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  computes  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 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) 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 elements of  a
               symmetric tridiagonal matrix which is orthogonally similar to the original matrix.

modified LAPACK routine                            12 May 1997                                        SSTEQR2(l)