DragonFly On-Line Manual Pages

PSLAED1(l)                             )                            PSLAED1(l)

NAME
       PSLAED1 - compute the updated eigensystem of a diagonal matrix after
       modification by a rank-one symmetric matrix,

SYNOPSIS
       SUBROUTINE PSLAED1( N, N1, D, ID, Q, IQ, JQ, DESCQ, RHO, WORK, IWORK,
                           INFO )
           INTEGER ID, INFO, IQ, JQ, N, N1
           REAL RHO
           INTEGER DESCQ( * ), IWORK( * )
           REAL D( * ), Q( * ), WORK( * )

PURPOSE
       PSLAED1 computes the updated eigensystem of a diagonal matrix after
       modification by a rank-one symmetric matrix, in parallel.

         T = Q(in) ( D(in) + RHO * Z*Z' ) Q'(in) = Q(out) * D(out) * Q'(out)

          where Z = Q'u, u is a vector of length N with ones in the
          N1 and N1 + 1 th elements and zeros elsewhere.

          The eigenvectors of the original matrix are stored in Q, and the
          eigenvalues are in D.  The algorithm consists of three stages:

             The first stage consists of deflating the size of the problem
             when there are multiple eigenvalues or if there is a zero in
             the Z vector.  For each such occurence the dimension of the
             secular equation problem is reduced by one.  This stage is
             performed by the routine PSLAED2.

             The second stage consists of calculating the updated
             eigenvalues. This is done by finding the roots of the secular
             equation via the routine SLAED4 (as called by PSLAED3).
             This routine also calculates the eigenvectors of the current
             problem.

             The final stage consists of computing the updated eigenvectors
             directly using the updated eigenvalues.  The eigenvectors for
             the current problem are multiplied with the eigenvectors from
             the overall problem.

ARGUMENTS
       N       (global input) INTEGER
               The order of the tridiagonal matrix T.  N >= 0.

       N1      (input) INTEGER
               The location of the last eigenvalue in the leading sub-matrix.
               min(1,N) <= N1 <= N.

       D       (global input/output) REAL array, dimension (N)
               On entry,the eigenvalues of the rank-1-perturbed matrix.  On
               exit, the eigenvalues of the repaired matrix.

       ID      (global input) INTEGER
               Q's global row/col index, which points to the beginning of the
               submatrix which is to be operated on.

       Q       (local output) REAL array,
               global dimension (N, N), local dimension ( LLD_Q, LOCc(JQ+N-1))
               Q  contains the orthonormal eigenvectors of the symmetric
               tridiagonal matrix.

       IQ      (global input) INTEGER
               Q's global row index, which points to the beginning of the
               submatrix which is to be operated on.

       JQ      (global input) INTEGER
               Q's global column index, which points to the beginning of the
               submatrix which is to be operated on.

       DESCQ   (global and local input) INTEGER array of dimension DLEN_.
               The array descriptor for the distributed matrix Z.

       RHO    (input) REAL
              The subdiagonal entry used to create the rank-1 modification.

       WORK    (local workspace/output) REAL array,
               dimension 6*N + 2*NP*NQ

       IWORK   (local workspace/output) INTEGER array,
               dimension 7*N + 8*NPCOL + 2

       INFO    (global output) INTEGER
               = 0:  successful exit
               < 0:  If the i-th argument is an array and the j-entry had an
               illegal value, then INFO = -(i*100+j), if the i-th argument is
               a scalar and had an illegal value, then INFO = -i.  > 0:  The
               algorithm failed to compute the ith eigenvalue.

ScaLAPACK version 1.7           13 August 2001                      PSLAED1(l)