On 9/21/07, Randall Mackie <randy at geosystem.us> wrote: > Matt, > > There may be very valid reasons why one would want a matrix inverse, > say in the context of solving optimizations where you need the > inverse of the covariance matrix, and you need those values to > embed in another system.
I do not agree. What are you going to do with the inverse that cannot be done with LU? Backsolves against a matrix are commonplace. Matt > In this case, just getting the action of the inverse times a vector > is not what is needed. > > Randy > > > Matthew Knepley wrote: > > On 9/21/07, Randall Mackie <randy at geosystem.us> wrote: > >> I have a collegue who is considering to use PETSc for programming > >> his problem. He would like to know if you can get the actual > >> inverse of a matrix, because this inverse is embedded in another > >> larger system that is then solved. > >> > >> I suppose that if you solve the system directly, say with SuperLU > >> or MUMPS, you could get the actual inverse? I looked briefly through > >> the manual and documentation, but it didn't jump out at me. > > > > This makes several time this week people have asked for inverses. Never > > never never use the inverse. Its not stable. The storage blows up. That is > > why factorizations were invented. Numerical analysis courses have failed > > the country entirely. Just use a KSP and tell it -ksp_type preonly -pc_type > > lu > > and then you can customize the matrix type for different solvers. KSPSolve() > > will apply the matrix. If you want to get the action on an entire matrix, > > this > > is dense and we would have to put in the BLAS3 triangular solve calls. > > > > Matt > > > >> Any advice here? > >> > >> Thanks, Randy > >> > >> -- > >> Randall Mackie > >> GSY-USA, Inc. > >> PMB# 643 > >> 2261 Market St., > >> San Francisco, CA 94114-1600 > >> Tel (415) 469-8649 > >> Fax (415) 469-5044 > >> > >> California Registered Geophysicist > >> License No. GP 1034 > >> > >> > > > > > > -- > Randall Mackie > GSY-USA, Inc. > PMB# 643 > 2261 Market St., > San Francisco, CA 94114-1600 > Tel (415) 469-8649 > Fax (415) 469-5044 > > California Registered Geophysicist > License No. GP 1034 > > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
