SSTEV (3lapack)

compute all eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix A

SYNOPSIS

  SUBROUTINE SSTEV( JOBZ, N, D, E, Z, LDZ, WORK, INFO )

      CHARACTER     JOBZ

      INTEGER       INFO, LDZ, N

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

PURPOSE

  SSTEV computes all eigenvalues and, optionally, eigenvectors of a real
  symmetric tridiagonal matrix A.

ARGUMENTS

  JOBZ    (input) CHARACTER*1
          = 'N':  Compute eigenvalues only;
          = 'V':  Compute eigenvalues and eigenvectors.

  N       (input) INTEGER
          The order of the matrix.  N >= 0.

  D       (input/output) REAL array, dimension (N)
          On entry, the n diagonal elements of the tridiagonal matrix A.  On
          exit, if INFO = 0, the eigenvalues in ascending order.

  E       (input/output) REAL array, dimension (N)
          On entry, the (n-1) subdiagonal elements of the tridiagonal matrix
          A, stored in elements 1 to N-1 of E; E(N) need not be set, but is
          used by the routine.  On exit, the contents of E are destroyed.

  Z       (output) REAL array, dimension (LDZ, N)
          If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal
          eigenvectors of the matrix A, with the i-th column of Z holding the
          eigenvector associated with D(i).  If JOBZ = 'N', then Z is not
          referenced.

  LDZ     (input) INTEGER
          The leading dimension of the array Z.  LDZ >= 1, and if JOBZ = 'V',
          LDZ >= max(1,N).

  WORK    (workspace) REAL array, dimension (max(1,2*N-2))
          If JOBZ = 'N', WORK is not referenced.

  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 E did not converge to zero.