          SUBROUTINE SLAEXC( WANTQ, N, T, LDT, Q, LDQ, J1, N1, N2, WORK,
         $                   INFO )
    *
    *  -- LAPACK auxiliary routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          LOGICAL            WANTQ
          INTEGER            INFO, J1, LDQ, LDT, N, N1, N2
    *     ..
    *     .. Array Arguments ..
          REAL               Q( LDQ, * ), T( LDT, * ), WORK( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  SLAEXC 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) REAL 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) REAL 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) REAL 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.
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          REAL               ZERO, ONE
          PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0 )
          REAL               TEN
          PARAMETER          ( TEN = 1.0E+1 )
          INTEGER            LDD, LDX
          PARAMETER          ( LDD = 4, LDX = 2 )
    *     ..
    *     .. Local Scalars ..
          INTEGER            IERR, J2, J3, J4, K, ND
          REAL               CS, DNORM, EPS, SCALE, SMLNUM, SN, T11, T22,
         $                   T33, TAU, TAU1, TAU2, TEMP, THRESH, WI1, WI2,
         $                   WR1, WR2, XNORM
    *     ..
    *     .. Local Arrays ..
          REAL               D( LDD, 4 ), U( 3 ), U1( 3 ), U2( 3 ),
         $                   X( LDX, 2 )
    *     ..
    *     .. External Functions ..
          REAL               SLAMCH, SLANGE
          EXTERNAL           SLAMCH, SLANGE
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           SLACPY, SLANV2, SLARFG, SLARFX, SLARTG, SLASY2,
         $                   SROT
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          ABS, MAX
    *     ..
    *     .. Executable Statements ..
    *
          INFO = 0
    *
    *     Quick return if possible
    *
          IF( N.EQ.0 .OR. N1.EQ.0 .OR. N2.EQ.0 )
         $   RETURN
          IF( J1+N1.GT.N )
         $   RETURN
    *
          J2 = J1 + 1
          J3 = J1 + 2
          J4 = J1 + 3
    *
          IF( N1.EQ.1 .AND. N2.EQ.1 ) THEN
    *
    *        Swap two 1-by-1 blocks.
    *
             T11 = T( J1, J1 )
             T22 = T( J2, J2 )
    *
    *        Determine the transformation to perform the interchange.
    *
             CALL SLARTG( T( J1, J2 ), T22-T11, CS, SN, TEMP )
    *
    *        Apply transformation to the matrix T.
    *
             IF( J3.LE.N )
         $      CALL SROT( N-J1-1, T( J1, J3 ), LDT, T( J2, J3 ), LDT, CS,
         $                 SN )
             CALL SROT( J1-1, T( 1, J1 ), 1, T( 1, J2 ), 1, CS, SN )
    *
             T( J1, J1 ) = T22
             T( J2, J2 ) = T11
    *
             IF( WANTQ ) THEN
    *
    *           Accumulate transformation in the matrix Q.
    *
                CALL SROT( N, Q( 1, J1 ), 1, Q( 1, J2 ), 1, CS, SN )
             END IF
    *
          ELSE
    *
    *        Swapping involves at least one 2-by-2 block.
    *
    *        Copy the diagonal block of order N1+N2 to the local array D
    *        and compute its norm.
    *
             ND = N1 + N2
             CALL SLACPY( 'Full', ND, ND, T( J1, J1 ), LDT, D, LDD )
             DNORM = SLANGE( 'Max', ND, ND, D, LDD, WORK )
    *
    *        Compute machine-dependent threshold for test for accepting
    *        swap.
    *
             EPS = SLAMCH( 'P' )
             SMLNUM = SLAMCH( 'S' ) / EPS
             THRESH = MAX( TEN*EPS*DNORM, SMLNUM )
    *
    *        Solve T11*X - X*T22 = scale*T12 for X.
    *
             CALL SLASY2( .FALSE., .FALSE., -1, N1, N2, D, LDD,
         $                D( N1+1, N1+1 ), LDD, D( 1, N1+1 ), LDD, SCALE, X,
         $                LDX, XNORM, IERR )
    *
    *        Swap the adjacent diagonal blocks.
    *
             K = N1 + N1 + N2 - 3
             GO TO ( 10, 20, 30 )K
    *
       10    CONTINUE
    *
    *        N1 = 1, N2 = 2: generate elementary reflector H so that:
    *
    *        ( scale, X11, X12 ) H = ( 0, 0, * )
    *
             U( 1 ) = SCALE
             U( 2 ) = X( 1, 1 )
             U( 3 ) = X( 1, 2 )
             CALL SLARFG( 3, U( 3 ), U, 1, TAU )
             U( 3 ) = ONE
             T11 = T( J1, J1 )
    *
    *        Perform swap provisionally on diagonal block in D.
    *
             CALL SLARFX( 'L', 3, 3, U, TAU, D, LDD, WORK )
             CALL SLARFX( 'R', 3, 3, U, TAU, D, LDD, WORK )
    *
    *        Test whether to reject swap.
    *
             IF( MAX( ABS( D( 3, 1 ) ), ABS( D( 3, 2 ) ), ABS( D( 3,
         $       3 )-T11 ) ).GT.THRESH )GO TO 50
    *
    *        Accept swap: apply transformation to the entire matrix T.
    *
             CALL SLARFX( 'L', 3, N-J1+1, U, TAU, T( J1, J1 ), LDT, WORK )
             CALL SLARFX( 'R', J2, 3, U, TAU, T( 1, J1 ), LDT, WORK )
    *
             T( J3, J1 ) = ZERO
             T( J3, J2 ) = ZERO
             T( J3, J3 ) = T11
    *
             IF( WANTQ ) THEN
    *
    *           Accumulate transformation in the matrix Q.
    *
                CALL SLARFX( 'R', N, 3, U, TAU, Q( 1, J1 ), LDQ, WORK )
             END IF
             GO TO 40
    *
       20    CONTINUE
    *
    *        N1 = 2, N2 = 1: generate elementary reflector H so that:
    *
    *        H (  -X11 ) = ( * )
    *          (  -X21 ) = ( 0 )
    *          ( scale ) = ( 0 )
    *
             U( 1 ) = -X( 1, 1 )
             U( 2 ) = -X( 2, 1 )
             U( 3 ) = SCALE
             CALL SLARFG( 3, U( 1 ), U( 2 ), 1, TAU )
             U( 1 ) = ONE
             T33 = T( J3, J3 )
    *
    *        Perform swap provisionally on diagonal block in D.
    *
             CALL SLARFX( 'L', 3, 3, U, TAU, D, LDD, WORK )
             CALL SLARFX( 'R', 3, 3, U, TAU, D, LDD, WORK )
    *
    *        Test whether to reject swap.
    *
             IF( MAX( ABS( D( 2, 1 ) ), ABS( D( 3, 1 ) ), ABS( D( 1,
         $       1 )-T33 ) ).GT.THRESH )GO TO 50
    *
    *        Accept swap: apply transformation to the entire matrix T.
    *
             CALL SLARFX( 'R', J3, 3, U, TAU, T( 1, J1 ), LDT, WORK )
             CALL SLARFX( 'L', 3, N-J1, U, TAU, T( J1, J2 ), LDT, WORK )
    *
             T( J1, J1 ) = T33
             T( J2, J1 ) = ZERO
             T( J3, J1 ) = ZERO
    *
             IF( WANTQ ) THEN
    *
    *           Accumulate transformation in the matrix Q.
    *
                CALL SLARFX( 'R', N, 3, U, TAU, Q( 1, J1 ), LDQ, WORK )
             END IF
             GO TO 40
    *
       30    CONTINUE
    *
    *        N1 = 2, N2 = 2: generate elementary reflectors H(1) and H(2) so
    *        that:
    *
    *        H(2) H(1) (  -X11  -X12 ) = (  *  * )
    *                  (  -X21  -X22 )   (  0  * )
    *                  ( scale    0  )   (  0  0 )
    *                  (    0  scale )   (  0  0 )
    *
             U1( 1 ) = -X( 1, 1 )
             U1( 2 ) = -X( 2, 1 )
             U1( 3 ) = SCALE
             CALL SLARFG( 3, U1( 1 ), U1( 2 ), 1, TAU1 )
             U1( 1 ) = ONE
    *
             TEMP = -TAU1*( X( 1, 2 )+U1( 2 )*X( 2, 2 ) )
             U2( 1 ) = -TEMP*U1( 2 ) - X( 2, 2 )
             U2( 2 ) = -TEMP*U1( 3 )
             U2( 3 ) = SCALE
             CALL SLARFG( 3, U2( 1 ), U2( 2 ), 1, TAU2 )
             U2( 1 ) = ONE
    *
    *        Perform swap provisionally on diagonal block in D.
    *
             CALL SLARFX( 'L', 3, 4, U1, TAU1, D, LDD, WORK )
             CALL SLARFX( 'R', 4, 3, U1, TAU1, D, LDD, WORK )
             CALL SLARFX( 'L', 3, 4, U2, TAU2, D( 2, 1 ), LDD, WORK )
             CALL SLARFX( 'R', 4, 3, U2, TAU2, D( 1, 2 ), LDD, WORK )
    *
    *        Test whether to reject swap.
    *
             IF( MAX( ABS( D( 3, 1 ) ), ABS( D( 3, 2 ) ), ABS( D( 4, 1 ) ),
         $       ABS( D( 4, 2 ) ) ).GT.THRESH )GO TO 50
    *
    *        Accept swap: apply transformation to the entire matrix T.
    *
             CALL SLARFX( 'L', 3, N-J1+1, U1, TAU1, T( J1, J1 ), LDT, WORK )
             CALL SLARFX( 'R', J4, 3, U1, TAU1, T( 1, J1 ), LDT, WORK )
             CALL SLARFX( 'L', 3, N-J1+1, U2, TAU2, T( J2, J1 ), LDT, WORK )
             CALL SLARFX( 'R', J4, 3, U2, TAU2, T( 1, J2 ), LDT, WORK )
    *
             T( J3, J1 ) = ZERO
             T( J3, J2 ) = ZERO
             T( J4, J1 ) = ZERO
             T( J4, J2 ) = ZERO
    *
             IF( WANTQ ) THEN
    *
    *           Accumulate transformation in the matrix Q.
    *
                CALL SLARFX( 'R', N, 3, U1, TAU1, Q( 1, J1 ), LDQ, WORK )
                CALL SLARFX( 'R', N, 3, U2, TAU2, Q( 1, J2 ), LDQ, WORK )
             END IF
    *
       40    CONTINUE
    *
             IF( N2.EQ.2 ) THEN
    *
    *           Standardize new 2-by-2 block T11
    *
                CALL SLANV2( T( J1, J1 ), T( J1, J2 ), T( J2, J1 ),
         $                   T( J2, J2 ), WR1, WI1, WR2, WI2, CS, SN )
                CALL SROT( N-J1-1, T( J1, J1+2 ), LDT, T( J2, J1+2 ), LDT,
         $                 CS, SN )
                CALL SROT( J1-1, T( 1, J1 ), 1, T( 1, J2 ), 1, CS, SN )
                IF( WANTQ )
         $         CALL SROT( N, Q( 1, J1 ), 1, Q( 1, J2 ), 1, CS, SN )
             END IF
    *
             IF( N1.EQ.2 ) THEN
    *
    *           Standardize new 2-by-2 block T22
    *
                J3 = J1 + N2
                J4 = J3 + 1
                CALL SLANV2( T( J3, J3 ), T( J3, J4 ), T( J4, J3 ),
         $                   T( J4, J4 ), WR1, WI1, WR2, WI2, CS, SN )
                IF( J3+2.LE.N )
         $         CALL SROT( N-J3-1, T( J3, J3+2 ), LDT, T( J4, J3+2 ),
         $                    LDT, CS, SN )
                CALL SROT( J3-1, T( 1, J3 ), 1, T( 1, J4 ), 1, CS, SN )
                IF( WANTQ )
         $         CALL SROT( N, Q( 1, J3 ), 1, Q( 1, J4 ), 1, CS, SN )
             END IF
    *
          END IF
          RETURN
    *
    *     Exit with INFO = 1 if swap was rejected.
    *
       50 INFO = 1
          RETURN
    *
    *     End of SLAEXC
    *
          END