Hi Wolfgang, Thanks very much for your clarification.
The code works fine now with the help of TrilinosWrappers::SparseMatrix::mmult(), though, I understand, mmult is expensive. Regards, Charlie On Wednesday, August 28, 2019 at 12:43:04 AM UTC+2, Wolfgang Bangerth wrote: > > On 8/27/19 2:23 PM, Charlie Z wrote: > > > > I find a conflict here is that:: > > 1) TrilinosWrappers::PreconditionAMG can only be initialized with a > > matrix, but not a ApproximateSchurComplement; > > 2) step-20 uses ApproximateSchurComplement because we actually don't > > want to/can't assemble the Schur matrix or its inverse. > > Yes, you correctly identified the issue. step-20 gets away with using > ApproximateSchurComplement as a preconditioner because, from the > perspective of GMRES, a preconditioner only has be able to provide a > matrix-vector product (i.e., apply itself to a vector). > > But if you want to use an object to initialize a algebraic multigrid, > the AMG implementation needs to be able to do much more than just apply > the object to a vector -- it needs to build a multilevel hierarchy, and > for that it needs access to the elements that make up that matrix. The > problem is of course that ApproximateSchurComplement does not offer > access to the elements of the matrix it represents (because it never > explicitly computes them). > > > > Could you please give me a hint on how to solve this conflict in > > deal.II? Is it actually possible to assemble this B(DA^-1)BT directly? > > Yes, of course. You can just multiply it out as the matrix-matrix > product of three matrices. The result is a square matrix of the right > size, with a rather large number of nonzero entries per row. > > Best > W. > > > -- > ------------------------------------------------------------------------ > Wolfgang Bangerth email: bang...@colostate.edu > <javascript:> > www: http://www.math.colostate.edu/~bangerth/ > -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to dealii+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/dealii/4fadbe93-ff6c-4653-8b22-180bf71f095f%40googlegroups.com.
