[petsc-users] working-with-submatrix

Julian Andrej juan at tf.uni-kiel.de
Wed Oct 7 07:27:09 CDT 2015

Especially in eigenproblems, I would suggest using Matthews approach, or
applying Dirichlet boundary conditions via the penalty method (penalized
robin for example). This would avoid spurious eigenvalues which occur when
you use MatZeroRows().

On Wed, Oct 7, 2015 at 2:17 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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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