Re: Invert Matrix

Hi,

On Aug 5, 2009, at 1:01 PM, Julia Campa Romero wrote:

> Hi,
>
> I want to invert a 39x39 symmetric matrix. But I have some
> difficulties.
> First, I fill the upper diagonal elements and then I complete it.
> The programa to fill the Matrix is:
> ------------------------------------------------------------------
> TMatrixD h(39,39);
> //CON are the values of the elements
> for (Int_t i=0;i<39;i++){
> for (Int_t j=i;j<39;j++){
> h[i][j]=CON[i][j];
> }
> //completed the low diagonal elements
> for (Int_t j=0;j<i;j++){
> h[i][j]=CON[j][i];
> }
> }
> ----------------------------------------------
> --Then to check whether the matrix is singular before attempting to
> invert it I write:
> -------------------------
> TDecompLU lu(h);
> TMatrixD p;
> if (!lu.Decompose()) {
> cout << "Decomposition failed, matrix singular ?" << endl;
> } else {
> lu.Invert(p);
> }

you need to declare correctly the size of p; Do:

TMatrixD p(39,39);

> ----
> And it gives me the following error:
>
> >>Error in <TDecompLU::Invert(TMatrixD &>: Input matrix has wrong
> shape
>
> --Then when I made
>
> >TMatrixD hi=h.Invert();
> It is gives me:
> >>Error in <TDecompLU::InvertLU>: matrix is singular, 1 diag
> elements < tolerance of 2.2204e-16
>
>

Some diagonal elements are too small. You can try either increasing the tolerance using lu.SetTol( tol) , if you are confident that the matrix is not singular or try to use another method like the SVD decomposition.

  Best Regards

  Lorenzo

> Could you explain me what it is happening?
>
> Julia Campa
>
>
>
>
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Received on Wed Aug 05 2009 - 15:22:37 CEST