CXML
        	    
LAPACK version 3.0
dlaexc(3)

PURPOSE
  DLAEXC - swap adjacent diagonal blocks T11 and T22 of order 1 or 2 in an
  upper quasi-triangular matrix T by an orthogonal similarity transformation

SYNTAX
  SUBROUTINE DLAEXC( WANTQ, N, T, LDT, Q, LDQ, J1, N1, N2, WORK, INFO )

      LOGICAL	     WANTQ

      INTEGER	     INFO, J1, LDQ, LDT, N, N1, N2

      DOUBLE	     PRECISION Q( LDQ, * ), T( LDT, * ), WORK( * )

DESCRIPTION
  DLAEXC swaps adjacent diagonal blocks T11 and T22 of order 1 or 2 in an
  upper quasi-triangular matrix T by an orthogonal similarity transformation.
  T must be in Schur canonical form, that is, block upper triangular with 1-
  by-1 and 2-by-2 diagonal blocks; each 2-by-2 diagonal block has its
  diagonal elemnts equal and its off-diagonal elements of opposite sign.

ARGUMENTS
  WANTQ	  (input) LOGICAL
	  = .TRUE. : accumulate the transformation in the matrix Q;
	  = .FALSE.: do not accumulate the transformation.

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

  T	  (input/output) DOUBLE PRECISION array, dimension (LDT,N)
	  On entry, the upper quasi-triangular matrix T, in Schur canonical
	  form.	 On exit, the updated matrix T, again in Schur canonical
	  form.

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

  Q	  (input/output) DOUBLE PRECISION array, dimension (LDQ,N)
	  On entry, if WANTQ is .TRUE., the orthogonal matrix Q.  On exit, if
	  WANTQ is .TRUE., the updated matrix Q.  If WANTQ is .FALSE., Q is
	  not referenced.

  LDQ	  (input) INTEGER
	  The leading dimension of the array Q.	 LDQ >= 1; and if WANTQ is
	  .TRUE., LDQ >= N.

  J1	  (input) INTEGER
	  The index of the first row of the first block T11.

  N1	  (input) INTEGER
	  The order of the first block T11. N1 = 0, 1 or 2.

  N2	  (input) INTEGER
	  The order of the second block T22. N2 = 0, 1 or 2.

  WORK	  (workspace) DOUBLE PRECISION array, dimension (N)

  INFO	  (output) INTEGER
	  = 0: successful exit
	  = 1: the transformed matrix T would be too far from Schur form; the
	  blocks are not swapped and T and Q are unchanged.