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### 39.8 Search Stopping Parameters

A minimization procedure should stop when one of the following conditions is true:

• A minimum has been found to within the user-specified precision.
• A user-specified maximum number of iterations has been reached.
• An error has occurred.

The handling of these conditions is under user control. The functions below allow the user to test the current estimate of the best-fit parameters in several standard ways.

Function: int gsl_multifit_fdfsolver_test (const gsl_multifit_fdfsolver * s, const double xtol, const double gtol, const double ftol, int * info)

This function tests for convergence of the minimization method using the following criteria:

• Testing for a small step size relative to the current parameter vector
|\delta_i| <= xtol (|x_i| + xtol)


for each 0 <= i < p. Each element of the step vector \delta is tested individually in case the different parameters have widely different scales. Adding xtol to |x_i| helps the test avoid breaking down in situations where the true solution value x_i = 0. If this test succeeds, info is set to 1 and the function returns GSL_SUCCESS.

A general guideline for selecting the step tolerance is to choose xtol = 10^{-d} where d is the number of accurate decimal digits desired in the solution x. See Dennis and Schnabel for more information.

• Testing for a small gradient (g = \nabla \Phi(x) = J^T f) indicating a local function minimum:
||g||_inf <= gtol


This expression tests whether the ratio (\nabla \Phi)_i x_i / \Phi is small. Testing this scaled gradient is a better than \nabla \Phi alone since it is a dimensionless quantity and so independent of the scale of the problem. The max arguments help ensure the test doesn’t break down in regions where x_i or \Phi(x) are close to 0. If this test succeeds, info is set to 2 and the function returns GSL_SUCCESS.

A general guideline for choosing the gradient tolerance is to set gtol = GSL_DBL_EPSILON^(1/3). See Dennis and Schnabel for more information.

If none of the tests succeed, info is set to 0 and the function returns GSL_CONTINUE, indicating further iterations are required.

Function: int gsl_multifit_test_delta (const gsl_vector * dx, const gsl_vector * x, double epsabs, double epsrel)

This function tests for the convergence of the sequence by comparing the last step dx with the absolute error epsabs and relative error epsrel to the current position x. The test returns GSL_SUCCESS if the following condition is achieved,

|dx_i| < epsabs + epsrel |x_i|


for each component of x and returns GSL_CONTINUE otherwise.

Function: int gsl_multifit_test_gradient (const gsl_vector * g, double epsabs)

This function tests the residual gradient g against the absolute error bound epsabs. Mathematically, the gradient should be exactly zero at the minimum. The test returns GSL_SUCCESS if the following condition is achieved,

\sum_i |g_i| < epsabs


and returns GSL_CONTINUE otherwise. This criterion is suitable for situations where the precise location of the minimum, x, is unimportant provided a value can be found where the gradient is small enough.

Function: int gsl_multifit_gradient (const gsl_matrix * J, const gsl_vector * f, gsl_vector * g)

This function computes the gradient g of \Phi(x) = (1/2) ||f(x)||^2 from the Jacobian matrix J and the function values f, using the formula g = J^T f.

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