# MATSNESMF_DEFAULT

the default code for compute the "h" used in the finite difference matrix-free matrix vector product

### Options Database Keys

-snes_mf_umin <umin> see MatSNESMFDefaultSetUmin() -

Notes: Requires 2 norms and 1 inner product, but they are computed together so only one parallel collective operation is needed. See MATSNESMF_WP for a method (with GMRES) that requires NO collective operations.

### Formula used

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 otherwise where error_rel = square root of relative error in function evaluation umin = minimum iterate parameter

### See Also

MATMFFD, MatCreateMF(), MatCreateSNESMF(), MATSNESMF_WP, MatSNESMFDefaultSetUmin()

Level:intermediate
Location:src/snes/mf/snesmfjdef.c
Index of all SNESMF routines
Table of Contents for all manual pages
Index of all manual pages