          SUBROUTINE CTREVC( SIDE, HOWMNY, SELECT, N, T, LDT, VL, LDVL, VR,
         $                   LDVR, MM, M, WORK, RWORK, INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          CHARACTER          HOWMNY, SIDE
          INTEGER            INFO, LDT, LDVL, LDVR, M, MM, N
    *     ..
    *     .. Array Arguments ..
          LOGICAL            SELECT( * )
          REAL               RWORK( * )
          COMPLEX            T( LDT, * ), VL( LDVL, * ), VR( LDVR, * ),
         $                   WORK( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  CTREVC computes some or all of the right and/or left eigenvectors of
    *  a complex upper triangular matrix T.
    *  Matrices of this type are produced by the Schur factorization of
    *  a complex general matrix:  A = Q*T*Q**H, as computed by CHSEQR.
    *
    *  The right eigenvector x and the left eigenvector y of T corresponding
    *  to an eigenvalue w are defined by:
    *
    *               T*x = w*x,     (y**H)*T = w*(y**H)
    *
    *  where y**H denotes the conjugate transpose of the vector y.
    *  The eigenvalues are not input to this routine, but are read directly
    *  from the diagonal of T.
    *
    *  This routine returns the matrices X and/or Y of right and left
    *  eigenvectors of T, or the products Q*X and/or Q*Y, where Q is an
    *  input matrix.  If Q is the unitary factor that reduces a matrix A to
    *  Schur form T, then Q*X and Q*Y are the matrices of right and left
    *  eigenvectors of A.
    *
    *  Arguments
    *  =========
    *
    *  SIDE    (input) CHARACTER*1
    *          = 'R':  compute right eigenvectors only;
    *          = 'L':  compute left eigenvectors only;
    *          = 'B':  compute both right and left eigenvectors.
    *
    *  HOWMNY  (input) CHARACTER*1
    *          = 'A':  compute all right and/or left eigenvectors;
    *          = 'B':  compute all right and/or left eigenvectors,
    *                  backtransformed using the matrices supplied in
    *                  VR and/or VL;
    *          = 'S':  compute selected right and/or left eigenvectors,
    *                  as indicated by the logical array SELECT.
    *
    *  SELECT  (input) LOGICAL array, dimension (N)
    *          If HOWMNY = 'S', SELECT specifies the eigenvectors to be
    *          computed.
    *          The eigenvector corresponding to the j-th eigenvalue is
    *          computed if SELECT(j) = .TRUE..
    *          Not referenced if HOWMNY = 'A' or 'B'.
    *
    *  N       (input) INTEGER
    *          The order of the matrix T. N >= 0.
    *
    *  T       (input/output) COMPLEX array, dimension (LDT,N)
    *          The upper triangular matrix T.  T is modified, but restored
    *          on exit.
    *
    *  LDT     (input) INTEGER
    *          The leading dimension of the array T. LDT >= max(1,N).
    *
    *  VL      (input/output) COMPLEX array, dimension (LDVL,MM)
    *          On entry, if SIDE = 'L' or 'B' and HOWMNY = 'B', VL must
    *          contain an N-by-N matrix Q (usually the unitary matrix Q of
    *          Schur vectors returned by CHSEQR).
    *          On exit, if SIDE = 'L' or 'B', VL contains:
    *          if HOWMNY = 'A', the matrix Y of left eigenvectors of T;
    *          if HOWMNY = 'B', the matrix Q*Y;
    *          if HOWMNY = 'S', the left eigenvectors of T specified by
    *                           SELECT, stored consecutively in the columns
    *                           of VL, in the same order as their
    *                           eigenvalues.
    *          Not referenced if SIDE = 'R'.
    *
    *  LDVL    (input) INTEGER
    *          The leading dimension of the array VL.  LDVL >= 1, and if
    *          SIDE = 'L' or 'B', LDVL >= N.
    *
    *  VR      (input/output) COMPLEX array, dimension (LDVR,MM)
    *          On entry, if SIDE = 'R' or 'B' and HOWMNY = 'B', VR must
    *          contain an N-by-N matrix Q (usually the unitary matrix Q of
    *          Schur vectors returned by CHSEQR).
    *          On exit, if SIDE = 'R' or 'B', VR contains:
    *          if HOWMNY = 'A', the matrix X of right eigenvectors of T;
    *          if HOWMNY = 'B', the matrix Q*X;
    *          if HOWMNY = 'S', the right eigenvectors of T specified by
    *                           SELECT, stored consecutively in the columns
    *                           of VR, in the same order as their
    *                           eigenvalues.
    *          Not referenced if SIDE = 'L'.
    *
    *  LDVR    (input) INTEGER
    *          The leading dimension of the array VR.  LDVR >= 1, and if
    *          SIDE = 'R' or 'B'; LDVR >= N.
    *
    *  MM      (input) INTEGER
    *          The number of columns in the arrays VL and/or VR. MM >= M.
    *
    *  M       (output) INTEGER
    *          The number of columns in the arrays VL and/or VR actually
    *          used to store the eigenvectors.  If HOWMNY = 'A' or 'B', M
    *          is set to N.  Each selected eigenvector occupies one
    *          column.
    *
    *  WORK    (workspace) COMPLEX array, dimension (2*N)
    *
    *  RWORK   (workspace) REAL array, dimension (N)
    *
    *  INFO    (output) INTEGER
    *          = 0:  successful exit
    *          < 0:  if INFO = -i, the i-th argument had an illegal value
    *
    *  Further Details
    *  ===============
    *
    *  The algorithm used in this program is basically backward (forward)
    *  substitution, with scaling to make the the code robust against
    *  possible overflow.
    *
    *  Each eigenvector is normalized so that the element of largest
    *  magnitude has magnitude 1; here the magnitude of a complex number
    *  (x,y) is taken to be |x| + |y|.
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          REAL               ZERO, ONE
          PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0 )
          COMPLEX            CMZERO, CMONE
          PARAMETER          ( CMZERO = ( 0.0E+0, 0.0E+0 ),
         $                   CMONE = ( 1.0E+0, 0.0E+0 ) )
    *     ..
    *     .. Local Scalars ..
          LOGICAL            ALLV, BOTHV, LEFTV, OVER, RIGHTV, SOMEV
          INTEGER            I, II, IS, J, K, KI
          REAL               OVFL, REMAX, SCALE, SMIN, SMLNUM, ULP, UNFL
          COMPLEX            CDUM
    *     ..
    *     .. External Functions ..
          LOGICAL            LSAME
          INTEGER            ICAMAX
          REAL               SCASUM, SLAMCH
          EXTERNAL           LSAME, ICAMAX, SCASUM, SLAMCH
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           CCOPY, CGEMV, CLATRS, CSSCAL, SLABAD, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          ABS, AIMAG, CMPLX, CONJG, MAX, REAL
    *     ..
    *     .. Statement Functions ..
          REAL               CABS1
    *     ..
    *     .. Statement Function definitions ..
          CABS1( CDUM ) = ABS( REAL( CDUM ) ) + ABS( AIMAG( CDUM ) )
    *     ..
    *     .. Executable Statements ..
    *
    *     Decode and test the input parameters
    *
          BOTHV = LSAME( SIDE, 'B' )
          RIGHTV = LSAME( SIDE, 'R' ) .OR. BOTHV
          LEFTV = LSAME( SIDE, 'L' ) .OR. BOTHV
    *
          ALLV = LSAME( HOWMNY, 'A' )
          OVER = LSAME( HOWMNY, 'B' )
          SOMEV = LSAME( HOWMNY, 'S' )
    *
    *     Set M to the number of columns required to store the selected
    *     eigenvectors.
    *
          IF( SOMEV ) THEN
             M = 0
             DO 10 J = 1, N
                IF( SELECT( J ) )
         $         M = M + 1
       10    CONTINUE
          ELSE
             M = N
          END IF
    *
          INFO = 0
          IF( .NOT.RIGHTV .AND. .NOT.LEFTV ) THEN
             INFO = -1
          ELSE IF( .NOT.ALLV .AND. .NOT.OVER .AND. .NOT.SOMEV ) THEN
             INFO = -2
          ELSE IF( N.LT.0 ) THEN
             INFO = -4
          ELSE IF( LDT.LT.MAX( 1, N ) ) THEN
             INFO = -6
          ELSE IF( LDVL.LT.1 .OR. ( LEFTV .AND. LDVL.LT.N ) ) THEN
             INFO = -8
          ELSE IF( LDVR.LT.1 .OR. ( RIGHTV .AND. LDVR.LT.N ) ) THEN
             INFO = -10
          ELSE IF( MM.LT.M ) THEN
             INFO = -11
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'CTREVC', -INFO )
             RETURN
          END IF
    *
    *     Quick return if possible.
    *
          IF( N.EQ.0 )
         $   RETURN
    *
    *     Set the constants to control overflow.
    *
          UNFL = SLAMCH( 'Safe minimum' )
          OVFL = ONE / UNFL
          CALL SLABAD( UNFL, OVFL )
          ULP = SLAMCH( 'Precision' )
          SMLNUM = UNFL*( N / ULP )
    *
    *     Store the diagonal elements of T in working array WORK.
    *
          DO 20 I = 1, N
             WORK( I+N ) = T( I, I )
       20 CONTINUE
    *
    *     Compute 1-norm of each column of strictly upper triangular
    *     part of T to control overflow in triangular solver.
    *
          RWORK( 1 ) = ZERO
          DO 30 J = 2, N
             RWORK( J ) = SCASUM( J-1, T( 1, J ), 1 )
       30 CONTINUE
    *
          IF( RIGHTV ) THEN
    *
    *        Compute right eigenvectors.
    *
             IS = M
             DO 80 KI = N, 1, -1
    *
                IF( SOMEV ) THEN
                   IF( .NOT.SELECT( KI ) )
         $            GO TO 80
                END IF
                SMIN = MAX( ULP*( CABS1( T( KI, KI ) ) ), SMLNUM )
    *
                WORK( 1 ) = CMONE
    *
    *           Form right-hand side.
    *
                DO 40 K = 1, KI - 1
                   WORK( K ) = -T( K, KI )
       40       CONTINUE
    *
    *           Solve the triangular system:
    *              (T(1:KI-1,1:KI-1) - T(KI,KI))*X = SCALE*WORK.
    *
                DO 50 K = 1, KI - 1
                   T( K, K ) = T( K, K ) - T( KI, KI )
                   IF( CABS1( T( K, K ) ).LT.SMIN )
         $            T( K, K ) = SMIN
       50       CONTINUE
    *
                IF( KI.GT.1 ) THEN
                   CALL CLATRS( 'Upper', 'No transpose', 'Non-unit', 'Y',
         $                      KI-1, T, LDT, WORK( 1 ), SCALE, RWORK,
         $                      INFO )
                   WORK( KI ) = SCALE
                END IF
    *
    *           Copy the vector x or Q*x to VR and normalize.
    *
                IF( .NOT.OVER ) THEN
                   CALL CCOPY( KI, WORK( 1 ), 1, VR( 1, IS ), 1 )
    *
                   II = ICAMAX( KI, VR( 1, IS ), 1 )
                   REMAX = ONE / CABS1( VR( II, IS ) )
                   CALL CSSCAL( KI, REMAX, VR( 1, IS ), 1 )
    *
                   DO 60 K = KI + 1, N
                      VR( K, IS ) = CMZERO
       60          CONTINUE
                ELSE
                   IF( KI.GT.1 )
         $            CALL CGEMV( 'N', N, KI-1, CMONE, VR, LDVR, WORK( 1 ),
         $                        1, CMPLX( SCALE ), VR( 1, KI ), 1 )
    *
                   II = ICAMAX( N, VR( 1, KI ), 1 )
                   REMAX = ONE / CABS1( VR( II, KI ) )
                   CALL CSSCAL( N, REMAX, VR( 1, KI ), 1 )
                END IF
    *
    *           Set back the original diagonal elements of T.
    *
                DO 70 K = 1, KI - 1
                   T( K, K ) = WORK( K+N )
       70       CONTINUE
    *
                IS = IS - 1
       80    CONTINUE
          END IF
    *
          IF( LEFTV ) THEN
    *
    *        Compute left eigenvectors.
    *
             IS = 1
             DO 130 KI = 1, N
    *
                IF( SOMEV ) THEN
                   IF( .NOT.SELECT( KI ) )
         $            GO TO 130
                END IF
                SMIN = MAX( ULP*( CABS1( T( KI, KI ) ) ), SMLNUM )
    *
                WORK( N ) = CMONE
    *
    *           Form right-hand side.
    *
                DO 90 K = KI + 1, N
                   WORK( K ) = -CONJG( T( KI, K ) )
       90       CONTINUE
    *
    *           Solve the triangular system:
    *              (T(KI+1:N,KI+1:N) - T(KI,KI))'*X = SCALE*WORK.
    *
                DO 100 K = KI + 1, N
                   T( K, K ) = T( K, K ) - T( KI, KI )
                   IF( CABS1( T( K, K ) ).LT.SMIN )
         $            T( K, K ) = SMIN
      100       CONTINUE
    *
                IF( KI.LT.N ) THEN
                   CALL CLATRS( 'Upper', 'Conjugate transpose', 'Non-unit',
         $                      'Y', N-KI, T( KI+1, KI+1 ), LDT,
         $                      WORK( KI+1 ), SCALE, RWORK, INFO )
                   WORK( KI ) = SCALE
                END IF
    *
    *           Copy the vector x or Q*x to VL and normalize.
    *
                IF( .NOT.OVER ) THEN
                   CALL CCOPY( N-KI+1, WORK( KI ), 1, VL( KI, IS ), 1 )
    *
                   II = ICAMAX( N-KI+1, VL( KI, IS ), 1 ) + KI - 1
                   REMAX = ONE / CABS1( VL( II, IS ) )
                   CALL CSSCAL( N-KI+1, REMAX, VL( KI, IS ), 1 )
    *
                   DO 110 K = 1, KI - 1
                      VL( K, IS ) = CMZERO
      110          CONTINUE
                ELSE
                   IF( KI.LT.N )
         $            CALL CGEMV( 'N', N, N-KI, CMONE, VL( 1, KI+1 ), LDVL,
         $                        WORK( KI+1 ), 1, CMPLX( SCALE ),
         $                        VL( 1, KI ), 1 )
    *
                   II = ICAMAX( N, VL( 1, KI ), 1 )
                   REMAX = ONE / CABS1( VL( II, KI ) )
                   CALL CSSCAL( N, REMAX, VL( 1, KI ), 1 )
                END IF
    *
    *           Set back the original diagonal elements of T.
    *
                DO 120 K = KI + 1, N
                   T( K, K ) = WORK( K+N )
      120       CONTINUE
    *
                IS = IS + 1
      130    CONTINUE
          END IF
    *
          RETURN
    *
    *     End of CTREVC
    *
          END