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.