SSTEV - compute all eigenvalues and, optionally, eigenvec
tors of a real symmetric tridiagonal matrix A
SYNOPSIS
SUBROUTINE SSTEV( JOBZ, N, D, E, Z, LDZ, WORK, INFO )
CHARACTER JOBZ
INTEGER INFO, LDZ, N
REAL D( * ), E( * ), WORK( * ), Z( LDZ, * )
PURPOSE
SSTEV computes all eigenvalues and, optionally, eigenvec
tors 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) REAL array, dimension (N)
On entry, the n diagonal elements of the tridiago
nal matrix A. On exit, if INFO = 0, the eigenval
ues in ascending order.
E (input/output) REAL array, dimension (N)
On entry, the (n-1) subdiagonal elements of the
tridiagonal matrix A, stored in elements 1 to N-1
of E; E(N) need not be set, but is used by the
routine. On exit, the contents of E are
destroyed.
Z (output) REAL 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 associ
ated with D(i). If JOBZ = 'N', then Z is not ref
erenced.
LDZ (input) INTEGER
The leading dimension of the array Z. LDZ >= 1,
and if JOBZ = 'V', LDZ >= max(1,N).
WORK (workspace) REAL 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
> 0: if INFO = i, the algorithm failed to con
verge; i off-diagonal elements of E did not con
verge to zero.
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