CTREXC (3lapack)

reorder the Schur factorization of a complex matrix A = Q*T*Q**H, so that the diagonal element of T with row index IFST is moved to row ILST

SYNOPSIS

  SUBROUTINE CTREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, INFO )

      CHARACTER      COMPQ

      INTEGER        IFST, ILST, INFO, LDQ, LDT, N

      COMPLEX        Q( LDQ, * ), T( LDT, * )

PURPOSE

  CTREXC reorders the Schur factorization of a complex matrix A = Q*T*Q**H,
  so that the diagonal element of T with row index IFST is moved to row ILST.

  The Schur form T is reordered by a unitary similarity transformation
  Z**H*T*Z, and optionally the matrix Q of Schur vectors is updated by
  postmultplying it with Z.

ARGUMENTS

  COMPQ   (input) CHARACTER*1
          = 'V':  update the matrix Q of Schur vectors;
          = 'N':  do not update Q.

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

  T       (input/output) COMPLEX array, dimension (LDT,N)
          On entry, the upper triangular matrix T.  On exit, the reordered
          upper triangular matrix.

  LDT     (input) INTEGER
          The leading dimension of the array T. LDT >= max(1,N).

  Q       (input/output) COMPLEX array, dimension (LDQ,N)
          On entry, if COMPQ = 'V', the matrix Q of Schur vectors.  On exit,
          if COMPQ = 'V', Q has been postmultiplied by the unitary
          transformation matrix Z which reorders T.  If COMPQ = 'N', Q is not
          referenced.

  LDQ     (input) INTEGER
          The leading dimension of the array Q.  LDQ >= max(1,N).

  IFST    (input) INTEGER
          ILST    (input) INTEGER Specify the reordering of the diagonal
          elements of T: The element with row index IFST is moved to row ILST
          by a sequence of transpositions between adjacent elements.  1 <=
          IFST <= N; 1 <= ILST <= N.

  INFO    (output) INTEGER
          = 0:  successful exit
          < 0:  if INFO = -i, the i-th argument had an illegal value