debian man page for dsteqr2

DSTEQR2(l)						   LAPACK routine (version 2.0) 						DSTEQR2(l)

NAME
       DSTEQR2 - i a modified version of LAPACK routine DSTEQR

SYNOPSIS
       SUBROUTINE DSTEQR2( COMPZ, N, D, E, Z, LDZ, NR, WORK, INFO )

	   CHARACTER	   COMPZ

	   INTEGER	   INFO, LDZ, N, NR

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

PURPOSE
       DSTEQR2	is  a  modified  version  of LAPACK routine DSTEQR.  DSTEQR2 computes all eigenvalues and, optionally, eigenvectors of a symmetric
       tridiagonal matrix using the implicit QL or QR method.  DSTEQR2 is modified from DSTEQR to allow each ScaLAPACK process running DSTEQR2	to
       perform updates on a distributed matrix Q.  Proper usage of DSTEQR2 can be gleaned from examination of ScaLAPACK's PDSYEV.

ARGUMENTS
       COMPZ   (input) CHARACTER*1
	       = 'N':  Compute eigenvalues only.
	       = 'I':  Compute eigenvalues and eigenvectors of the tridiagonal matrix.	Z must be initialized to the identity matrix by PDLASET or
	       DLASET prior to entering this subroutine.

       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       (local input/local output) DOUBLE PRECISION array, global
	       dimension (N, N), local dimension (LDZ, NR).  On entry, if  COMPZ = 'V', then Z contains the orthogonal matrix used in  the  reduc-
	       tion to tridiagonal form.  On exit, if INFO = 0, then if  COMPZ = 'V', Z contains the orthonormal eigenvectors of the original sym-
	       metric 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).

       NR      (input) INTEGER
	       NR = MAX(1, NUMROC( N, NB, MYPROW, 0, NPROCS ) ).  If COMPZ = 'N', then NR is not referenced.

       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 orthogonally similar to the
	       original matrix.

modified LAPACK routine 					    12 May 1997 							DSTEQR2(l)