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DSTEQR2(l)                             )                            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
               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) 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.

LAPACK version 2.0              13 August 2001                      DSTEQR2(l)

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