SSTEQR2(3)    ScaLAPACK routine of NEC Numeric Library Collection   SSTEQR2(3)



NAME
       SSTEQR2 - i a modified version of LAPACK routine SSTEQR

SYNOPSIS
       SUBROUTINE SSTEQR2( COMPZ, N, D, E, Z, LDZ, NR, WORK, INFO )

           CHARACTER       COMPZ

           INTEGER         INFO, LDZ, N, NR

           REAL            D( * ), E( * ), WORK( * ), Z( LDZ, * )

PURPOSE
       SSTEQR2  is  a  modified version of LAPACK routine SSTEQR. SSTEQR2 com-
       putes all eigenvalues and,  optionally,  eigenvectors  of  a  symmetric
       tridiagonal matrix using the implicit QL or QR method.  running SSTEQR2
       to perform updates on a distributed matrix Q.  Proper usage of  SSTEQR2
       can be gleaned from examination of ScaLAPACK's PSSYEV.


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) REAL 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) REAL 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) REAL array, global
               dimension  (N,  N),  local  dimension  (LDZ, NR).  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.

       LDZ     (input) INTEGER
               The  leading dimension of the array Z.  LDZ >= 1, and if eigen-
               vectors 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) REAL 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 ele-
               ments of a symmetric tridiagonal matrix which  is  orthogonally
               similar to the original matrix.



ScaLAPACK routine               31 October 2017                     SSTEQR2(3)