If you are using DMMGSetKSP() then just have the third argument (func) compute A+B as the first matrix and A as the second matrix. Then the smoothing and coarse grid solve will only use the A entries. That is A+B will be preconditioned with geometric multigrid applied to A.
But, I suspect that you will do just as well using A+B. Barry On Sep 28, 2010, at 8:19 PM, Shao-Ching Huang wrote: > Hi > > I can trying to solve a linear system > > (A+B)x=b > > where A is the standard finite difference Laplacian matrix and B has > only a few elements (i.e. B's number of non-zero entries is much much > smaller than A's). > Is it possible to precondition by applying DMMG on A (like in ex34.c) > in a KSP solver? Is there an example that I can follow to set this up? > > Thanks, > > Shao-Ching
