PDPTTRF computes a Cholesky factorization
of an N-by-N real tridiagonal
symmetric positive definite distributed matrix
A(1:N, JA:JA+N-1).
Reordering is used to increase parallelism in the factorization.
This reordering results in factors that are DIFFERENT from those
produced by equivalent sequential codes. These factors cannot
be used directly by users; however, they can be used in
subsequent calls to PDPTTRS to solve linear systems.

The factorization has the form

P A(1:N, JA:JA+N-1) P^T = U' D U or

P A(1:N, JA:JA+N-1) P^T = L D L',

where U is a tridiagonal upper triangular matrix and L is tridiagonal
lower triangular, and P is a permutation matrix.