# SNESMatrixFreeCreate2

Creates a matrix-free matrix context for use with a SNES solver. This matrix can be used as the Jacobian argument for the routine SNESSetJacobian().

### Synopsis

    #include "petscsnes.h"
    PetscErrorCode PETSCSNES_DLLEXPORT SNESDefaultMatrixFreeCreate2(SNES snes,Vec x,Mat *J)

### Input Parameters

snes -the SNES context
x -vector where SNES solution is to be stored.

### Output Parameter

J -the matrix-free matrix

### Notes

The matrix-free matrix context merely contains the function pointers and work space for performing finite difference approximations of Jacobian-vector products, J(u)*a, via

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

      where by default

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

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

      where

           error_rel = square root of relative error in

                       function evaluation

           umin = minimum iterate parameter

      Alternatively, the differencing parameter, h, can be set using

      Jorge's nifty new strategy if one specifies the option

             -snes_mf_jorge

The user can set these parameters via MatMFFDSetFunctionError(). See the nonlinear solvers chapter of the users manual for details.

The user should call MatDestroy() when finished with the matrix-free matrix context.

### Options Database Keys

     -snes_mf_err <error_rel>

     -snes_mf_unim <umin>

     -snes_mf_compute_err

     -snes_mf_freq_err <freq>

     -snes_mf_jorge

### Keywords

SNES, default, matrix-free, create, matrix

### See Also

MatDestroy(), MatMFFDSetFunctionError()

Level:advanced
Location:src/snes/interface/noise/snesmfj2.c
Index of all SNES routines
Table of Contents for all manual pages
Index of all manual pages