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# NEPComputeError

Computes the error (based on the residual norm) associated with the `i`-th computed eigenpair.

## Synopsis

    #include "slepcnep.h"
    PetscErrorCode NEPComputeError(NEP nep,PetscInt i,NEPErrorType type,PetscReal *error)

Collective

## Input Parameters

- nep - the nonlinear eigensolver context

- i - the solution index

- type - the type of error to compute, see `NEPErrorType`

## Output Parameter

- error - the error

## Notes

The error can be computed in various ways, all of them based on the residual norm computed as \(\|T(\lambda)x\|_2\) where \((\lambda,x)\) is the approximate eigenpair.

If the computation of left eigenvectors was enabled with `NEPSetTwoSided()`, then the error will be computed using the maximum of the value above and the left residual norm \(\|y^*T(\lambda)\|_2\), where \(y\) is the approximate left eigenvector.

## See Also

NEP: Nonlinear Eigenvalue Problems, `NEPErrorType`, `NEPSolve()`, `NEPGetErrorEstimate()`, `NEPSetTwoSided()`

## Level

beginner

## Location

src/nep/interface/nepsolve.c

## Examples

src/nep/tutorials/ex20f.F90
src/nep/tutorials/ex20.c

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