[petsc-users] working-with-submatrix

[Matthew Knepley]

On Wed, Oct 7, 2015 at 7:08 AM, Giuntoli Guido Alfredo <ggiuntoli at tecna.com>
wrote:

> Hi! I have a problem of the form Ax=kBx were x is a scalar variable that
> is 0 in the border of a domain (x_D=0).
>
> Using FEM the problem that you have to solve is a reduced problem were you
> don’t have to include the coefficients of the matrix that multiply the
> x_Ds. So, I have two alternatives here, one is to construct a reduce matrix
> A and B from the beginning… this could result in a slow code. The other
> alternative I want to try is to use a sub matrix of A and B and pass them
> to slepc. How can I do this is I have Mat A and Mat b ?
>

1) Can't you just use MatZeroRows() or MatZeroRowsColumns() to apply your
BC?

2) I am not sure why you think eliminating variables will be slower. My
experiments and theoretical understanding say it is at least as fast as
zeroing the rows.

3) If you want to extract a submatrix, you can use MatGetSubmatrix().

  Thanks,

     Matt


>
>
> Thank you, Guido!
>
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