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RedHat 9 (Linux i386) - man page for dsteqr (redhat section l)

DSTEQR(l)					)					DSTEQR(l)

NAME
       DSTEQR  - compute all eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal
       matrix using the implicit QL or QR method

SYNOPSIS
       SUBROUTINE DSTEQR( COMPZ, N, D, E, Z, LDZ, WORK, INFO )

	   CHARACTER	  COMPZ

	   INTEGER	  INFO, LDZ, N

	   DOUBLE	  PRECISION D( * ), E( * ), WORK( * ), Z( LDZ, * )

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.

ARGUMENTS
       COMPZ   (input) CHARACTER*1
	       = 'N':  Compute eigenvalues only.
	       = '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.

       N       (input) INTEGER
	       The order of the matrix.  N >= 0.

       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.

       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.

       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 con-
	       tains 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.

       LDZ     (input) INTEGER
	       The leading dimension of the array Z.  LDZ >= 1, and if eigenvectors are  desired,
	       then  LDZ >= max(1,N).

       WORK    (workspace) DOUBLE PRECISION array, dimension (max(1,2*N-2))
	       If COMPZ = 'N', then WORK is not referenced.

       INFO    (output) INTEGER
	       = 0:  successful exit
	       < 0:  if INFO = -i, the i-th argument had an illegal value
	       >  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 orthogo-
	       nally similar to the original matrix.

LAPACK version 3.0			   15 June 2000 				DSTEQR(l)


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