[petsc-dev] Computing inverse of a small matrix: MatLUFactor() usage

[Gautam Bisht]

Jed,

On Nov 21, 2012, at 12:09 PM, Jed Brown wrote:

> On Wed, Nov 21, 2012 at 9:03 PM, Gautam Bisht <gbisht at lbl.gov> wrote:
> I need to compute an inverse of M, a small matrix (3x3), and am coding in Fortran 90. Following the suggestion under FAQ on the website, I'm trying to use MatMatSolve to obtain the inverse of matrix. I did the following:
> 
> - Created three sequential dense matrices: M, I, Minv
> - Filled values in M and I using MatSetValues
> - Tried factoring Matrix M before calling MatMatSolve(M,I,Minv,ierr)
> 
> I'm not sure what arguments needs to be passed as arguments in MatLUFactor() corresponding to IS row, IS col, and MatFactorInfo. I noticed the following note on website that states "Developer Note: fortran interface is not autogenerated as the f90 interface defintion cannot be generated correctly [due to MatFactorInfo]".
> Is there a Fortran90 example demonstrating the use of MatMatSolve()?
> 
> For a 3x3 matrix, you should just use the explicit formula for the inverse. Going through the PETSc interface doesn't make sense at all.

The 3x3 matrix is being generated using MatMult of 3xN and Nx3 matrices. I wanted to give MatMatSolve a try, but could not figure how to use it via Fortran. I will now code up the explicit formula.

>  
> 
> Additionally, I have been trying to update my copy of petsc-dev, and am getting timed out from the server. 
> 
> The server died so it has moved to https://bitbucket.org/petsc/petsc-dev.


-Gautam.
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