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Iterative Krylov-subspace solvers generally only require matrix-vector products and don't care whether or where there are zeros in the matrix. In your case, unless you have other information about the matrix (e.g., symmetry), you could for example use GMRES. What you probably had in mind is the question of preconditioning, and that you can't use things such ...


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Conclusion: I have tried CG, BiCG, and GMRES with no pre-conditioner, and these are all at least twice as slow as LU decomposition for the 121x121 matrix. Further, even for convergence tolerances near machine-precision, I do not get the accuracy that I get with LU. So I guess I'm sticking with LU :)


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Base case: Computing the factorization requires $\frac 2 3n^3$ operations and the inverse requires $\frac 4 3n^3$ operations. Computing the trace adds $O(n)$. Let's round that to $2n^3$. LU case: Computing the factorization requires $\frac 2 3n^3$ operations. If you were to compute $L^{-1}$ and $U^{-1}$, those operations each require $\frac 1 3n^3$ ...


1

PETSC might be a good option. Not super user friendly, but its good with lots of options.


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