On 8/18/22 10:38, Simon wrote:
Say, I have in total 20 dofs and the dof with global dof index Zero is
constrained.
As a consequence, the first component of the rhs b is set to Zero as well as
the first row and column of the system matrix A (except the diagonal value).
In a postprecessing step, I have to solve another linear system, however, with
the system matrix being only the 19x19 matrix associated with the 19
unconstrained dofs; I do not need the rhs anymore.
Is there a way to get this portion of the system matrix based on the existing
system matrix (sparsity pattern)?
Both the sparsity pattern and the sparse matrix have iterators that allow you
to iterate over all entries of a matrix. You can do that and just filter out
those rows and columns you're not interested in, and then copy the rest into
output objects with translated indices.
But the easier approach may be to use the same 20x20 matrix and just copy the
new rhs you want to solve with into a vector with size 20, leaving the entries
of the rhs vector that correspond to constrained DoFs zero (or, in fact,
whatever you want -- the value there doesn't matter). By zeroing out rows and
columns, you are in essence solving a linear system with only the 19x19
matrix. You then copy the 19 DoFs you care about out of the solution vector
into an output vector.
Best
W.
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Wolfgang Bangerth email: bange...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/
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