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# PCFieldSplitSetSchurPre

Indicates from what operator the preconditioner is constructucted for the Schur complement. The default is the A11 matrix.

### Synopsis

    #include "petscpc.h"
    PetscErrorCode PCFieldSplitSetSchurPre(PC pc,PCFieldSplitSchurPreType ptype,Mat pre)

Collective on PC

### Input Parameters

| pc | - the preconditioner context
| ptype | - which matrix to use for preconditioning the Schur complement: PC_FIELDSPLIT_SCHUR_PRE_A11 (default), PC_FIELDSPLIT_SCHUR_PRE_SELF, PC_FIELDSPLIT_SCHUR_PRE_USER PC_FIELDSPLIT_SCHUR_PRE_SELFP, and PC_FIELDSPLIT_SCHUR_PRE_FULL
| userpre | - matrix to use for preconditioning, or NULL

### Options Database

| -pc_fieldsplit_schur_precondition <self,selfp,user,a11,full> | - default is a11. See notes for meaning of various arguments
| -fieldsplit_1_pc_type <pctype> | - the preconditioner algorithm that is used to construct the preconditioner from the operator

### Notes

       If ptype is

           a11 - the preconditioner for the Schur complement is generated from the block diagonal part of the preconditioner

           matrix associated with the Schur complement (i.e. A11), not the Schur complement matrix

           self - the preconditioner for the Schur complement is generated from the symbolic representation of the Schur complement matrix:

                The only preconditioner that currently works with this symbolic respresentation matrix object is the PCLSC

                preconditioner

           user - the preconditioner for the Schur complement is generated from the user provided matrix (pre argument

                to this function).

           selfp - the preconditioning for the Schur complement is generated from an explicitly-assembled approximation Sp = A11 - A10 inv(diag(A00)) A01

                This is only a good preconditioner when diag(A00) is a good preconditioner for A00. Optionally, A00 can be

                lumped before extracting the diagonal using the additional option -fieldsplit_1_mat_schur_complement_ainv_type lump

           full - the preconditioner for the Schur complement is generated from the exact Schur complement matrix representation computed internally by PCFIELDSPLIT (this is expensive)

                useful mostly as a test that the Schur complement approach can work for your problem

When solving a saddle point problem, where the A11 block is identically zero, using a11 as the ptype only makes sense with the additional option -fieldsplit_1_pc_type none. Usually for saddle point problems one would use a ptype of self and -fieldsplit_1_pc_type lsc which uses the least squares commutator to compute a preconditioner for the Schur complement.

### See Also

PCFieldSplitGetSchurPre(), PCFieldSplitGetSubKSP(), PCFIELDSPLIT, PCFieldSplitSetFields(), PCFieldSplitSchurPreType,
MatSchurComplementSetAinvType(), PCLSC

### Level

intermediate

### Location

src/ksp/pc/impls/fieldsplit/fieldsplit.c

### Examples

src/snes/tutorials/ex70.c.html

### Implementations

PCFieldSplitSetSchurPre_FieldSplit in src/ksp/pc/impls/fieldsplit/fieldsplit.c
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