Matrix-vector product for symmetric tridiagonal matrices?

[johnr]

Hi!

I was wondering whether there is a LAPACK/BLAS routine for matrix-vector product where the matrix is a symmetric tridiagonal matrix? I was able to find an auxiliary LAPACK routine (xLAGTM) for a general tridiagonal matrix, but I would also like one for symmetric tridiagonal matrices. It's strange if there is no matrix-vector product for symmetric tridiagonal matrices, when the matrix-format is generally supported?

Thanks!
johnr

[Julie]

John,

The matrix-vector product is coming from the Blas library (Level2), not Lapack.
Blas supplies a symmetric band matrix-vector product: xsbmv.f but no symmetric tridiagonal matrix-vector product.
The input matrix is stored in one array instead of using two vectors as for Lapack symmetric tridiagonal routines.

Julie

[johnr]

Julie, thanks for your reply!

Yes, I know there is a BLAS 2 subroutine for a symmetric band matrix-vector product and I could of course use that for a symmetric tridiagnoal matrix-vector product, but I would really like one for that particular matrix-format. As I said in my previous post, I found this auxiliary LAPACK routine, xLAGTM, which gives the matrix-vector product for general tridiagonal matrices. So, I was wondering whether there also might be one for symmetric tridiagonal matrices. I couldn't find one.

Regards,
johnr