MueLu builds zero column prolongator for diagonal matrix
Created by: tawiesn
By default, MueLu drops "Dirichlet" nodes from aggregation. In the worst case it builds a prolongation operator with zero columns.
There is a "aggregation: preserve Dirichlet points" option (default=off) to keep all Dirichlet nodes unaggregated. The problem is, that there would be no progress in coarsening if we activate that.
What are the options for a smarter handling of this special case?
- We could detect the problem during the aggregation (all nodes "aggregated" with 0 aggregates) and then build a dummy one node aggregate (the coarsening would terminate on the next level)
- We check the size of two consecutive levels and if the matrix size is not shrinking (by a factor X) we stop coarsening...