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# SlepcEigenvalueComparisonFn

A prototype of an eigenvalue comparison function that would be passed to `EPSSetEigenvalueComparison()` and analogue functions in other solver types.

## Synopsis

    PETSC_EXTERN_TYPEDEF typedef PetscErrorCode SlepcEigenvalueComparisonFn(PetscScalar ar,PetscScalar ai,PetscScalar br,PetscScalar bi,PetscInt *res,void *ctx);

## Calling Sequence

- ar - real part of the 1st eigenvalue

- ai - imaginary part of the 1st eigenvalue

- br - real part of the 2nd eigenvalue

- bi - imaginary part of the 2nd eigenvalue

- res - [output] result of comparison

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

## Note

The return value `res` can be

- negative - if the 1st value is preferred to the 2st one

- zero - if both values are equally preferred

- positive - if the 2st value is preferred to the 1st one

## See Also

`SlepcSC`, `EPSSetEigenvalueComparison()`

## Level

advanced

## Location

include/slepcsc.h

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