.TH DSTEQR l "15 June 2000" "LAPACK version 3.0" ")"
.SH NAME
DSTEQR - compute all eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal matrix using the implicit QL or QR method
.SH SYNOPSIS
.TP 19
SUBROUTINE DSTEQR(
COMPZ, N, D, E, Z, LDZ, WORK, INFO )
.TP 19
.ti +4
CHARACTER
COMPZ
.TP 19
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INTEGER
INFO, LDZ, N
.TP 19
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DOUBLE
PRECISION D( * ), E( * ), WORK( * ), Z( LDZ, * )
.SH PURPOSE
DSTEQR computes all eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal matrix using the implicit QL or QR method. The eigenvectors of a full or band symmetric matrix can also be found
if DSYTRD or DSPTRD or DSBTRD has been used to reduce this matrix to
tridiagonal form.
.br

.SH ARGUMENTS
.TP 8
COMPZ   (input) CHARACTER*1
= 'N':  Compute eigenvalues only.
.br
= 'V':  Compute eigenvalues and eigenvectors of the original
symmetric matrix.  On entry, Z must contain the
orthogonal matrix used to reduce the original matrix
to tridiagonal form.
= 'I':  Compute eigenvalues and eigenvectors of the
tridiagonal matrix.  Z is initialized to the identity
matrix.
.TP 8
N       (input) INTEGER
The order of the matrix.  N >= 0.
.TP 8
D       (input/output) DOUBLE PRECISION array, dimension (N)
On entry, the diagonal elements of the tridiagonal matrix.
On exit, if INFO = 0, the eigenvalues in ascending order.
.TP 8
E       (input/output) DOUBLE PRECISION array, dimension (N-1)
On entry, the (n-1) subdiagonal elements of the tridiagonal
matrix.
On exit, E has been destroyed.
.TP 8
Z       (input/output) DOUBLE PRECISION array, dimension (LDZ, N)
On entry, if  COMPZ = 'V', then Z contains the orthogonal
matrix used in the reduction to tridiagonal form.
On exit, if INFO = 0, then if  COMPZ = 'V', Z contains the
orthonormal eigenvectors of the original symmetric matrix,
and if COMPZ = 'I', Z contains the orthonormal eigenvectors
of the symmetric tridiagonal matrix.
If COMPZ = 'N', then Z is not referenced.
.TP 8
LDZ     (input) INTEGER
The leading dimension of the array Z.  LDZ >= 1, and if
eigenvectors are desired, then  LDZ >= max(1,N).
.TP 8
WORK    (workspace) DOUBLE PRECISION array, dimension (max(1,2*N-2))
If COMPZ = 'N', then WORK is not referenced.
.TP 8
INFO    (output) INTEGER
= 0:  successful exit
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< 0:  if INFO = -i, the i-th argument had an illegal value
.br
> 0:  the algorithm has failed to find all the eigenvalues in
a total of 30*N iterations; if INFO = i, then i
elements of E have not converged to zero; on exit, D
and E contain the elements of a symmetric tridiagonal
matrix which is orthogonally similar to the original
matrix.