petsc-3.8.4 2018-03-24

SNESDefaultMatrixFreeSetParameters2

Sets the parameters for the approximation of matrix-vector products using finite differences.

#include "petscsnes.h"   
PetscErrorCode  SNESDefaultMatrixFreeSetParameters2(Mat mat,PetscReal error,PetscReal umin,PetscReal h)

      J(u)*a = [J(u+h*a) - J(u)]/h where

either the user sets h directly here, or this parameter is computed via

       h = error_rel*u'a/||a||^2                        if  |u'a| > umin*||a||_{1}

         = error_rel*umin*sign(u'a)*||a||_{1}/||a||^2   else

	mat	- the matrix
	error_rel	- relative error (should be set to the square root of the relative error in the function evaluations)
	umin	- minimum allowable u-value
	h	- differencing parameter

If the user sets the parameter h directly, then this value will be used instead of the default computation indicated above.

SNES, matrix-free, parameters