PETSc version 3.17.4
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# PCApplyRichardson

Applies several steps of Richardson iteration with the particular preconditioner. This routine is usually used by the Krylov solvers and not the application code directly.

### Synopsis

    #include "petscksp.h"
    PetscErrorCode  PCApplyRichardson(PC pc,Vec b,Vec y,Vec w,PetscReal rtol,PetscReal abstol, PetscReal dtol,PetscInt its,PetscBool guesszero,PetscInt *outits,PCRichardsonConvergedReason *reason)

Collective on PC

### Input Parameters

| pc | - the preconditioner context
| b | - the right hand side
| w | - one work vector
| rtol | - relative decrease in residual norm convergence criteria
| abstol | - absolute residual norm convergence criteria
| dtol | - divergence residual norm increase criteria
| its | - the number of iterations to apply.
| guesszero | - if the input x contains nonzero initial guess

### Output Parameters

| outits | - number of iterations actually used (for SOR this always equals its)
| reason | - the reason the apply terminated
| y | - the solution (also contains initial guess if guesszero is PETSC_FALSE

### Notes

Most preconditioners do not support this function. Use the command PCApplyRichardsonExists() to determine if one does.

Except for the multigrid PC this routine ignores the convergence tolerances and always runs for the number of iterations

### See Also

PCApplyRichardsonExists()

### Level

developer

### Location

src/ksp/pc/interface/precon.c

### Implementations

PCApplyRichardson_PFMG in src/ksp/pc/impls/hypre/hypre.c
PCApplyRichardson_SysPFMG in src/ksp/pc/impls/hypre/hypre.c
PCApplyRichardson_MG in src/ksp/pc/impls/mg/mg.c
PCApplyRichardson_Shell in src/ksp/pc/impls/shell/shellpc.c
PCApplyRichardson_SOR in src/ksp/pc/impls/sor/sor.c
PCApplyRichardson_Telescope in src/ksp/pc/impls/telescope/telescope.c
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