> I want to push this and provide you with news... What I did now, was to > export the systemMatrix and RHS to Matlab and tried to solve it there, > as it obviously is a problem with the solver. > Matlab did also have some problems, but it was possible to solve the > problem using fsolve. (The direct solver, bicgstab failed, where gmres > was able to get the right solution for one of the unknowns but not for > the other one). The matrix is singular and I suppose this can be the > reason why the solvers terminate during step 0. Does this sounds > reasonable to you?
Yes. It sometimes happens that one iterative solver or another is able to solve a singular system (essentially by luck) but that another one is unable to do the same. That doesn't mean that there isn't a problem: if the matrix is singular, then there are many solutions to the problem and you happened to find exactly one of those. > I now attempted to transform the problem to a second order problem. Here > I expierence another problem. When applying the gradient to the first > order equations I get a delta function in the right hand side. How can > this be treated in assembling the system? You have to know where this is and evaluate the value of a shape function at this location. This transformation is not usually a useful strategy, though. W. ------------------------------------------------------------------------- Wolfgang Bangerth email: [email protected] www: http://www.math.tamu.edu/~bangerth/ _______________________________________________ dealii mailing list http://poisson.dealii.org/mailman/listinfo/dealii
