# KSPInitialResidual

Computes the residual. Either b - A*C*x with right preconditioning or C*b - C*A*x with left preconditioning; that later residual is often called the "preconditioned residual".

### Synopsis

    #include "petscksp.h"
    PetscErrorCode PETSCKSP_DLLEXPORT KSPInitialResidual(KSP ksp,Vec vsoln,Vec vt1,Vec vt2,Vec vres,Vec vb)

Collective on KSP

### Input Parameters

| vsoln | - solution to use in computing residual
| vt1, vt2 | - temporary work vectors
| vb | - right-hand-side vector

### Output Parameters

vres -calculated residual

### Notes

This routine assumes that an iterative method, designed for

        A x = b

will be used with a preconditioner, C, such that the actual problem is either

        AC u = b (right preconditioning) or

        CA x = Cb (left preconditioning).

This means that the calculated residual will be scaled and/or preconditioned; the true residual

        b-Ax

is returned in the vt2 temporary.

### Keywords

KSP, residual

### See Also

KSPMonitor()

Level:developer
Location:src/ksp/ksp/interface/itres.c
Index of all KSP routines
Table of Contents for all manual pages
Index of all manual pages