[petsc-users] Question on some PETSc functions.

[Matthew Knepley]

On Tue, Jan 4, 2011 at 12:30 AM, Gaurish Telang <gaurish108 at gmail.com>wrote:

> I have 2 semi sparse matrices (see sparsity plots) attached and I wanted to
> get to know some basic information about them.


There are no attachments.


> 1. How does one calculate the rank of a matrix in PETSc. Is the answer
> returned approximate or exact for matrices of dimension say 2500X1200
>

You need a rank revealing factorization. I know of no sparse packages for
this.


> 2. How good are the sparse sovlers for Ax=b that PETSc employs when A is a
> dense full rank square matrix of size 2000x2000.
>

I think you mean, "how well do Krylov solvers work for dense matrices?".
Solver performance depends
heavily on the characteristics of the matrix. There are no general
statements that can be made about
Krylov solvers.


> 3. In my case the matrices are mostly sparse but they tend to get dense
> towards the bottom. What matrix format is most efficent for handling  such
> matrices?
>

You might be able to handle this with a sparse matrix A + an outer product
of vectors. However, it
depends on your problem.

    Matt
-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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