An unsymmetric tridiagonal matrix of order n is stored in three one-dimensional arrays, one of length n containing the diagonal elements, and two of length n - 1 containing the subdiagonal and superdiagonal elements in elements 1 : n - 1.
A symmetric tridiagonal or bidiagonal matrix is stored in two one-dimensional arrays, one of length n containing the diagonal elements, and one of length n containing the off-diagonal elements. (EISPACK routines store the off-diagonal elements in elements 2 : n of a vector of length n.)