redhat man page for ssyevd

SSYEVD(l)								 )								 SSYEVD(l)

NAME
       SSYEVD - compute all eigenvalues and, optionally, eigenvectors of a real symmetric matrix A

SYNOPSIS
       SUBROUTINE SSYEVD( JOBZ, UPLO, N, A, LDA, W, WORK, LWORK, IWORK, LIWORK, INFO )

	   CHARACTER	  JOBZ, UPLO

	   INTEGER	  INFO, LDA, LIWORK, LWORK, N

	   INTEGER	  IWORK( * )

	   REAL 	  A( LDA, * ), W( * ), WORK( * )

PURPOSE
       SSYEVD  computes  all eigenvalues and, optionally, eigenvectors of a real symmetric matrix A. If eigenvectors are desired, it uses a divide
       and conquer algorithm.

       The divide and conquer algorithm makes very mild assumptions about floating point arithmetic. It will work on machines with a  guard  digit
       in  add/subtract,  or  on those binary machines without guard digits which subtract like the Cray X-MP, Cray Y-MP, Cray C-90, or Cray-2. It
       could conceivably fail on hexadecimal or decimal machines without guard digits, but we know of none.

       Because of large use of BLAS of level 3, SSYEVD needs N**2 more workspace than SSYEVX.

ARGUMENTS
       JOBZ    (input) CHARACTER*1
	       = 'N':  Compute eigenvalues only;
	       = 'V':  Compute eigenvalues and eigenvectors.

       UPLO    (input) CHARACTER*1
	       = 'U':  Upper triangle of A is stored;
	       = 'L':  Lower triangle of A is stored.

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

       A       (input/output) REAL array, dimension (LDA, N)
	       On entry, the symmetric matrix A.  If UPLO = 'U', the leading N-by-N upper triangular part of A contains the upper triangular  part
	       of  the matrix A.  If UPLO = 'L', the leading N-by-N lower triangular part of A contains the lower triangular part of the matrix A.
	       On exit, if JOBZ = 'V', then if INFO = 0, A contains the orthonormal eigenvectors of the matrix A.  If JOBZ = 'N', then on exit the
	       lower triangle (if UPLO='L') or the upper triangle (if UPLO='U') of A, including the diagonal, is destroyed.

       LDA     (input) INTEGER
	       The leading dimension of the array A.  LDA >= max(1,N).

       W       (output) REAL array, dimension (N)
	       If INFO = 0, the eigenvalues in ascending order.

       WORK    (workspace/output) REAL array,
	       dimension (LWORK) On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

       LWORK   (input) INTEGER
	       The  dimension  of  the	array WORK.  If N <= 1, 	      LWORK must be at least 1.  If JOBZ = 'N' and N > 1, LWORK must be at
	       least 2*N+1.  If JOBZ = 'V' and N > 1, LWORK must be at least 1 + 6*N + 2*N**2.

	       If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of  the  WORK  array,  returns  this
	       value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA.

       IWORK   (workspace/output) INTEGER array, dimension (LIWORK)
	       On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.

       LIWORK  (input) INTEGER
	       The  dimension  of the array IWORK.  If N <= 1,		      LIWORK must be at least 1.  If JOBZ  = 'N' and N > 1, LIWORK must be
	       at least 1.  If JOBZ  = 'V' and N > 1, LIWORK must be at least 3 + 5*N.

	       If LIWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the IWORK  array,  returns  this
	       value as the first entry of the IWORK array, and no error message related to LIWORK is issued by XERBLA.

       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 an intermediate tridiagonal form did not converge
	       to zero.

FURTHER DETAILS
       Based on contributions by
	  Jeff Rutter, Computer Science Division, University of California
	  at Berkeley, USA
       Modified by Francoise Tisseur, University of Tennessee.

LAPACK version 3.0						   15 June 2000 							 SSYEVD(l)