ichol encountered a nonpositive pivot

I have 2527676x2527676 sparse matrix K that ichol gives up easily with the ''ichol encountered a nonpositive pivot'' error. I tried re-odering, decreasing drop tol, level of fil and type of fil. I checked that diag(diag(K)==0) shows three zeros on the main diagonal. Regularization with speye function eliminates them but the solution with pcg is not correct. Any recommendations?

Answer 1

[aniket singh]

from the documentation: https://in.mathworks.com/help/matlab/ref/ichol.html

they suggest using the diagcomp option.

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If ichol breaks down as above, you can use the diagcomp option to construct a shifted incomplete Cholesky factorization. That is, instead of constructing L such that L*L' approximates A, ichol with diagonal compensation constructs L such that L*L' approximates M = A + alpha*diag(diag(A)) without explicitly forming M. As incomplete factorizations always exist for diagonally dominant matrices, alpha can be found to make M diagonally dominant

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