmax, int m, double x, double result_array[])
int gsl_sf_legendre_Plm_deriv_array(int lmax, int m, double x, double result_array[], double result_deriv_array[])

These functions are now deprecated and will be removed in a future release; see gsl_sf_legendre_array() and gsl_sf_legendre_deriv_array().

int gsl_sf_legendre_sphPlm_array(int lmax, int m, double x, double result_array[])
int gsl_sf_legendre_sphPlm_deriv_array(int lmax, int m, double x, double result_array[], double result_deriv_array[])

These functions are now deprecated and will be removed in a future release; see gsl_sf_legendre_array() and gsl_sf_legendre_deriv_array().

int gsl_sf_legendre_array_size(const int lmax, const int m)

This function is now deprecated and will be removed in a future release.

Conical Functions

The Conical Functions P^\mu_{-(1/2)+i\lambda}(x) and Q^\mu_{-(1/2)+i\lambda} are described in Abramowitz & Stegun, Section 8.12.

double gsl_sf_conicalP_half(double lambda, double x)
int gsl_sf_conicalP_half_e(double lambda, double x, gsl_sf_result * result)

These routines compute the irregular Spherical Conical Function P^{1/2}_{-1/2 + i \lambda}(x) for x > -1.

double gsl_sf_conicalP_mhalf(double lambda, double x)
int gsl_sf_conicalP_mhalf_e(double lambda, double x, gsl_sf_result * result)

These routines compute the regular Spherical Conical Function P^{-1/2}_{-1/2 + i \lambda}(x) for x > -1.

double gsl_sf_conicalP_0(double lambda, double x)
int gsl_sf_conicalP_0_e(double lambda, double x, gsl_sf_result * result)

These routines compute the conical function P^0_{-1/2 + i \lambda}(x) for x > -1.

double gsl_sf_conicalP_1(double lambda, double x)
int gsl_sf_conicalP_1_e(double lambda, double x, gsl_sf_result * result)

These routines compute the conical function P^1_{-1/2 + i \lambda}(x) for x > -1.

double gsl_sf_conicalP_sph_reg(int l, double lambda, double x)
int gsl_sf_conicalP_sph_reg_e(int l, double lambda, double x, gsl_sf_result * result)

These routines compute the Regular Spherical Conical Function P^{-1/2-l}_{-1/2 + i \lambda}(x) for x > -1 and l \ge -1.

double gsl_sf_conicalP_cyl_reg(int m, double lambda, double x)
int gsl_sf_conicalP_cyl_reg_e(int m, double lambda, double x, gsl_sf_result * result)

These routines compute the Regular Cylindrical Conical Function P^{-m}_{-1/2 + i \lambda}(x) for x > -1 and m \ge -1.

Radial Functions for Hyperbolic Space

The following spherical functions are specializations of Legendre functions which give the regular eigenfunctions of the Laplacian on a 3-dimensional hyperbolic space H^3. Of particular interest is the flat limit, \lambda \to \infty, \eta \to 0, \lambda\eta fixed.

double gsl_sf_legendre_H3d_0(double lambda, double eta)
int gsl_sf_legendre_H3d_0_e(double lambda, double eta, gsl_sf_result * result)

These routines compute the zeroth radial eigenfunction of the Laplacian on the 3-dimensional hyperbolic space,

L^{H3d}_0(\lambda,\eta) := {\sin(\lambda\eta) \over \lambda\sinh(\eta)}

for \eta \ge 0. In the flat limit this takes the form L^{H3d}_0(\lambda,\eta) = j_0(\lambda\eta).

double gsl_sf_legendre_H3d_1(double lambda, double eta)
int gsl_sf_legendre_H3d_1_e(double lambda, double eta, gsl_sf_result * result)

These routines compute the first radial eigenfunction of the Laplacian on the 3-dimensional hyperbolic space,

L^{H3d}_1(\lambda,\eta) := {1\over\sqrt{\lambda^2 + 1}} {\left(\sin(\lambda \eta)\over \lambda \sinh(\eta)\right)} \left(\coth(\eta) - \lambda \cot(\lambda\eta)\right)

for \eta \ge 0 In the flat limit this takes the form L^{H3d}_1(\lambda,\eta) = j_1(\lambda\eta).

double gsl_sf_legendre_H3d(int l, double lambda, double eta)
int gsl_sf_legendre_H3d_e(int l, double lambda, double eta, gsl_sf_result * result)

These routines compute the l-th radial eigenfunction of the Laplacian on the 3-dimensional hyperbolic space \eta \ge 0 and l \ge 0. In the flat limit this takes the form L^{H3d}_l(\lambda,\eta) = j_l(\lambda\eta).

int gsl_sf_legendre_H3d_array(int lmax, double lambda, double eta, double result_array[])

This function computes an array of radial eigenfunctions L^{H3d}_l( \lambda, \eta) for 0 \le l \le lmax.

Mathieu Functions

The routines described in this section compute the angular and radial Mathieu functions, and their characteristic values. Mathieu functions are the solutions of the following two differential equations:

{{d^2 y}\over{d v^2}}& + (a - 2q\cos 2v)y  = 0 \\
{{d^2 f}\over{d u^2}}& - (a - 2q\cosh 2u)f  = 0

The angular Mathieu functions ce_r(x,q), se_r(x,q) are the even and odd periodic solutions of the first equation, which is known as Mathieu’s equation. These exist only for the discrete sequence of characteristic values a = a_r(q) (even-periodic) and a = b_r(q) (odd-periodic).

The radial Mathieu functions Mc^{(j)}_{r}(z,q) and Ms^{(j)}_{r}(z,q) are the solutions of the second equation, which is referred to as Mathieu’s modified equation. The radial Mathieu functions of the first, second, third and fourth kind are denoted by the parameter j, which takes the value 1, 2, 3 or 4.

For more information on the Mathieu functions, see Abramowitz and Stegun, Chapter 20. These functions are defined in the header file gsl_sf_mathieu.h.

Mathieu Function Workspace

The Mathieu functions can be computed for a single order or for multiple orders, using array-based routines. The array-based routines require a preallocated workspace.

gsl_sf_mathieu_workspace

Workspace required for array-based routines

gsl_sf_mathieu_workspace * gsl_sf_mathieu_alloc(size_t n, double qmax)

This function returns a workspace for the array versions of the Mathieu routines. The arguments n and qmax specify the maximum order and q-value of Mathieu functions which can be computed with this workspace.

void gsl_sf_mathieu_free(gsl_sf_mathieu_workspace * work)

This function frees the workspace work.

Mathieu Function Characteristic Values

int gsl_sf_mathieu_a(int n, double q)
int gsl_sf_mathieu_a_e(int n, double q, gsl_sf_result * result)
int gsl_sf_mathieu_b(int n, double q)
int gsl_sf_mathieu_b_e(int n, double q, gsl_sf_result * result)

These routines compute the characteristic values a_n(q), b_n(q) of the Mathieu functions ce_n(q,x) and se_n(q,x), respectively.

int gsl_sf_mathieu_a_array(int order_min, int order_max, double q, gsl_sf_mathieu_workspace * work, double result_array[])
int gsl_sf_mathieu_b_array(int order_min, int order_max, double q, gsl_sf_mathieu_workspace * work, double result_array[])

These routines compute a series of Mathieu characteristic values a_n(q), b_n(q) for n from order_min to order_max inclusive, storing the results in the array result_array.

Angular Mathieu Functions

int gsl_sf_mathieu_ce(int n, double q, double x)
int gsl_sf_mathieu_ce_e(int n, double q, double x, gsl_sf_result * result)
int gsl_sf_mathieu_se(int n, double q, double x)
int gsl_sf_mathieu_se_e(int n, double q, double x, gsl_sf_result * result)

These routines compute the angular Mathieu functions ce_n(q,x) and se_n(q,x), respectively.

int gsl_sf_mathieu_ce_array(int nmin, int nmax, double q, double x, gsl_sf_mathieu_workspace * work, double result_array[])
int gsl_sf_mathieu_se_array(int nmin, int nmax, double q, double x, gsl_sf_mathieu_workspace * work, double result_array[])

These routines compute a series of the angular Mathieu functions ce_n(q,x) and se_n(q,x) of order n from nmin to nmax inclusive, storing the results in the array result_array.

Radial Mathieu Functions

int gsl_sf_mathieu_Mc(int j, int n, double q, double x)
int gsl_sf_mathieu_Mc_e(int j, int n, double q, double x, gsl_sf_result * result)
int gsl_sf_mathieu_Ms(int j, int n, double q, double x)
int gsl_sf_mathieu_Ms_e(int j, int n, double q, double x, gsl_sf_result * result)

These routines compute the radial j-th kind Mathieu functions Mc_n^{(j)}(q,x) and Ms_n^{(j)}(q,x) of order n.

The allowed values of j are 1 and 2. The functions for j = 3,4 can be computed as M_n^{(3)} = M_n^{(1)} + iM_n^{(2)} and M_n^{(4)} = M_n^{(1)} - iM_n^{(2)}, where M_n^{(j)} = Mc_n^{(j)} or Ms_n^{(j)}.

int gsl_sf_mathieu_Mc_array(int j, int nmin, int nmax, double q, double x, gsl_sf_mathieu_workspace * work, double result_array[])
int gsl_sf_mathieu_Ms_array(int j, int nmin, int nmax, double q, double x, gsl_sf_mathieu_workspace * work, double result_array[])

These routines compute a series of the radial Mathieu functions of kind j, with order from nmin to nmax inclusive, storing the results in the array result_array.

Power Function

The following functions are equivalent to the function gsl_pow_int() with an error estimate. These functions are declared in the header file gsl_sf_pow_int.h.

double gsl_sf_pow_int(double x, int n)
int gsl_sf_pow_int_e(double x, int n, gsl_sf_result * result)

These routines compute the power x^n for integer n. The power is computed using the minimum number of multiplications. For example, x^8 is computed as ((x^2)^2)^2, requiring only 3 multiplications. For reasons of efficiency, these functions do not check for overflow or underflow conditions. The following is a simple example:

#include <gsl/gsl_sf_pow_int.h>
/* compute 3.0**12 */
double y = gsl_sf_pow_int(3.0, 12);

Psi (Digamma) Function

The polygamma functions of order n are defined by

\psi^{(n)}(x) = \left(d \over dx\right)^n \psi(x) = \left(d \over dx\right)^{n+1} \log(\Gamma(x))

where \psi(x) = \Gamma'(x)/\Gamma(x) is known as the digamma function. These functions are declared in the header file gsl_sf_psi.h.

Digamma Function

double gsl_sf_psi_int(int n)
int gsl_sf_psi_int_e(int n, gsl_sf_result * result)

These routines compute the digamma function \psi(n) for positive integer n. The digamma function is also called the Psi function.

double gsl_sf_psi(double x)
int gsl_sf_psi_e(double x, gsl_sf_result * result)

These routines compute the digamma function \psi(x) for general x, x \ne 0.

double gsl_sf_psi_1piy(double y)
int gsl_sf_psi_1piy_e(double y, gsl_sf_result * result)

These routines compute the real part of the digamma function on the line 1 + i y, \Re[\psi(1 + i y)].

Trigamma Function

double gsl_sf_psi_1_int(int n)
int gsl_sf_psi_1_int_e(int n, gsl_sf_result * result)

These routines compute the Trigamma function \psi'(n) for positive integer n.

double gsl_sf_psi_1(double x)
int gsl_sf_psi_1_e(double x, gsl_sf_result * result)

These routines compute the Trigamma function \psi'(x) for general x.

Polygamma Function

double gsl_sf_psi_n(int n, double x)
int gsl_sf_psi_n_e(int n, double x, gsl_sf_result * result)

These routines compute the polygamma function \psi^{(n)}(x) for n \ge 0, x > 0.

Synchrotron Functions

The functions described in this section are declared in the header file gsl_sf_synchrotron.h.

double gsl_sf_synchrotron_1(double x)
int gsl_sf_synchrotron_1_e(double x, gsl_sf_result * result)

These routines compute the first synchrotron function x \int_x^\infty dt K_{5/3}(t) for x \ge 0.

double gsl_sf_synchrotron_2(double x)
int gsl_sf_synchrotron_2_e(double x, gsl_sf_result * result)

These routines compute the second synchrotron function x K_{2/3}(x) for x \ge 0.

Transport Functions

The transport functions J(n,x) are defined by the integral representations

J(n,x) = \int_0^x t^n e^t /(e^t - 1)^2 dt

They are declared in the header file gsl_sf_transport.h.

double gsl_sf_transport_2(double x)
int gsl_sf_transport_2_e(double x, gsl_sf_result * result)

These routines compute the transport function J(2,x).

double gsl_sf_transport_3(double x)
int gsl_sf_transport_3_e(double x, gsl_sf_result * result)

These routines compute the transport function J(3,x).

double gsl_sf_transport_4(double x)
int gsl_sf_transport_4_e(double x, gsl_sf_result * result)

These routines compute the transport function J(4,x).

double gsl_sf_transport_5(double x)
int gsl_sf_transport_5_e(double x, gsl_sf_result * result)

These routines compute the transport function J(5,x).

Trigonometric Functions

The library includes its own trigonometric functions in order to provide consistency across platforms and reliable error estimates. These functions are declared in the header file gsl_sf_trig.h.

Circular Trigonometric Functions

double gsl_sf_sin(double x)
int gsl_sf_sin_e(double x, gsl_sf_result * result)

These routines compute the sine function \sin(x).

double gsl_sf_cos(double x)
int gsl_sf_cos_e(double x, gsl_sf_result * result)

These routines compute the cosine function \cos(x).

double gsl_sf_hypot(double x, double y)
int gsl_sf_hypot_e(double x, double y, gsl_sf_result * result)

These routines compute the hypotenuse function \sqrt{x^2 + y^2} avoiding overflow and underflow.

double gsl_sf_sinc(double x)
int gsl_sf_sinc_e(double x, gsl_sf_result * result)

These routines compute \sinc(x) = \sin(\pi x) / (\pi x) for any value of x.

Trigonometric Functions for Complex Arguments

int gsl_sf_complex_sin_e(double zr, double zi, gsl_sf_result * szr, gsl_sf_result * szi)

This function computes the complex sine, \sin(z_r + i z_i) storing the real and imaginary parts in szr, szi.

int gsl_sf_complex_cos_e(double zr, double zi, gsl_sf_result * czr, gsl_sf_result * czi)

This function computes the complex cosine, \cos(z_r + i z_i) storing the real and imaginary parts in czr, czi.

int gsl_sf_complex_logsin_e(double zr, double zi, gsl_sf_result * lszr, gsl_sf_result * lszi)

This function computes the logarithm of the complex sine, \log(\sin(z_r + i z_i)) storing the real and imaginary parts in lszr, lszi.

Hyperbolic Trigonometric Functions

double gsl_sf_lnsinh(double x)
int gsl_sf_lnsinh_e(double x, gsl_sf_result * result)

These routines compute \log(\sinh(x)) for x > 0.

double gsl_sf_lncosh(double x)
int gsl_sf_lncosh_e(double x, gsl_sf_result * result)

These routines compute \log(\cosh(x)) for any x.

Conversion Functions

int gsl_sf_polar_to_rect(double r, double theta, gsl_sf_result * x, gsl_sf_result * y)

This function converts the polar coordinates (r, theta) to rectilinear coordinates (x, y), x = r\cos(\theta), y = r\sin(\theta).

int gsl_sf_rect_to_polar(double x, double y, gsl_sf_result * r, gsl_sf_result * theta)

This function converts the rectilinear coordinates (x, y) to polar coordinates (r, theta), such that x = r\cos(\theta), y = r\sin(\theta). The argument theta lies in the range [-\pi, \pi].

Restriction Functions

double gsl_sf_angle_restrict_symm(double theta)
int gsl_sf_angle_restrict_symm_e(double * theta)

These routines force the angle theta to lie in the range (-\pi,\pi].

Note that the mathematical value of \pi is slightly greater than M_PI, so the machine numbers M_PI and -M_PI are included in the range.

double gsl_sf_angle_restrict_pos(double theta)
int gsl_sf_angle_restrict_pos_e(double * theta)

These routines force the angle theta to lie in the range [0, 2\pi).

Note that the mathematical value of 2\pi is slightly greater than 2*M_PI, so the machine number 2*M_PI is included in the range.

Trigonometric Functions With Error Estimates

int gsl_sf_sin_err_e(double x, double dx, gsl_sf_result * result)

This routine computes the sine of an angle x with an associated absolute error dx, \sin(x \pm dx). Note that this function is provided in the error-handling form only since its purpose is to compute the propagated error.

int gsl_sf_cos_err_e(double x, double dx, gsl_sf_result * result)

This routine computes the cosine of an angle x with an associated absolute error dx, \cos(x \pm dx). Note that this function is provided in the error-handling form only since its purpose is to compute the propagated error.

Zeta Functions

The Riemann zeta function is defined in Abramowitz & Stegun, Section 23.2. The functions described in this section are declared in the header file gsl_sf_zeta.h.

Riemann Zeta Function

The Riemann zeta function is defined by the infinite sum

\zeta(s) = \sum_{k=1}^\infty k^{-s}

double gsl_sf_zeta_int(int n)
int gsl_sf_zeta_int_e(int n, gsl_sf_result * result)

These routines compute the Riemann zeta function \zeta(n) for integer n, n \ne 1.

double gsl_sf_zeta(double s)
int gsl_sf_zeta_e(double s, gsl_sf_result * result)

These routines compute the Riemann zeta function \zeta(s) for arbitrary s, s \ne 1.

Riemann Zeta Function Minus One

For large positive argument, the Riemann zeta function approaches one. In this region the fractional part is interesting, and therefore we need a function to evaluate it explicitly.

double gsl_sf_zetam1_int(int n)
int gsl_sf_zetam1_int_e(int n, gsl_sf_result * result)

These routines compute \zeta(n) - 1 for integer n, n \ne 1.

double gsl_sf_zetam1(double s)
int gsl_sf_zetam1_e(double s, gsl_sf_result * result)

These routines compute \zeta(s) - 1 for arbitrary s, s \ne 1.

Hurwitz Zeta Function

The Hurwitz zeta function is defined by

\zeta(s,q) = \sum_0^\infty (k+q)^{-s}

double gsl_sf_hzeta(double s, double q)
int gsl_sf_hzeta_e(double s, double q, gsl_sf_result * result)

These routines compute the Hurwitz zeta function \zeta(s,q) for s > 1, q > 0.

Eta Function

The eta function is defined by

\eta(s) = (1-2^{1-s}) \zeta(s)

double gsl_sf_eta_int(int n)
int gsl_sf_eta_int_e(int n, gsl_sf_result * result)

These routines compute the eta function \eta(n) for integer n.

double gsl_sf_eta(double s)
int gsl_sf_eta_e(double s, gsl_sf_result * result)

These routines compute the eta function \eta(s) for arbitrary s.

Examples

The following example demonstrates the use of the error handling form of the special functions, in this case to compute the Bessel function J_0(5.0),

#include <stdio.h>
#include <gsl/gsl_errno.h>
#include <gsl/gsl_sf_bessel.h>

int
main (void)
{
  double x = 5.0;
  gsl_sf_result result;

  double expected = -0.17759677131433830434739701;

  int status = gsl_sf_bessel_J0_e (x, &result);

  printf ("status  = %s\n", gsl_strerror(status));
  printf ("J0(5.0) = %.18f\n"
          "      +/- % .18f\n",
          result.val, result.err);
  printf ("exact   = %.18f\n", expected);
  return status;
}

Here are the results of running the program,

status  = success
J0(5.0) = -0.177596771314338264
      +/-  0.000000000000000193
exact   = -0.177596771314338292

The next program computes the same quantity using the natural form of the function. In this case the error term result.err and return status are not accessible.

#include <stdio.h>
#include <gsl/gsl_sf_bessel.h>

int
main (void)
{
  double x = 5.0;
  double expected = -0.17759677131433830434739701;

  double y = gsl_sf_bessel_J0 (x);

  printf ("J0(5.0) = %.18f\n", y);
  printf ("exact   = %.18f\n", expected);
  return 0;
}

The results of the function are the same,

J0(5.0) = -0.177596771314338264
exact   = -0.177596771314338292

References and Further Reading

The library follows the conventions of the following book where possible,

  • Handbook of Mathematical Functions, edited by Abramowitz & Stegun, Dover, ISBN 0486612724.

The following papers contain information on the algorithms used to compute the special functions,

  • Allan J. MacLeod, MISCFUN: A software package to compute uncommon special functions. ACM Trans. Math. Soft., vol.: 22, 1996, 288–301
  • G.N. Watson, A Treatise on the Theory of Bessel Functions, 2nd Edition (Cambridge University Press, 1944).
  • G. Nemeth, Mathematical Approximations of Special Functions, Nova Science Publishers, ISBN 1-56072-052-2
  • B.C. Carlson, Special Functions of Applied Mathematics (1977)
  • N. M. Temme, Special Functions: An Introduction to the Classical Functions of Mathematical Physics (1996), ISBN 978-0471113133.
  • W.J. Thompson, Atlas for Computing Mathematical Functions, John Wiley & Sons, New York (1997).
  • Y.Y. Luke, Algorithms for the Computation of Mathematical Functions, Academic Press, New York (1977).
  • S. A. Holmes and W. E. Featherstone, A unified approach to the Clenshaw summation and the recursive computation of very high degree and order normalised associated Legendre functions, Journal of Geodesy, 76, pg. 279-299, 2002.