petsc-3.7.3 2016-07-24
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# MatGetOrdering

Gets a reordering for a matrix to reduce fill or to improve numerical stability of LU factorization.

### Synopsis

    #include "petscmat.h"
    PetscErrorCode  MatGetOrdering(Mat mat,MatOrderingType type,IS *rperm,IS *cperm)

Collective on Mat

### Input Parameters

| mat | - the matrix
| type | - type of reordering, one of the following:

         MATORDERINGNATURAL - Natural

         MATORDERINGND - Nested Dissection

         MATORDERING1WD - One-way Dissection

         MATORDERINGRCM - Reverse Cuthill-McKee

         MATORDERINGQMD - Quotient Minimum Degree

### Output Parameters

| rperm | - row permutation indices
| cperm | - column permutation indices

### Options Database Key

-mat_view_ordering draw -plots matrix nonzero structure in new ordering

### Notes

This DOES NOT actually reorder the matrix; it merely returns two index sets that define a reordering. This is usually not used directly, rather use the options PCFactorSetMatOrderingType()

The user can define additional orderings; see MatOrderingRegister().

These are generally only implemented for sequential sparse matrices.

The external packages that PETSc can use for direct factorization such as SuperLU do not accept orderings provided by this call.

### Keywords

matrix, set, ordering, factorization, direct, ILU, LU,
fill, reordering, natural, Nested Dissection, One-way Dissection, Cholesky, Reverse Cuthill-McKee, Quotient Minimum Degree

### See Also

MatOrderingRegister(), PCFactorSetMatOrderingType()

Level:intermediate
Location: src/mat/order/sorder.c
Index of all MatOrderings routines
Table of Contents for all manual pages
Index of all manual pages

### Examples

src/mat/examples/tutorials/ex1.c.html
src/ksp/ksp/examples/tutorials/ex18.c.html