[Trilinos-Users] Solving a Reduced System

Mike Heroux maherou at sandia.gov
Tue Oct 18 16:58:14 MDT 2005

> I have two questions and two notes:
> Q1- Do you have a version where a view is created instead of 
> a copy (such that no need for fwd and rvc)?
>     Do you think it is possible the way you code it?

Making submatrices that are views of an existing matrix is not generally
possible given some of the implicit assumption about underlying data
structures.  We have had discussions about how to possibly support this, but
have not made progress on any concrete implementation.  There is a transform
class in EpetraExt called CrsMatrix_View that does exactly what you want,
but because of data structure restrictions, the rows of the submatrix must
be stored contiguously. 

Your request is a reasonable one, and one that could be accomplished at
least partially by providing an implementation of Epetra_RowMatrix that
supports a submatrix view of an Epetra_CrsMatrix.  This view would not be a
full-fledged Epetra_CrsMatrix, but often an Epetra_RowMatrix is sufficient.

I have added this to the feature list for the next Trilinos release.

> Q2- Do you have a class that does the opposite, i.e., from 
> several matrices, create a global matrix? E.g., in matlab:
> subA11, subA12, subA21, subA22 are given, then create A= [ 
> subA11, subA12
>      subA21, subA22];

There is nothing explicitly like this in Epetra or EpetraExt.  There is
functionality that has been part of the Trilinos abstract interfaces, the
TSF packages that are now part way through a transition into Thyra.  These
classes support a light-weight (no data copy) aggregation of multiple
operators to be used as a single global operator.  I have copied Kevin Long
and Ross Bartlett who are the primary developers of these tools, in case
they have more to say.

I have also attached a class that I wrote some time ago to support Schur
Complement operators built from Epetra objects.  Your subscripts suggest
that you might be trying something similar, so I thought you might find this
class of use.

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