NAME¶

DSTEV - computes all eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix A

SYNOPSIS¶

SUBROUTINE DSTEV(

JOBZ, N, D, E, Z, LDZ, WORK, INFO )

CHARACTER JOBZ INTEGER INFO, LDZ, N DOUBLE PRECISION D( * ), E( * ), WORK( * ), Z( LDZ, * )

PURPOSE¶

DSTEV computes all eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix A.

ARGUMENTS¶

JOBZ (input) CHARACTER*1

= 'N': Compute eigenvalues only;
= 'V': Compute eigenvalues and eigenvectors.

N (input) INTEGER

The order of the matrix. N >= 0.

D (input/output) DOUBLE PRECISION array, dimension (N)

On entry, the n diagonal elements of the tridiagonal matrix A. On exit, if INFO = 0, the eigenvalues in ascending order.

E (input/output) DOUBLE PRECISION array, dimension (N-1)

On entry, the (n-1) subdiagonal elements of the tridiagonal matrix A, stored in elements 1 to N-1 of E. On exit, the contents of E are destroyed.

Z (output) DOUBLE PRECISION array, dimension (LDZ, N)

If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal eigenvectors of the matrix A, with the i-th column of Z holding the eigenvector associated with D(i). If JOBZ = 'N', then Z is not referenced.

LDZ (input) INTEGER

The leading dimension of the array Z. LDZ >= 1, and if JOBZ = 'V', LDZ >= max(1,N).

WORK (workspace) DOUBLE PRECISION array, dimension (max(1,2*N-2))

If JOBZ = 'N', WORK is not referenced.

INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: if INFO = i, the algorithm failed to converge; i off-diagonal elements of E did not converge to zero.