The reason for getting wrong results was a forgotten '+'. I just inserted the values of the right hand side into the cell vector, but never added them up, unlike what I did with the matrix. Therefore I got wrong values for the right hand side. After this was a simple writing error, and (as far as I can see) nothing which can be used by others, I decided to delete the question. Therefore, the original question is not relevant anymore, either. Thanks!
Am Mittwoch, 29. Januar 2020 23:19:50 UTC+1 schrieb Wolfgang Bangerth: > > On 1/20/20 7:59 AM, 'Maxi Miller' via deal.II User Group wrote: > > > > I wrote a short test program which should solve the diffusion equation > > using the time-stepping class, and implemented both methods. When > > calculating the matrix and applying it to my solution vector, I get a > > different result compared to reading the gradients from the solution > > with get_function_gradients() and multiplying it with the gradients > > returned by shape_grad(). The results obtained from the matrix > > multiplication are correct compared to the expected solution, the > > results obtained from the direct approach are not. Why is that? > > Maxi -- if I understand you correctly, you're asking what the difference > is between computing > > F_i = \int \grad\varphi_i . grad u_h > > and > > F_i = (AU)_i > > where A=Laplace matrix, U=coefficient vector corresponding to u_h. > > There shouldn't be a difference in principle, but you have to pay > attention to what hanging nodes and Dirichlet boundary conditions do. In > particular, you might have to call F.condense() in the first case. > > You only say that the results are different, but not *how* they are > different. Have you looked at that? Are these two vectors different only > in hanging nodes? Only for shape functions at the boundary? > > 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/327036e0-7343-488e-949e-8ce1bb42f829%40googlegroups.com.
