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# dstev

```
NAME
DSTEV - compute all eigenvalues and, optionally, eigenvec-
tors 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/workspace) DOUBLE PRECISION array, dimension (N)
On entry, the (n-1) subdiagonal elements of the tri-
diagonal 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) 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).

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