Thus I intended to use a GMG-preconditioner, with a Chebyshev-smoother. Usually the GMG-preconditioner does not require the main matrix either (yes, small matrices are still required, but they do not require the same amount of storage space), but here I run into the problem which is mentioned in a separate question, i.e. I can not get the gradients/values of my function on non-active cells while having to loop over both non-active and active cells for building the MG-matrices, if that makes sense?
Am Mittwoch, 24. Juli 2019 21:18:13 UTC+2 schrieb Wolfgang Bangerth: > > On 7/24/19 12:23 PM, 'Maxi Miller' via deal.II User Group wrote: > > My LinearOperator only provides a vmult-interface, nothing else, but for > > initializing the preconditioner I still need a sparse matrix, thus I can > not > > use the LinearOperator, as far as I understand. Or is there a way to use > it? > > You just found the problem everyone faces. The LinearOperator interface is > useful if you only know how to express a linear operator through its > action, > without knowing the entries of the matrix that represent this action. This > is > all you need for the iterative solvers we have. > > But to work well, these iterators all need preconditioners, and most of > the > preconditioners we have require knowledge of the entries of the matrix -- > i.e., there is no longer any advantage to using LinearOperator over > building a > matrix. The ways you can work around this are Chebyshev iterations, for > example; step-20 also in great detail explores this issue. But the point > is > that it's a difficult issue. > > 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/6f2c93b2-f316-4a99-b86b-4775d450b2dd%40googlegroups.com.
