[petsc-users] Eliminating rows and columns which are zeros

[Karthikeyan Chockalingam - STFC UKRI]

Hello,

I am assembling a MATIJ of size N, where a very large number of rows (and corresponding columns), are zeros. I would like to potentially eliminate them before the solve.

For instance say N=7


0 0  0  0 0 0 0

0 1 -1  0 0 0 0

0 -1 2  0 0 0 -1

0 0  0  0 0 0 0

0 0  0  0 0 0 0

0 0  0  0 0 0 0

0 0  -1 0 0 0 1

I would like to reduce it to a 3x3

1 -1 0
-1 2 -1
0 -1 1

I do know the size N.

Q1) How do I do it?
Q2) Is it better to eliminate them as it would save a lot of memory?
Q3) At the moment, I don’t know which rows (and columns) have the zero entries but with some effort I probably can find them. Should I know which rows (and columns) I am eliminating?

Thank you.

Karthik.

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