          SUBROUTINE DTREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK,
         $                   INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          CHARACTER          COMPQ
          INTEGER            IFST, ILST, INFO, LDQ, LDT, N
    *     ..
    *     .. Array Arguments ..
          DOUBLE PRECISION   Q( LDQ, * ), T( LDT, * ), WORK( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  DTREXC 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 DHSEQR), 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) DOUBLE PRECISION 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) DOUBLE PRECISION 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) DOUBLE PRECISION 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.
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          DOUBLE PRECISION   ZERO
          PARAMETER          ( ZERO = 0.0D+0 )
    *     ..
    *     .. Local Scalars ..
          LOGICAL            WANTQ
          INTEGER            HERE, NBF, NBL, NBNEXT
    *     ..
    *     .. External Functions ..
          LOGICAL            LSAME
          EXTERNAL           LSAME
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           DLAEXC, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          MAX
    *     ..
    *     .. Executable Statements ..
    *
    *     Decode and test the input arguments.
    *
          INFO = 0
          WANTQ = LSAME( COMPQ, 'V' )
          IF( .NOT.WANTQ .AND. .NOT.LSAME( COMPQ, 'N' ) ) THEN
             INFO = -1
          ELSE IF( N.LT.0 ) THEN
             INFO = -2
          ELSE IF( LDT.LT.MAX( 1, N ) ) THEN
             INFO = -4
          ELSE IF( LDQ.LT.1 .OR. ( WANTQ .AND. LDQ.LT.MAX( 1, N ) ) ) THEN
             INFO = -6
          ELSE IF( IFST.LT.1 .OR. IFST.GT.N ) THEN
             INFO = -7
          ELSE IF( ILST.LT.1 .OR. ILST.GT.N ) THEN
             INFO = -8
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'DTREXC', -INFO )
             RETURN
          END IF
    *
    *     Quick return if possible
    *
          IF( N.LE.1 )
         $   RETURN
    *
    *     Determine the first row of specified block
    *     and find out it is 1 by 1 or 2 by 2.
    *
          IF( IFST.GT.1 ) THEN
             IF( T( IFST, IFST-1 ).NE.ZERO )
         $      IFST = IFST - 1
          END IF
          NBF = 1
          IF( IFST.LT.N ) THEN
             IF( T( IFST+1, IFST ).NE.ZERO )
         $      NBF = 2
          END IF
    *
    *     Determine the first row of the final block
    *     and find out it is 1 by 1 or 2 by 2.
    *
          IF( ILST.GT.1 ) THEN
             IF( T( ILST, ILST-1 ).NE.ZERO )
         $      ILST = ILST - 1
          END IF
          NBL = 1
          IF( ILST.LT.N ) THEN
             IF( T( ILST+1, ILST ).NE.ZERO )
         $      NBL = 2
          END IF
    *
          IF( IFST.EQ.ILST )
         $   RETURN
    *
          IF( IFST.LT.ILST ) THEN
    *
    *        Update ILST
    *
             IF( NBF.EQ.2 .AND. NBL.EQ.1 )
         $      ILST = ILST - 1
             IF( NBF.EQ.1 .AND. NBL.EQ.2 )
         $      ILST = ILST + 1
    *
             HERE = IFST
    *
       10    CONTINUE
    *
    *        Swap block with next one below
    *
             IF( NBF.EQ.1 .OR. NBF.EQ.2 ) THEN
    *
    *           Current block either 1 by 1 or 2 by 2
    *
                NBNEXT = 1
                IF( HERE+NBF+1.LE.N ) THEN
                   IF( T( HERE+NBF+1, HERE+NBF ).NE.ZERO )
         $            NBNEXT = 2
                END IF
                CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE, NBF, NBNEXT,
         $                   WORK, INFO )
                IF( INFO.NE.0 ) THEN
                   ILST = HERE
                   RETURN
                END IF
                HERE = HERE + NBNEXT
    *
    *           Test if 2 by 2 block breaks into two 1 by 1 blocks
    *
                IF( NBF.EQ.2 ) THEN
                   IF( T( HERE+1, HERE ).EQ.ZERO )
         $            NBF = 3
                END IF
    *
             ELSE
    *
    *           Current block consists of two 1 by 1 blocks each of which
    *           must be swapped individually
    *
                NBNEXT = 1
                IF( HERE+3.LE.N ) THEN
                   IF( T( HERE+3, HERE+2 ).NE.ZERO )
         $            NBNEXT = 2
                END IF
                CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE+1, 1, NBNEXT,
         $                   WORK, INFO )
                IF( INFO.NE.0 ) THEN
                   ILST = HERE
                   RETURN
                END IF
                IF( NBNEXT.EQ.1 ) THEN
    *
    *              Swap two 1 by 1 blocks, no problems possible
    *
                   CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE, 1, NBNEXT,
         $                      WORK, INFO )
                   HERE = HERE + 1
                ELSE
    *
    *              Recompute NBNEXT in case 2 by 2 split
    *
                   IF( T( HERE+2, HERE+1 ).EQ.ZERO )
         $            NBNEXT = 1
                   IF( NBNEXT.EQ.2 ) THEN
    *
    *                 2 by 2 Block did not split
    *
                      CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE, 1,
         $                         NBNEXT, WORK, INFO )
                      IF( INFO.NE.0 ) THEN
                         ILST = HERE
                         RETURN
                      END IF
                      HERE = HERE + 2
                   ELSE
    *
    *                 2 by 2 Block did split
    *
                      CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE, 1, 1,
         $                         WORK, INFO )
                      CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE+1, 1, 1,
         $                         WORK, INFO )
                      HERE = HERE + 2
                   END IF
                END IF
             END IF
             IF( HERE.LT.ILST )
         $      GO TO 10
    *
          ELSE
    *
             HERE = IFST
       20    CONTINUE
    *
    *        Swap block with next one above
    *
             IF( NBF.EQ.1 .OR. NBF.EQ.2 ) THEN
    *
    *           Current block either 1 by 1 or 2 by 2
    *
                NBNEXT = 1
                IF( HERE.GE.3 ) THEN
                   IF( T( HERE-1, HERE-2 ).NE.ZERO )
         $            NBNEXT = 2
                END IF
                CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE-NBNEXT, NBNEXT,
         $                   NBF, WORK, INFO )
                IF( INFO.NE.0 ) THEN
                   ILST = HERE
                   RETURN
                END IF
                HERE = HERE - NBNEXT
    *
    *           Test if 2 by 2 block breaks into two 1 by 1 blocks
    *
                IF( NBF.EQ.2 ) THEN
                   IF( T( HERE+1, HERE ).EQ.ZERO )
         $            NBF = 3
                END IF
    *
             ELSE
    *
    *           Current block consists of two 1 by 1 blocks each of which
    *           must be swapped individually
    *
                NBNEXT = 1
                IF( HERE.GE.3 ) THEN
                   IF( T( HERE-1, HERE-2 ).NE.ZERO )
         $            NBNEXT = 2
                END IF
                CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE-NBNEXT, NBNEXT,
         $                   1, WORK, INFO )
                IF( INFO.NE.0 ) THEN
                   ILST = HERE
                   RETURN
                END IF
                IF( NBNEXT.EQ.1 ) THEN
    *
    *              Swap two 1 by 1 blocks, no problems possible
    *
                   CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE, NBNEXT, 1,
         $                      WORK, INFO )
                   HERE = HERE - 1
                ELSE
    *
    *              Recompute NBNEXT in case 2 by 2 split
    *
                   IF( T( HERE, HERE-1 ).EQ.ZERO )
         $            NBNEXT = 1
                   IF( NBNEXT.EQ.2 ) THEN
    *
    *                 2 by 2 Block did not split
    *
                      CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE-1, 2, 1,
         $                         WORK, INFO )
                      IF( INFO.NE.0 ) THEN
                         ILST = HERE
                         RETURN
                      END IF
                      HERE = HERE - 2
                   ELSE
    *
    *                 2 by 2 Block did split
    *
                      CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE, 1, 1,
         $                         WORK, INFO )
                      CALL DLAEXC( WANTQ, N, T, LDT, Q, LDQ, HERE-1, 1, 1,
         $                         WORK, INFO )
                      HERE = HERE - 2
                   END IF
                END IF
             END IF
             IF( HERE.GT.ILST )
         $      GO TO 20
          END IF
          ILST = HERE
    *
          RETURN
    *
    *     End of DTREXC
    *
          END