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LAPACK version 3.0
strexc(3)

PURPOSE
  STREXC - reorder the real Schur factorization of a real matrix A =
  Q*T*Q**T, so that the diagonal block of T with row index IFST is moved to
  row ILST

SYNTAX
  SUBROUTINE STREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, INFO )

      CHARACTER	     COMPQ

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

      REAL	     Q( LDQ, * ), T( LDT, * ), WORK( * )

DESCRIPTION
  STREXC reorders the real Schur factorization of a real matrix A = Q*T*Q**T,
  so that the diagonal block of T with row index IFST is moved to row ILST.
  The real Schur form T is reordered by an orthogonal similarity
  transformation Z**T*T*Z, and optionally the matrix Q of Schur vectors is
  updated by postmultiplying it with Z.

  T must be in Schur canonical form (as returned by SHSEQR), that is, block
  upper triangular with 1-by-1 and 2-by-2 diagonal blocks; each 2-by-2
  diagonal block has its diagonal elements equal and its off-diagonal
  elements of opposite sign.

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) REAL array, dimension (LDT,N)
	  On entry, the upper quasi-triangular matrix T, in Schur Schur
	  canonical form.  On exit, the reordered upper quasi-triangular
	  matrix, again in Schur canonical form.

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

  Q	  (input/output) REAL 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 orthogonal
	  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/output) INTEGER
	  ILST	  (input/output) INTEGER Specify the reordering of the
	  diagonal blocks of T.	 The block with row index IFST is moved to
	  row ILST, by a sequence of transpositions between adjacent blocks.
	  On exit, if IFST pointed on entry to the second row of a 2-by-2
	  block, it is changed to point to the first row; ILST always points
	  to the first row of the block in its final position (which may
	  differ from its input value by +1 or -1).  1 <= IFST <= N; 1 <=
	  ILST <= N.

  WORK	  (workspace) REAL array, dimension (N)

  INFO	  (output) INTEGER
	  = 0:	successful exit
	  < 0:	if INFO = -i, the i-th argument had an illegal value
	  = 1:	two adjacent blocks were too close to swap (the problem is
	  very ill-conditioned); T may have been partially reordered, and
	  ILST points to the first row of the current position of the block
	  being moved.