Hi,
I'm solving the system Ax=b using GMRES. I use block-diagonal preconditioning, which makes a lot of sense in my case because there is a finite element block and a boundary element block in that matrix A.
The GMRES solution is correct: For smaller systems I have compared against the solution from mldivide. For bigger systems I have compared against results using other software.
But I do get a little confused when I look at the residual after GMRES has solved the system. For the last few iterations the residual suddenly increases before the GMRES solver stops. This behaviour is independent of system size. A screenshot can be found at http://www.varg.unsw.edu.au/Assets/Images/helloworld/GMRESres.png
So, the question is, why does the residual do that?
Cheers Herwig
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