Please send the code. This should work fine. Barry
> On Jun 25, 2020, at 2:43 PM, Rakesh Halder <[email protected]> wrote: > > Hi all, > > I'm using PETSc for matrix calculations as part of a model order reduction > code. An algorithm I'm using requires that I compute the explicit inverse of > a matrix, as it needs to be used in matrix-matrix products. > > The matrix is small (I'll show a 5x5 example), and I first find the LU > factorization of it as follows: > > MatGetOrdering(A,MATORDERINGNATURAL,&perm,&iperm); > MatFactorInfoInitialize(&info); > MatLUFactor(A,perm,iperm,&info); > > I then find the inverse of it: > > MatMatSolve(A,I,B); > > Where I is the identity matrix, and B is the inverse. The subroutine does > calculate an inverse, although not accurately; the matrix A I looked at is: > > 5.23E-02 1.86E-02 2.67E-02 4.58E-02 3.55E-02 > 6.37E-03 5.86E-02 5.07E-03 1.64E-02 1.36E-02 > -4.07E-02 3.99E-03 5.50E-02 1.77E-02 -3.21E-02 > 1.96E-02 -5.53E-03 4.02E-02 -5.37E-02 1.80E-02 > 1.51E-02 1.70E-02 -1.57E-02 -3.75E-03 -5.64E-02 > > And when I take the product A*B, I don't recover the identity matrix, but > rather: > > 9.97E-01 2.77E-04 -1.66E-03 1.25E-04 -1.10E-03 > -6.79E-04 9.99E-01 -1.72E-04 6.24E-04 1.16E-03 > 2.70E-03 -3.98E-04 9.98E-01 -1.51E-04 6.82E-04 > -3.35E-04 -4.82E-04 -1.03E-03 9.99E-01 9.80E-05 > -9.50E-04 -8.70E-04 8.37E-04 -3.68E-04 9.99E-01 > > I believe this is causing large inaccuracies in my program, as the diagonal > and off-diagonal entries have large errors associated with them. I am > wondering if there is a way to perhaps tighten the tolerance of the > subroutine, or if there are other methods I can use. I would like to avoid > using KSP and solving for the inverse vector-by-vector if possible. > > Thanks, > > Rakesh Halder
