ssyevd(3S)

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( * )

IMPLEMENTATION
     These routines are part of the SCSL Scientific Library and can be loaded
     using either the -lscs or the -lscs_mp option.  The -lscs_mp option
     directs the linker to use the multi-processor version of the library.

     When linking to SCSL with -lscs or -lscs_mp, the default integer size is
     4 bytes (32 bits). Another version of SCSL is available in which integers
     are 8 bytes (64 bits).  This version allows the user access to larger
     memory sizes and helps when porting legacy Cray codes.  It can be loaded
     by using the -lscs_i8 option or the -lscs_i8_mp option. A program may use
     only one of the two versions; 4-byte integer and 8-byte integer library
     calls cannot be mixed.

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.


SEE ALSO
     INTRO_LAPACK(3S), INTRO_SCSL(3S)

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