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Positiveindefinite rank deficient matrix http://mumpster.org/viewtopic.php?f=8&t=1808 
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Author:  michoski [ Tue Jul 05, 2016 9:04 am ] 
Post subject:  Positiveindefinite rank deficient matrix 
Hi all, I am solving a matrix problem that corresponds to doubly periodic boundary condition in a FEM representation. It leads to a positiveindefinite rank deficient matrix, the null space dimension is one, and it's a vector of some constant C, I think. As far as I am thinking, the system can be written Ax=B, but since the boundaries, there is a null vector y, such that A(x+y) = B, for some Ay=0. I would like to solve the system under the global constraint that y (or, the effect of y on the solution x) vanishes. I am not sure how to make this happen in MUMPS. I think in petsc something like KSPSetNullSpace() might work...? Right now I set: MatMumpsSetIcntl (F, 7, 2); MatMumpsSetIcntl (F, 24, 1); MatMumpsSetCntl (F, 1, 0.1); MatMumpsSetCntl (F,4, 1.E6); But the solution I get back is still off by a constant. Is there a way of determining a unique solution? Thanks, Craig 
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