      SUBROUTINE DTREVC( SIDE, HOWMNY, SELECT, N, T, LDT, VL, LDVL, VR,
     $                   LDVR, MM, M, WORK, INFO )
*
*  -- LAPACK routine (version 3.0) --
*     Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
*     Courant Institute, Argonne National Lab, and Rice University
*     June 30, 1999
*
*     .. Scalar Arguments ..
      CHARACTER          HOWMNY, SIDE
      INTEGER            INFO, LDT, LDVL, LDVR, M, MM, N
*     ..
*     .. Array Arguments ..
      LOGICAL            SELECT( * )
      DOUBLE PRECISION   T( LDT, * ), VL( LDVL, * ), VR( LDVR, * ),
     $                   WORK( * )
*     ..
*
*  Purpose
*  =======
*
*  DTREVC computes some or all of the right and/or left eigenvectors of
*  a real upper quasi-triangular matrix T.
*
*  The right eigenvector x and the left eigenvector y of T corresponding
*  to an eigenvalue w are defined by:
*
*               T*x = w*x,     y'*T = w*y'
*
*  where y' denotes the conjugate transpose of the vector y.
*
*  If all eigenvectors are requested, the routine may either return the
*  matrices X and/or Y of right or left eigenvectors of T, or the
*  products Q*X and/or Q*Y, where Q is an input orthogonal
*  matrix. If T was obtained from the real-Schur factorization of an
*  original matrix A = Q*T*Q', then Q*X and Q*Y are the matrices of
*  right or left eigenvectors of A.
*
*  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.  Corresponding to each 2-by-2
*  diagonal block is a complex conjugate pair of eigenvalues and
*  eigenvectors; only one eigenvector of the pair is computed, namely
*  the one corresponding to the eigenvalue with positive imaginary part.
*
*  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,
*                  and backtransform them using the input matrices
*                  supplied in VR and/or VL;
*          = 'S':  compute selected right and/or left eigenvectors,
*                  specified by the logical array SELECT.
*
*  SELECT  (input/output) LOGICAL array, dimension (N)
*          If HOWMNY = 'S', SELECT specifies the eigenvectors to be
*          computed.
*          If HOWMNY = 'A' or 'B', SELECT is not referenced.
*          To select the real eigenvector corresponding to a real
*          eigenvalue w(j), SELECT(j) must be set to .TRUE..  To select
*          the complex eigenvector corresponding to a complex conjugate
*          pair w(j) and w(j+1), either SELECT(j) or SELECT(j+1) must be
*          set to .TRUE.; then on exit SELECT(j) is .TRUE. and
*          SELECT(j+1) is .FALSE..
*
*  N       (input) INTEGER
*          The order of the matrix T. N >= 0.
*
*  T       (input) DOUBLE PRECISION array, dimension (LDT,N)
*          The upper quasi-triangular matrix T in Schur canonical form.
*
*  LDT     (input) INTEGER
*          The leading dimension of the array T. LDT >= max(1,N).
*
*  VL      (input/output) DOUBLE PRECISION 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 orthogonal matrix Q
*          of Schur vectors returned by DHSEQR).
*          On exit, if SIDE = 'L' or 'B', VL contains:
*          if HOWMNY = 'A', the matrix Y of left eigenvectors of T;
*                           VL has the same quasi-lower triangular form
*                           as T'. If T(i,i) is a real eigenvalue, then
*                           the i-th column VL(i) of VL  is its
*                           corresponding eigenvector. If T(i:i+1,i:i+1)
*                           is a 2-by-2 block whose eigenvalues are
*                           complex-conjugate eigenvalues of T, then
*                           VL(i)+sqrt(-1)*VL(i+1) is the complex
*                           eigenvector corresponding to the eigenvalue
*                           with positive real part.
*          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.
*          A complex eigenvector corresponding to a complex eigenvalue
*          is stored in two consecutive columns, the first holding the
*          real part, and the second the imaginary part.
*          If SIDE = 'R', VL is not referenced.
*
*  LDVL    (input) INTEGER
*          The leading dimension of the array VL.  LDVL >= max(1,N) if
*          SIDE = 'L' or 'B'; LDVL >= 1 otherwise.
*
*  VR      (input/output) DOUBLE PRECISION 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 orthogonal matrix Q
*          of Schur vectors returned by DHSEQR).
*          On exit, if SIDE = 'R' or 'B', VR contains:
*          if HOWMNY = 'A', the matrix X of right eigenvectors of T;
*                           VR has the same quasi-upper triangular form
*                           as T. If T(i,i) is a real eigenvalue, then
*                           the i-th column VR(i) of VR  is its
*                           corresponding eigenvector. If T(i:i+1,i:i+1)
*                           is a 2-by-2 block whose eigenvalues are
*                           complex-conjugate eigenvalues of T, then
*                           VR(i)+sqrt(-1)*VR(i+1) is the complex
*                           eigenvector corresponding to the eigenvalue
*                           with positive real part.
*          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.
*          A complex eigenvector corresponding to a complex eigenvalue
*          is stored in two consecutive columns, the first holding the
*          real part and the second the imaginary part.
*          If SIDE = 'L', VR is not referenced.
*
*  LDVR    (input) INTEGER
*          The leading dimension of the array VR.  LDVR >= max(1,N) if
*          SIDE = 'R' or 'B'; LDVR >= 1 otherwise.
*
*  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 real eigenvector occupies one column and each
*          selected complex eigenvector occupies two columns.
*
*  WORK    (workspace) DOUBLE PRECISION array, dimension (3*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 ..
      DOUBLE PRECISION   ZERO, ONE
      PARAMETER          ( ZERO = 0.0D+0, ONE = 1.0D+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            ALLV, BOTHV, LEFTV, OVER, PAIR, RIGHTV, SOMEV
      INTEGER            I, IERR, II, IP, IS, J, J1, J2, JNXT, K, KI, N2
      DOUBLE PRECISION   BETA, BIGNUM, EMAX, OVFL, REC, REMAX, SCALE,
     $                   SMIN, SMLNUM, ULP, UNFL, VCRIT, VMAX, WI, WR,
     $                   XNORM
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      INTEGER            IDAMAX
      DOUBLE PRECISION   DDOT, DLAMCH
      EXTERNAL           LSAME, IDAMAX, DDOT, DLAMCH
*     ..
*     .. External Subroutines ..
      EXTERNAL           DAXPY, DCOPY, DGEMV, DLALN2, DSCAL, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          ABS, MAX, SQRT
*     ..
*     .. Local Arrays ..
      DOUBLE PRECISION   X( 2, 2 )
*     ..
*     .. 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' )
*
      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
*
*        Set M to the number of columns required to store the selected
*        eigenvectors, standardize the array SELECT if necessary, and
*        test MM.
*
         IF( SOMEV ) THEN
            M = 0
            PAIR = .FALSE.
            DO 10 J = 1, N
               IF( PAIR ) THEN
                  PAIR = .FALSE.
                  SELECT( J ) = .FALSE.
               ELSE
                  IF( J.LT.N ) THEN
                     IF( T( J+1, J ).EQ.ZERO ) THEN
                        IF( SELECT( J ) )
     $                     M = M + 1
                     ELSE
                        PAIR = .TRUE.
                        IF( SELECT( J ) .OR. SELECT( J+1 ) ) THEN
                           SELECT( J ) = .TRUE.
                           M = M + 2
                        END IF
                     END IF
                  ELSE
                     IF( SELECT( N ) )
     $                  M = M + 1
                  END IF
               END IF
   10       CONTINUE
         ELSE
            M = N
         END IF
*
         IF( MM.LT.M ) THEN
            INFO = -11
         END IF
      END IF
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'DTREVC', -INFO )
         RETURN
      END IF
*
*     Quick return if possible.
*
      IF( N.EQ.0 )
     $   RETURN
*
*     Set the constants to control overflow.
*
      UNFL = DLAMCH( 'Safe minimum' )
      OVFL = ONE / UNFL
      CALL DLABAD( UNFL, OVFL )
      ULP = DLAMCH( 'Precision' )
      SMLNUM = UNFL*( N / ULP )
      BIGNUM = ( ONE-ULP ) / SMLNUM
*
*     Compute 1-norm of each column of strictly upper triangular
*     part of T to control overflow in triangular solver.
*
      WORK( 1 ) = ZERO
      DO 30 J = 2, N
         WORK( J ) = ZERO
         DO 20 I = 1, J - 1
            WORK( J ) = WORK( J ) + ABS( T( I, J ) )
   20    CONTINUE
   30 CONTINUE
*
*     Index IP is used to specify the real or complex eigenvalue:
*       IP = 0, real eigenvalue,
*            1, first of conjugate complex pair: (wr,wi)
*           -1, second of conjugate complex pair: (wr,wi)
*
      N2 = 2*N
*
      IF( RIGHTV ) THEN
*
*        Compute right eigenvectors.
*
         IP = 0
         IS = M
         DO 140 KI = N, 1, -1
*
            IF( IP.EQ.1 )
     $         GO TO 130
            IF( KI.EQ.1 )
     $         GO TO 40
            IF( T( KI, KI-1 ).EQ.ZERO )
     $         GO TO 40
            IP = -1
*
   40       CONTINUE
            IF( SOMEV ) THEN
               IF( IP.EQ.0 ) THEN
                  IF( .NOT.SELECT( KI ) )
     $               GO TO 130
               ELSE
                  IF( .NOT.SELECT( KI-1 ) )
     $               GO TO 130
               END IF
            END IF
*
*           Compute the KI-th eigenvalue (WR,WI).
*
            WR = T( KI, KI )
            WI = ZERO
            IF( IP.NE.0 )
     $         WI = SQRT( ABS( T( KI, KI-1 ) ) )*
     $              SQRT( ABS( T( KI-1, KI ) ) )
            SMIN = MAX( ULP*( ABS( WR )+ABS( WI ) ), SMLNUM )
*
            IF( IP.EQ.0 ) THEN
*
*              Real right eigenvector
*
               WORK( KI+N ) = ONE
*
*              Form right-hand side
*
               DO 50 K = 1, KI - 1
                  WORK( K+N ) = -T( K, KI )
   50          CONTINUE
*
*              Solve the upper quasi-triangular system:
*                 (T(1:KI-1,1:KI-1) - WR)*X = SCALE*WORK.
*
               JNXT = KI - 1
               DO 60 J = KI - 1, 1, -1
                  IF( J.GT.JNXT )
     $               GO TO 60
                  J1 = J
                  J2 = J
                  JNXT = J - 1
                  IF( J.GT.1 ) THEN
                     IF( T( J, J-1 ).NE.ZERO ) THEN
                        J1 = J - 1
                        JNXT = J - 2
                     END IF
                  END IF
*
                  IF( J1.EQ.J2 ) THEN
*
*                    1-by-1 diagonal block
*
                     CALL DLALN2( .FALSE., 1, 1, SMIN, ONE, T( J, J ),
     $                            LDT, ONE, ONE, WORK( J+N ), N, WR,
     $                            ZERO, X, 2, SCALE, XNORM, IERR )
*
*                    Scale X(1,1) to avoid overflow when updating
*                    the right-hand side.
*
                     IF( XNORM.GT.ONE ) THEN
                        IF( WORK( J ).GT.BIGNUM / XNORM ) THEN
                           X( 1, 1 ) = X( 1, 1 ) / XNORM
                           SCALE = SCALE / XNORM
                        END IF
                     END IF
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE )
     $                  CALL DSCAL( KI, SCALE, WORK( 1+N ), 1 )
                     WORK( J+N ) = X( 1, 1 )
*
*                    Update right-hand side
*
                     CALL DAXPY( J-1, -X( 1, 1 ), T( 1, J ), 1,
     $                           WORK( 1+N ), 1 )
*
                  ELSE
*
*                    2-by-2 diagonal block
*
                     CALL DLALN2( .FALSE., 2, 1, SMIN, ONE,
     $                            T( J-1, J-1 ), LDT, ONE, ONE,
     $                            WORK( J-1+N ), N, WR, ZERO, X, 2,
     $                            SCALE, XNORM, IERR )
*
*                    Scale X(1,1) and X(2,1) to avoid overflow when
*                    updating the right-hand side.
*
                     IF( XNORM.GT.ONE ) THEN
                        BETA = MAX( WORK( J-1 ), WORK( J ) )
                        IF( BETA.GT.BIGNUM / XNORM ) THEN
                           X( 1, 1 ) = X( 1, 1 ) / XNORM
                           X( 2, 1 ) = X( 2, 1 ) / XNORM
                           SCALE = SCALE / XNORM
                        END IF
                     END IF
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE )
     $                  CALL DSCAL( KI, SCALE, WORK( 1+N ), 1 )
                     WORK( J-1+N ) = X( 1, 1 )
                     WORK( J+N ) = X( 2, 1 )
*
*                    Update right-hand side
*
                     CALL DAXPY( J-2, -X( 1, 1 ), T( 1, J-1 ), 1,
     $                           WORK( 1+N ), 1 )
                     CALL DAXPY( J-2, -X( 2, 1 ), T( 1, J ), 1,
     $                           WORK( 1+N ), 1 )
                  END IF
   60          CONTINUE
*
*              Copy the vector x or Q*x to VR and normalize.
*
               IF( .NOT.OVER ) THEN
                  CALL DCOPY( KI, WORK( 1+N ), 1, VR( 1, IS ), 1 )
*
                  II = IDAMAX( KI, VR( 1, IS ), 1 )
                  REMAX = ONE / ABS( VR( II, IS ) )
                  CALL DSCAL( KI, REMAX, VR( 1, IS ), 1 )
*
                  DO 70 K = KI + 1, N
                     VR( K, IS ) = ZERO
   70             CONTINUE
               ELSE
                  IF( KI.GT.1 )
     $               CALL DGEMV( 'N', N, KI-1, ONE, VR, LDVR,
     $                           WORK( 1+N ), 1, WORK( KI+N ),
     $                           VR( 1, KI ), 1 )
*
                  II = IDAMAX( N, VR( 1, KI ), 1 )
                  REMAX = ONE / ABS( VR( II, KI ) )
                  CALL DSCAL( N, REMAX, VR( 1, KI ), 1 )
               END IF
*
            ELSE
*
*              Complex right eigenvector.
*
*              Initial solve
*                [ (T(KI-1,KI-1) T(KI-1,KI) ) - (WR + I* WI)]*X = 0.
*                [ (T(KI,KI-1)   T(KI,KI)   )               ]
*
               IF( ABS( T( KI-1, KI ) ).GE.ABS( T( KI, KI-1 ) ) ) THEN
                  WORK( KI-1+N ) = ONE
                  WORK( KI+N2 ) = WI / T( KI-1, KI )
               ELSE
                  WORK( KI-1+N ) = -WI / T( KI, KI-1 )
                  WORK( KI+N2 ) = ONE
               END IF
               WORK( KI+N ) = ZERO
               WORK( KI-1+N2 ) = ZERO
*
*              Form right-hand side
*
               DO 80 K = 1, KI - 2
                  WORK( K+N ) = -WORK( KI-1+N )*T( K, KI-1 )
                  WORK( K+N2 ) = -WORK( KI+N2 )*T( K, KI )
   80          CONTINUE
*
*              Solve upper quasi-triangular system:
*              (T(1:KI-2,1:KI-2) - (WR+i*WI))*X = SCALE*(WORK+i*WORK2)
*
               JNXT = KI - 2
               DO 90 J = KI - 2, 1, -1
                  IF( J.GT.JNXT )
     $               GO TO 90
                  J1 = J
                  J2 = J
                  JNXT = J - 1
                  IF( J.GT.1 ) THEN
                     IF( T( J, J-1 ).NE.ZERO ) THEN
                        J1 = J - 1
                        JNXT = J - 2
                     END IF
                  END IF
*
                  IF( J1.EQ.J2 ) THEN
*
*                    1-by-1 diagonal block
*
                     CALL DLALN2( .FALSE., 1, 2, SMIN, ONE, T( J, J ),
     $                            LDT, ONE, ONE, WORK( J+N ), N, WR, WI,
     $                            X, 2, SCALE, XNORM, IERR )
*
*                    Scale X(1,1) and X(1,2) to avoid overflow when
*                    updating the right-hand side.
*
                     IF( XNORM.GT.ONE ) THEN
                        IF( WORK( J ).GT.BIGNUM / XNORM ) THEN
                           X( 1, 1 ) = X( 1, 1 ) / XNORM
                           X( 1, 2 ) = X( 1, 2 ) / XNORM
                           SCALE = SCALE / XNORM
                        END IF
                     END IF
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE ) THEN
                        CALL DSCAL( KI, SCALE, WORK( 1+N ), 1 )
                        CALL DSCAL( KI, SCALE, WORK( 1+N2 ), 1 )
                     END IF
                     WORK( J+N ) = X( 1, 1 )
                     WORK( J+N2 ) = X( 1, 2 )
*
*                    Update the right-hand side
*
                     CALL DAXPY( J-1, -X( 1, 1 ), T( 1, J ), 1,
     $                           WORK( 1+N ), 1 )
                     CALL DAXPY( J-1, -X( 1, 2 ), T( 1, J ), 1,
     $                           WORK( 1+N2 ), 1 )
*
                  ELSE
*
*                    2-by-2 diagonal block
*
                     CALL DLALN2( .FALSE., 2, 2, SMIN, ONE,
     $                            T( J-1, J-1 ), LDT, ONE, ONE,
     $                            WORK( J-1+N ), N, WR, WI, X, 2, SCALE,
     $                            XNORM, IERR )
*
*                    Scale X to avoid overflow when updating
*                    the right-hand side.
*
                     IF( XNORM.GT.ONE ) THEN
                        BETA = MAX( WORK( J-1 ), WORK( J ) )
                        IF( BETA.GT.BIGNUM / XNORM ) THEN
                           REC = ONE / XNORM
                           X( 1, 1 ) = X( 1, 1 )*REC
                           X( 1, 2 ) = X( 1, 2 )*REC
                           X( 2, 1 ) = X( 2, 1 )*REC
                           X( 2, 2 ) = X( 2, 2 )*REC
                           SCALE = SCALE*REC
                        END IF
                     END IF
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE ) THEN
                        CALL DSCAL( KI, SCALE, WORK( 1+N ), 1 )
                        CALL DSCAL( KI, SCALE, WORK( 1+N2 ), 1 )
                     END IF
                     WORK( J-1+N ) = X( 1, 1 )
                     WORK( J+N ) = X( 2, 1 )
                     WORK( J-1+N2 ) = X( 1, 2 )
                     WORK( J+N2 ) = X( 2, 2 )
*
*                    Update the right-hand side
*
                     CALL DAXPY( J-2, -X( 1, 1 ), T( 1, J-1 ), 1,
     $                           WORK( 1+N ), 1 )
                     CALL DAXPY( J-2, -X( 2, 1 ), T( 1, J ), 1,
     $                           WORK( 1+N ), 1 )
                     CALL DAXPY( J-2, -X( 1, 2 ), T( 1, J-1 ), 1,
     $                           WORK( 1+N2 ), 1 )
                     CALL DAXPY( J-2, -X( 2, 2 ), T( 1, J ), 1,
     $                           WORK( 1+N2 ), 1 )
                  END IF
   90          CONTINUE
*
*              Copy the vector x or Q*x to VR and normalize.
*
               IF( .NOT.OVER ) THEN
                  CALL DCOPY( KI, WORK( 1+N ), 1, VR( 1, IS-1 ), 1 )
                  CALL DCOPY( KI, WORK( 1+N2 ), 1, VR( 1, IS ), 1 )
*
                  EMAX = ZERO
                  DO 100 K = 1, KI
                     EMAX = MAX( EMAX, ABS( VR( K, IS-1 ) )+
     $                      ABS( VR( K, IS ) ) )
  100             CONTINUE
*
                  REMAX = ONE / EMAX
                  CALL DSCAL( KI, REMAX, VR( 1, IS-1 ), 1 )
                  CALL DSCAL( KI, REMAX, VR( 1, IS ), 1 )
*
                  DO 110 K = KI + 1, N
                     VR( K, IS-1 ) = ZERO
                     VR( K, IS ) = ZERO
  110             CONTINUE
*
               ELSE
*
                  IF( KI.GT.2 ) THEN
                     CALL DGEMV( 'N', N, KI-2, ONE, VR, LDVR,
     $                           WORK( 1+N ), 1, WORK( KI-1+N ),
     $                           VR( 1, KI-1 ), 1 )
                     CALL DGEMV( 'N', N, KI-2, ONE, VR, LDVR,
     $                           WORK( 1+N2 ), 1, WORK( KI+N2 ),
     $                           VR( 1, KI ), 1 )
                  ELSE
                     CALL DSCAL( N, WORK( KI-1+N ), VR( 1, KI-1 ), 1 )
                     CALL DSCAL( N, WORK( KI+N2 ), VR( 1, KI ), 1 )
                  END IF
*
                  EMAX = ZERO
                  DO 120 K = 1, N
                     EMAX = MAX( EMAX, ABS( VR( K, KI-1 ) )+
     $                      ABS( VR( K, KI ) ) )
  120             CONTINUE
                  REMAX = ONE / EMAX
                  CALL DSCAL( N, REMAX, VR( 1, KI-1 ), 1 )
                  CALL DSCAL( N, REMAX, VR( 1, KI ), 1 )
               END IF
            END IF
*
            IS = IS - 1
            IF( IP.NE.0 )
     $         IS = IS - 1
  130       CONTINUE
            IF( IP.EQ.1 )
     $         IP = 0
            IF( IP.EQ.-1 )
     $         IP = 1
  140    CONTINUE
      END IF
*
      IF( LEFTV ) THEN
*
*        Compute left eigenvectors.
*
         IP = 0
         IS = 1
         DO 260 KI = 1, N
*
            IF( IP.EQ.-1 )
     $         GO TO 250
            IF( KI.EQ.N )
     $         GO TO 150
            IF( T( KI+1, KI ).EQ.ZERO )
     $         GO TO 150
            IP = 1
*
  150       CONTINUE
            IF( SOMEV ) THEN
               IF( .NOT.SELECT( KI ) )
     $            GO TO 250
            END IF
*
*           Compute the KI-th eigenvalue (WR,WI).
*
            WR = T( KI, KI )
            WI = ZERO
            IF( IP.NE.0 )
     $         WI = SQRT( ABS( T( KI, KI+1 ) ) )*
     $              SQRT( ABS( T( KI+1, KI ) ) )
            SMIN = MAX( ULP*( ABS( WR )+ABS( WI ) ), SMLNUM )
*
            IF( IP.EQ.0 ) THEN
*
*              Real left eigenvector.
*
               WORK( KI+N ) = ONE
*
*              Form right-hand side
*
               DO 160 K = KI + 1, N
                  WORK( K+N ) = -T( KI, K )
  160          CONTINUE
*
*              Solve the quasi-triangular system:
*                 (T(KI+1:N,KI+1:N) - WR)'*X = SCALE*WORK
*
               VMAX = ONE
               VCRIT = BIGNUM
*
               JNXT = KI + 1
               DO 170 J = KI + 1, N
                  IF( J.LT.JNXT )
     $               GO TO 170
                  J1 = J
                  J2 = J
                  JNXT = J + 1
                  IF( J.LT.N ) THEN
                     IF( T( J+1, J ).NE.ZERO ) THEN
                        J2 = J + 1
                        JNXT = J + 2
                     END IF
                  END IF
*
                  IF( J1.EQ.J2 ) THEN
*
*                    1-by-1 diagonal block
*
*                    Scale if necessary to avoid overflow when forming
*                    the right-hand side.
*
                     IF( WORK( J ).GT.VCRIT ) THEN
                        REC = ONE / VMAX
                        CALL DSCAL( N-KI+1, REC, WORK( KI+N ), 1 )
                        VMAX = ONE
                        VCRIT = BIGNUM
                     END IF
*
                     WORK( J+N ) = WORK( J+N ) -
     $                             DDOT( J-KI-1, T( KI+1, J ), 1,
     $                             WORK( KI+1+N ), 1 )
*
*                    Solve (T(J,J)-WR)'*X = WORK
*
                     CALL DLALN2( .FALSE., 1, 1, SMIN, ONE, T( J, J ),
     $                            LDT, ONE, ONE, WORK( J+N ), N, WR,
     $                            ZERO, X, 2, SCALE, XNORM, IERR )
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE )
     $                  CALL DSCAL( N-KI+1, SCALE, WORK( KI+N ), 1 )
                     WORK( J+N ) = X( 1, 1 )
                     VMAX = MAX( ABS( WORK( J+N ) ), VMAX )
                     VCRIT = BIGNUM / VMAX
*
                  ELSE
*
*                    2-by-2 diagonal block
*
*                    Scale if necessary to avoid overflow when forming
*                    the right-hand side.
*
                     BETA = MAX( WORK( J ), WORK( J+1 ) )
                     IF( BETA.GT.VCRIT ) THEN
                        REC = ONE / VMAX
                        CALL DSCAL( N-KI+1, REC, WORK( KI+N ), 1 )
                        VMAX = ONE
                        VCRIT = BIGNUM
                     END IF
*
                     WORK( J+N ) = WORK( J+N ) -
     $                             DDOT( J-KI-1, T( KI+1, J ), 1,
     $                             WORK( KI+1+N ), 1 )
*
                     WORK( J+1+N ) = WORK( J+1+N ) -
     $                               DDOT( J-KI-1, T( KI+1, J+1 ), 1,
     $                               WORK( KI+1+N ), 1 )
*
*                    Solve
*                      [T(J,J)-WR   T(J,J+1)     ]'* X = SCALE*( WORK1 )
*                      [T(J+1,J)    T(J+1,J+1)-WR]             ( WORK2 )
*
                     CALL DLALN2( .TRUE., 2, 1, SMIN, ONE, T( J, J ),
     $                            LDT, ONE, ONE, WORK( J+N ), N, WR,
     $                            ZERO, X, 2, SCALE, XNORM, IERR )
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE )
     $                  CALL DSCAL( N-KI+1, SCALE, WORK( KI+N ), 1 )
                     WORK( J+N ) = X( 1, 1 )
                     WORK( J+1+N ) = X( 2, 1 )
*
                     VMAX = MAX( ABS( WORK( J+N ) ),
     $                      ABS( WORK( J+1+N ) ), VMAX )
                     VCRIT = BIGNUM / VMAX
*
                  END IF
  170          CONTINUE
*
*              Copy the vector x or Q*x to VL and normalize.
*
               IF( .NOT.OVER ) THEN
                  CALL DCOPY( N-KI+1, WORK( KI+N ), 1, VL( KI, IS ), 1 )
*
                  II = IDAMAX( N-KI+1, VL( KI, IS ), 1 ) + KI - 1
                  REMAX = ONE / ABS( VL( II, IS ) )
                  CALL DSCAL( N-KI+1, REMAX, VL( KI, IS ), 1 )
*
                  DO 180 K = 1, KI - 1
                     VL( K, IS ) = ZERO
  180             CONTINUE
*
               ELSE
*
                  IF( KI.LT.N )
     $               CALL DGEMV( 'N', N, N-KI, ONE, VL( 1, KI+1 ), LDVL,
     $                           WORK( KI+1+N ), 1, WORK( KI+N ),
     $                           VL( 1, KI ), 1 )
*
                  II = IDAMAX( N, VL( 1, KI ), 1 )
                  REMAX = ONE / ABS( VL( II, KI ) )
                  CALL DSCAL( N, REMAX, VL( 1, KI ), 1 )
*
               END IF
*
            ELSE
*
*              Complex left eigenvector.
*
*               Initial solve:
*                 ((T(KI,KI)    T(KI,KI+1) )' - (WR - I* WI))*X = 0.
*                 ((T(KI+1,KI) T(KI+1,KI+1))                )
*
               IF( ABS( T( KI, KI+1 ) ).GE.ABS( T( KI+1, KI ) ) ) THEN
                  WORK( KI+N ) = WI / T( KI, KI+1 )
                  WORK( KI+1+N2 ) = ONE
               ELSE
                  WORK( KI+N ) = ONE
                  WORK( KI+1+N2 ) = -WI / T( KI+1, KI )
               END IF
               WORK( KI+1+N ) = ZERO
               WORK( KI+N2 ) = ZERO
*
*              Form right-hand side
*
               DO 190 K = KI + 2, N
                  WORK( K+N ) = -WORK( KI+N )*T( KI, K )
                  WORK( K+N2 ) = -WORK( KI+1+N2 )*T( KI+1, K )
  190          CONTINUE
*
*              Solve complex quasi-triangular system:
*              ( T(KI+2,N:KI+2,N) - (WR-i*WI) )*X = WORK1+i*WORK2
*
               VMAX = ONE
               VCRIT = BIGNUM
*
               JNXT = KI + 2
               DO 200 J = KI + 2, N
                  IF( J.LT.JNXT )
     $               GO TO 200
                  J1 = J
                  J2 = J
                  JNXT = J + 1
                  IF( J.LT.N ) THEN
                     IF( T( J+1, J ).NE.ZERO ) THEN
                        J2 = J + 1
                        JNXT = J + 2
                     END IF
                  END IF
*
                  IF( J1.EQ.J2 ) THEN
*
*                    1-by-1 diagonal block
*
*                    Scale if necessary to avoid overflow when
*                    forming the right-hand side elements.
*
                     IF( WORK( J ).GT.VCRIT ) THEN
                        REC = ONE / VMAX
                        CALL DSCAL( N-KI+1, REC, WORK( KI+N ), 1 )
                        CALL DSCAL( N-KI+1, REC, WORK( KI+N2 ), 1 )
                        VMAX = ONE
                        VCRIT = BIGNUM
                     END IF
*
                     WORK( J+N ) = WORK( J+N ) -
     $                             DDOT( J-KI-2, T( KI+2, J ), 1,
     $                             WORK( KI+2+N ), 1 )
                     WORK( J+N2 ) = WORK( J+N2 ) -
     $                              DDOT( J-KI-2, T( KI+2, J ), 1,
     $                              WORK( KI+2+N2 ), 1 )
*
*                    Solve (T(J,J)-(WR-i*WI))*(X11+i*X12)= WK+I*WK2
*
                     CALL DLALN2( .FALSE., 1, 2, SMIN, ONE, T( J, J ),
     $                            LDT, ONE, ONE, WORK( J+N ), N, WR,
     $                            -WI, X, 2, SCALE, XNORM, IERR )
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE ) THEN
                        CALL DSCAL( N-KI+1, SCALE, WORK( KI+N ), 1 )
                        CALL DSCAL( N-KI+1, SCALE, WORK( KI+N2 ), 1 )
                     END IF
                     WORK( J+N ) = X( 1, 1 )
                     WORK( J+N2 ) = X( 1, 2 )
                     VMAX = MAX( ABS( WORK( J+N ) ),
     $                      ABS( WORK( J+N2 ) ), VMAX )
                     VCRIT = BIGNUM / VMAX
*
                  ELSE
*
*                    2-by-2 diagonal block
*
*                    Scale if necessary to avoid overflow when forming
*                    the right-hand side elements.
*
                     BETA = MAX( WORK( J ), WORK( J+1 ) )
                     IF( BETA.GT.VCRIT ) THEN
                        REC = ONE / VMAX
                        CALL DSCAL( N-KI+1, REC, WORK( KI+N ), 1 )
                        CALL DSCAL( N-KI+1, REC, WORK( KI+N2 ), 1 )
                        VMAX = ONE
                        VCRIT = BIGNUM
                     END IF
*
                     WORK( J+N ) = WORK( J+N ) -
     $                             DDOT( J-KI-2, T( KI+2, J ), 1,
     $                             WORK( KI+2+N ), 1 )
*
                     WORK( J+N2 ) = WORK( J+N2 ) -
     $                              DDOT( J-KI-2, T( KI+2, J ), 1,
     $                              WORK( KI+2+N2 ), 1 )
*
                     WORK( J+1+N ) = WORK( J+1+N ) -
     $                               DDOT( J-KI-2, T( KI+2, J+1 ), 1,
     $                               WORK( KI+2+N ), 1 )
*
                     WORK( J+1+N2 ) = WORK( J+1+N2 ) -
     $                                DDOT( J-KI-2, T( KI+2, J+1 ), 1,
     $                                WORK( KI+2+N2 ), 1 )
*
*                    Solve 2-by-2 complex linear equation
*                      ([T(j,j)   T(j,j+1)  ]'-(wr-i*wi)*I)*X = SCALE*B
*                      ([T(j+1,j) T(j+1,j+1)]             )
*
                     CALL DLALN2( .TRUE., 2, 2, SMIN, ONE, T( J, J ),
     $                            LDT, ONE, ONE, WORK( J+N ), N, WR,
     $                            -WI, X, 2, SCALE, XNORM, IERR )
*
*                    Scale if necessary
*
                     IF( SCALE.NE.ONE ) THEN
                        CALL DSCAL( N-KI+1, SCALE, WORK( KI+N ), 1 )
                        CALL DSCAL( N-KI+1, SCALE, WORK( KI+N2 ), 1 )
                     END IF
                     WORK( J+N ) = X( 1, 1 )
                     WORK( J+N2 ) = X( 1, 2 )
                     WORK( J+1+N ) = X( 2, 1 )
                     WORK( J+1+N2 ) = X( 2, 2 )
                     VMAX = MAX( ABS( X( 1, 1 ) ), ABS( X( 1, 2 ) ),
     $                      ABS( X( 2, 1 ) ), ABS( X( 2, 2 ) ), VMAX )
                     VCRIT = BIGNUM / VMAX
*
                  END IF
  200          CONTINUE
*
*              Copy the vector x or Q*x to VL and normalize.
*
  210          CONTINUE
               IF( .NOT.OVER ) THEN
                  CALL DCOPY( N-KI+1, WORK( KI+N ), 1, VL( KI, IS ), 1 )
                  CALL DCOPY( N-KI+1, WORK( KI+N2 ), 1, VL( KI, IS+1 ),
     $                        1 )
*
                  EMAX = ZERO
                  DO 220 K = KI, N
                     EMAX = MAX( EMAX, ABS( VL( K, IS ) )+
     $                      ABS( VL( K, IS+1 ) ) )
  220             CONTINUE
                  REMAX = ONE / EMAX
                  CALL DSCAL( N-KI+1, REMAX, VL( KI, IS ), 1 )
                  CALL DSCAL( N-KI+1, REMAX, VL( KI, IS+1 ), 1 )
*
                  DO 230 K = 1, KI - 1
                     VL( K, IS ) = ZERO
                     VL( K, IS+1 ) = ZERO
  230             CONTINUE
               ELSE
                  IF( KI.LT.N-1 ) THEN
                     CALL DGEMV( 'N', N, N-KI-1, ONE, VL( 1, KI+2 ),
     $                           LDVL, WORK( KI+2+N ), 1, WORK( KI+N ),
     $                           VL( 1, KI ), 1 )
                     CALL DGEMV( 'N', N, N-KI-1, ONE, VL( 1, KI+2 ),
     $                           LDVL, WORK( KI+2+N2 ), 1,
     $                           WORK( KI+1+N2 ), VL( 1, KI+1 ), 1 )
                  ELSE
                     CALL DSCAL( N, WORK( KI+N ), VL( 1, KI ), 1 )
                     CALL DSCAL( N, WORK( KI+1+N2 ), VL( 1, KI+1 ), 1 )
                  END IF
*
                  EMAX = ZERO
                  DO 240 K = 1, N
                     EMAX = MAX( EMAX, ABS( VL( K, KI ) )+
     $                      ABS( VL( K, KI+1 ) ) )
  240             CONTINUE
                  REMAX = ONE / EMAX
                  CALL DSCAL( N, REMAX, VL( 1, KI ), 1 )
                  CALL DSCAL( N, REMAX, VL( 1, KI+1 ), 1 )
*
               END IF
*
            END IF
*
            IS = IS + 1
            IF( IP.NE.0 )
     $         IS = IS + 1
  250       CONTINUE
            IF( IP.EQ.-1 )
     $         IP = 0
            IF( IP.EQ.1 )
     $         IP = -1
*
  260    CONTINUE
*
      END IF
*
      RETURN
*
*     End of DTREVC
*
      END
