Svetlana Tkachenko <[email protected]> writes: > I realised I have a set of dummy items to keep around the main matrix > (1, x_dim, with only items 2 ... x_dim-1 included into matrix A > itself). They are boundary conditions and the existing (serial) > preconditioner relies on them.
I'm not sure I parsed this description correctly, but it sounds like you're talking about Dirichlet boundary points. The most common approach is to assemble the matrix so that those rows of the matrix look like rows of the identity, then just put the known (perhaps 0.0) value in the right hand side. The ensures a nonsingular operator. There is another trick to preserve symmetry, though it's easiest to express in the context of nonlinear problems.
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