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# SlepcArbitrarySelectionFn

A prototype of an arbitrary selection function that would be passed to `EPSSetArbitrarySelection()` and analogue functions in other solver types.

## Synopsis

    PETSC_EXTERN_TYPEDEF typedef PetscErrorCode SlepcArbitrarySelectionFn(PetscScalar er,PetscScalar ei,Vec xr,Vec xi,PetscScalar *rr,PetscScalar *ri,void *ctx);

## Calling Sequence

- er - real part of the current eigenvalue approximation

- ei - imaginary part of the current eigenvalue approximation

- xr - real part of the current eigenvector approximation

- xi - imaginary part of the current eigenvector approximation

- rr - result of evaluation (real part)

- ri - result of evaluation (imaginary part)

- ctx - [optional] user-defined context for private data for the arbitrary selection routine (may be `NULL`)

## Notes

Given a scalar and a vector (usually an approximate eigenvalue and the corresponding eigenvector) this function evaluates the result `rr`, `ri` which is the value that will be used in a subsequent sorting.

This evaluation function is collective, that is, all processes call it and it can use collective operations; furthermore, the computed result must be the same in all processes.

The result is expressed as a complex number so that it is possible to use the standard eigenvalue sorting functions, but normally only `rr` is used. Set `ri` to zero unless it is meaningful in your application.

## See Also

`SlepcSC`, `SlepcEigenvalueComparisonFn`, `EPSSetArbitrarySelection()`

## Level

advanced

## Location

include/slepcsc.h

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