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# MATMFFD_WP
Implements an approach for computing the differencing parameter h used with the finite difference based matrix-free Jacobian. h = error_rel * sqrt(1 + ||U||) / ||a||


## Options Database Key

- ***-mat_mffd_compute_normu -*** Compute the norm of u every time see `MatMFFDWPSetComputeNormU()`





## Notes
|| U || does not change between linear iterations so is reused

In `KSPGMRES` || a || == 1 and so does not need to ever be computed except at restart
when it is recomputed.  Thus equires no global collectives when used with `KSPGMRES`


## Formula used
F'(u)*a = [F(u+h*a) - F(u)]/h where


## Reference

- **** -*** M. Pernice and H. F. Walker, "NITSOL: A Newton Iterative
Solver for Nonlinear Systems", SIAM J. Sci. Stat. Comput.", 1998,
vol 19, pp. 302--318.



## See Also
 `MATMFFD`, `MATMFFD_DS`, `MatCreateMFFD()`, `MatCreateSNESMF()`, `MATMFFD_DS`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/mffd/wp.c.html#MATMFFD_WP">src/mat/impls/mffd/wp.c</A>


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