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The Clausen function is defined by the following integral,

Cl_2(x) = - \int_0^x dt \log(2 \sin(t/2))

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`.

- Function:
*double***gsl_sf_clausen***(double*`x`) - Function:
*int***gsl_sf_clausen_e***(double*`x`, gsl_sf_result *`result`) These routines compute the Clausen integral

*Cl_2(x)*.