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[Nguyen Van Dang <question158688@xxxxxxxxxxxxxxxxxxxxx>]

Question #158688 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/158688

    Status: Answered => Open

Nguyen Van Dang is still having a problem:
*Dear Mr. James Avery,*
*In fact, I want to invert of the mass matrix. Can I find the inverse matrix
indirectly by solving the linear system MX=I? (where I is the Matrix unit)
Is this way better than using M.invert()? If yes, please tell me the way to
get the matrix unit in Fenics?*
*Thanks a lot.*
*Dang
*
On Tue, May 24, 2011 at 11:01 AM, James Avery <
question158688@xxxxxxxxxxxxxxxxxxxxx> wrote:

> Your question #158688 on DOLFIN changed:
> https://answers.launchpad.net/dolfin/+question/158688
>
>     Status: Open => Answered
>
> James Avery proposed the following answer:
> Are you certain that you really want to take the inverse of the matrix?
>
> The problem with doing that is that taking the inverse of a sparse
> matrix does not result in a sparse matrix. While the sparse matrix uses
> O(N) memory for N degrees of freedom, a dense one uses O(N^2), and so
> you restrict yourself to very coarse meshes if you do take the inverse.
>
> Usually, you will instead want to apply a preconditioner (for example
> SSOR) to the matrix to improve efficiency of solving linear systems, and
> then solve each system Ax = b individually using a sparse linear solver.
>
> If you really, really want to take the inverse, you can use the
> uBLASSparseMatrix class instead of the Matrix class. I.e.
>
> uBLASSparseMatrix T;
> assemble(T,stiffness);
> T.invert();
>
> Please see also my answer to the same question at the bottom of this
> thread: https://answers.launchpad.net/dolfin/+question/158315
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
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>
> If you still need help, you can reply to this email or go to the
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> https://answers.launchpad.net/dolfin/+question/158688
>
> You received this question notification because you asked the question.
>


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Nguyen Van Dang

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