Approximate the coefficient matrix

A preconditioner need not be derived from the coefficient matrix. For instance, in the case of a higher order finite element matrix, incomplete factorization preconditioner are better derived from a linear element discretization of the same problem, since this matrix will be an M-matrix.

This preprocessor can perform the following approximations:

• symmetric: take the symmetric part of the coefficient matrix
• gustafsson: apply the Gustafsson modified element matrix transformation (see reference [GUS] below).

[GUS]
@article{Gu:modified_element,
author = {Ivar Gustafsson},
title = {An Incomplete Factorization Preconditioning Method
                based on Modification of Element Matrices},
journal = {BIT},
year = {1996},
volume = {36},
pages = {86--100}
}