MatMFFDSetFunctionError#

Sets the error_rel for the approximation of matrix-vector products using finite differences with the MATMFFD matrix

Synopsis#

#include "petscmat.h"   
PetscErrorCode MatMFFDSetFunctionError(Mat mat, PetscReal error)

Logically Collective

Input Parameters#

mat - the MATMFFD matrix free matrix
error_rel - relative error (should be set to the square root of the relative error in the function evaluations)

Options Database Key#

-mat_mffd_err <error_rel> - Sets error_rel

Note#

The default matrix-free matrix-vector product routine computes

     F'(u)*a = [F(u+h*a) - F(u)]/h where
     h = error_rel*u'a/||a||^2                        if  |u'a| > umin*||a||_{1}
       = error_rel*umin*sign(u'a)*||a||_{1}/||a||^2   else