# TaoAppSetHessianRoutine

Sets the function to compute the Hessian as well as the location to store the matrix.

## Synopsis

    #include "tao.h"
    int TaoAppSetHessianRoutine(TAO_APPLICATION taoapp, int (*hess)(TAO_APPLICATION,Vec,Mat*,Mat*,MatStructure*,void*),void *ctx)

Collective on TAO_APPLICATION and Mat

## Input Parameters

| taoapp | - the TAO_APPLICATION context
| hess | - Hessian evaluation routine
| ctx | - [optional] user-defined context for private data for the Hessian evaluation routine (may be TAO_NULL)

## Calling sequence of hess

       hess (TAO_APPLICATION taoapp,Vec x,Mat *H,Mat *Hpre,int *flag,void *ctx);

| taoapp | - the TAO_APPLICATION context
| x | - input vector
| H | - Hessian matrix
| Hpre | - preconditioner matrix, usually the same as A
| flag | - flag indicating information about the preconditioner matrix structure (see below)
| ctx | - [optional] user-defined Hessian context

## Options Database Keys

-tao_view_hessian -view the hessian after each evaluation using PETSC_VIEWER_STDOUT_WORLD

## Notes

The function hess() takes Mat * as the matrix arguments rather than Mat. This allows the Hessian evaluation routine to replace A and/or B with a completely new new matrix structure (not just different matrix elements) when appropriate, for instance, if the nonzero structure is changing throughout the global iterations.

The flag can be used to eliminate unnecessary work in the preconditioner during the repeated solution of linear systems of the same size. The available options are

       SAME_PRECONDITIONER -

         B is identical during successive linear solves.

         This option is intended for folks who are using

         different Amat and Pmat matrices and want to reuse the

         same preconditioner matrix.  For example, this option

         saves work by not recomputing incomplete factorization

         for ILU/ICC preconditioners.

       SAME_NONZERO_PATTERN -

         B has the same nonzero structure during

         successive linear solves.

       DIFFERENT_NONZERO_PATTERN -

         B does not have the same nonzero structure.

## Caution

If you specify SAME_NONZERO_PATTERN, the software believes your assertion and does not check the structure of the matrix. If you erroneously claim that the structure is the same when it actually is not, the new preconditioner will not function correctly. Thus, use this optimization feature carefully!

If in doubt about whether your preconditioner matrix has changed structure or not, use the flag DIFFERENT_NONZERO_PATTERN.

## Keywords

TAO_APPLICATION, Hessian

## See Also

TaoAppSetObjectiveAndGradientRoutine(), TaoAppSetHessianMat(), KSPSetOperators()

Level:beginner
Location:src/petsctao/application/petscapp/tao_app_fg.c
TAO Solver Index
Table of Contents

### Examples

src/bound/examples/tutorials/plate2.c.html
src/bound/examples/tutorials/jbearing2.c.html
src/bound/examples/tutorials/plate2f.F.html
src/unconstrained/examples/tutorials/eptorsion1.c.html
src/unconstrained/examples/tutorials/eptorsion2.c.html
src/unconstrained/examples/tutorials/minsurf2.c.html
src/unconstrained/examples/tutorials/rosenbrock1.c.html
src/unconstrained/examples/tutorials/eptorsion2f.F.html
src/unconstrained/examples/tutorials/rosenbrock1f.F.html