path: root/src/fortran/lapack/ztgsen.f

      SUBROUTINE ZTGSEN( IJOB, WANTQ, WANTZ, SELECT, N, A, LDA, B, LDB,
     $                   ALPHA, BETA, Q, LDQ, Z, LDZ, M, PL, PR, DIF,
     $                   WORK, LWORK, IWORK, LIWORK, INFO )
*
*  -- LAPACK routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     Modified to call ZLACN2 in place of ZLACON, 10 Feb 03, SJH.
*
*     .. Scalar Arguments ..
      LOGICAL            WANTQ, WANTZ
      INTEGER            IJOB, INFO, LDA, LDB, LDQ, LDZ, LIWORK, LWORK,
     $                   M, N
      DOUBLE PRECISION   PL, PR
*     ..
*     .. Array Arguments ..
      LOGICAL            SELECT( * )
      INTEGER            IWORK( * )
      DOUBLE PRECISION   DIF( * )
      COMPLEX*16         A( LDA, * ), ALPHA( * ), B( LDB, * ),
     $                   BETA( * ), Q( LDQ, * ), WORK( * ), Z( LDZ, * )
*     ..
*
*  Purpose
*  =======
*
*  ZTGSEN reorders the generalized Schur decomposition of a complex
*  matrix pair (A, B) (in terms of an unitary equivalence trans-
*  formation Q' * (A, B) * Z), so that a selected cluster of eigenvalues
*  appears in the leading diagonal blocks of the pair (A,B). The leading
*  columns of Q and Z form unitary bases of the corresponding left and
*  right eigenspaces (deflating subspaces). (A, B) must be in
*  generalized Schur canonical form, that is, A and B are both upper
*  triangular.
*
*  ZTGSEN also computes the generalized eigenvalues
*
*           w(j)= ALPHA(j) / BETA(j)
*
*  of the reordered matrix pair (A, B).
*
*  Optionally, the routine computes estimates of reciprocal condition
*  numbers for eigenvalues and eigenspaces. These are Difu[(A11,B11),
*  (A22,B22)] and Difl[(A11,B11), (A22,B22)], i.e. the separation(s)
*  between the matrix pairs (A11, B11) and (A22,B22) that correspond to
*  the selected cluster and the eigenvalues outside the cluster, resp.,
*  and norms of "projections" onto left and right eigenspaces w.r.t.
*  the selected cluster in the (1,1)-block.
*
*
*  Arguments
*  =========
*
*  IJOB    (input) integer
*          Specifies whether condition numbers are required for the
*          cluster of eigenvalues (PL and PR) or the deflating subspaces
*          (Difu and Difl):
*           =0: Only reorder w.r.t. SELECT. No extras.
*           =1: Reciprocal of norms of "projections" onto left and right
*               eigenspaces w.r.t. the selected cluster (PL and PR).
*           =2: Upper bounds on Difu and Difl. F-norm-based estimate
*               (DIF(1:2)).
*           =3: Estimate of Difu and Difl. 1-norm-based estimate
*               (DIF(1:2)).
*               About 5 times as expensive as IJOB = 2.
*           =4: Compute PL, PR and DIF (i.e. 0, 1 and 2 above): Economic
*               version to get it all.
*           =5: Compute PL, PR and DIF (i.e. 0, 1 and 3 above)
*
*  WANTQ   (input) LOGICAL
*          .TRUE. : update the left transformation matrix Q;
*          .FALSE.: do not update Q.
*
*  WANTZ   (input) LOGICAL
*          .TRUE. : update the right transformation matrix Z;
*          .FALSE.: do not update Z.
*
*  SELECT  (input) LOGICAL array, dimension (N)
*          SELECT specifies the eigenvalues in the selected cluster. To
*          select an eigenvalue w(j), SELECT(j) must be set to
*          .TRUE..
*
*  N       (input) INTEGER
*          The order of the matrices A and B. N >= 0.
*
*  A       (input/output) COMPLEX*16 array, dimension(LDA,N)
*          On entry, the upper triangular matrix A, in generalized
*          Schur canonical form.
*          On exit, A is overwritten by the reordered matrix A.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A. LDA >= max(1,N).
*
*  B       (input/output) COMPLEX*16 array, dimension(LDB,N)
*          On entry, the upper triangular matrix B, in generalized
*          Schur canonical form.
*          On exit, B is overwritten by the reordered matrix B.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B. LDB >= max(1,N).
*
*  ALPHA   (output) COMPLEX*16 array, dimension (N)
*  BETA    (output) COMPLEX*16 array, dimension (N)
*          The diagonal elements of A and B, respectively,
*          when the pair (A,B) has been reduced to generalized Schur
*          form.  ALPHA(i)/BETA(i) i=1,...,N are the generalized
*          eigenvalues.
*
*  Q       (input/output) COMPLEX*16 array, dimension (LDQ,N)
*          On entry, if WANTQ = .TRUE., Q is an N-by-N matrix.
*          On exit, Q has been postmultiplied by the left unitary
*          transformation matrix which reorder (A, B); The leading M
*          columns of Q form orthonormal bases for the specified pair of
*          left eigenspaces (deflating subspaces).
*          If WANTQ = .FALSE., Q is not referenced.
*
*  LDQ     (input) INTEGER
*          The leading dimension of the array Q. LDQ >= 1.
*          If WANTQ = .TRUE., LDQ >= N.
*
*  Z       (input/output) COMPLEX*16 array, dimension (LDZ,N)
*          On entry, if WANTZ = .TRUE., Z is an N-by-N matrix.
*          On exit, Z has been postmultiplied by the left unitary
*          transformation matrix which reorder (A, B); The leading M
*          columns of Z form orthonormal bases for the specified pair of
*          left eigenspaces (deflating subspaces).
*          If WANTZ = .FALSE., Z is not referenced.
*
*  LDZ     (input) INTEGER
*          The leading dimension of the array Z. LDZ >= 1.
*          If WANTZ = .TRUE., LDZ >= N.
*
*  M       (output) INTEGER
*          The dimension of the specified pair of left and right
*          eigenspaces, (deflating subspaces) 0 <= M <= N.
*
*  PL      (output) DOUBLE PRECISION
*  PR      (output) DOUBLE PRECISION
*          If IJOB = 1, 4 or 5, PL, PR are lower bounds on the
*          reciprocal  of the norm of "projections" onto left and right
*          eigenspace with respect to the selected cluster.
*          0 < PL, PR <= 1.
*          If M = 0 or M = N, PL = PR  = 1.
*          If IJOB = 0, 2 or 3 PL, PR are not referenced.
*
*  DIF     (output) DOUBLE PRECISION array, dimension (2).
*          If IJOB >= 2, DIF(1:2) store the estimates of Difu and Difl.
*          If IJOB = 2 or 4, DIF(1:2) are F-norm-based upper bounds on
*          Difu and Difl. If IJOB = 3 or 5, DIF(1:2) are 1-norm-based
*          estimates of Difu and Difl, computed using reversed
*          communication with ZLACN2.
*          If M = 0 or N, DIF(1:2) = F-norm([A, B]).
*          If IJOB = 0 or 1, DIF is not referenced.
*
*  WORK    (workspace/output) COMPLEX*16 array, dimension (MAX(1,LWORK))
*          IF IJOB = 0, WORK is not referenced.  Otherwise,
*          on exit, if INFO = 0, WORK(1) returns the optimal LWORK.
*
*  LWORK   (input) INTEGER
*          The dimension of the array WORK. LWORK >=  1
*          If IJOB = 1, 2 or 4, LWORK >=  2*M*(N-M)
*          If IJOB = 3 or 5, LWORK >=  4*M*(N-M)
*
*          If LWORK = -1, then a workspace query is assumed; the routine
*          only calculates the optimal size of the WORK array, returns
*          this value as the first entry of the WORK array, and no error
*          message related to LWORK is issued by XERBLA.
*
*  IWORK   (workspace/output) INTEGER array, dimension (MAX(1,LIWORK))
*          IF IJOB = 0, IWORK is not referenced.  Otherwise,
*          on exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.
*
*  LIWORK  (input) INTEGER
*          The dimension of the array IWORK. LIWORK >= 1.
*          If IJOB = 1, 2 or 4, LIWORK >=  N+2;
*          If IJOB = 3 or 5, LIWORK >= MAX(N+2, 2*M*(N-M));
*
*          If LIWORK = -1, then a workspace query is assumed; the
*          routine only calculates the optimal size of the IWORK array,
*          returns this value as the first entry of the IWORK array, and
*          no error message related to LIWORK is issued by XERBLA.
*
*  INFO    (output) INTEGER
*            =0: Successful exit.
*            <0: If INFO = -i, the i-th argument had an illegal value.
*            =1: Reordering of (A, B) failed because the transformed
*                matrix pair (A, B) would be too far from generalized
*                Schur form; the problem is very ill-conditioned.
*                (A, B) may have been partially reordered.
*                If requested, 0 is returned in DIF(*), PL and PR.
*
*
*  Further Details
*  ===============
*
*  ZTGSEN first collects the selected eigenvalues by computing unitary
*  U and W that move them to the top left corner of (A, B). In other
*  words, the selected eigenvalues are the eigenvalues of (A11, B11) in
*
*                U'*(A, B)*W = (A11 A12) (B11 B12) n1
*                              ( 0  A22),( 0  B22) n2
*                                n1  n2    n1  n2
*
*  where N = n1+n2 and U' means the conjugate transpose of U. The first
*  n1 columns of U and W span the specified pair of left and right
*  eigenspaces (deflating subspaces) of (A, B).
*
*  If (A, B) has been obtained from the generalized real Schur
*  decomposition of a matrix pair (C, D) = Q*(A, B)*Z', then the
*  reordered generalized Schur form of (C, D) is given by
*
*           (C, D) = (Q*U)*(U'*(A, B)*W)*(Z*W)',
*
*  and the first n1 columns of Q*U and Z*W span the corresponding
*  deflating subspaces of (C, D) (Q and Z store Q*U and Z*W, resp.).
*
*  Note that if the selected eigenvalue is sufficiently ill-conditioned,
*  then its value may differ significantly from its value before
*  reordering.
*
*  The reciprocal condition numbers of the left and right eigenspaces
*  spanned by the first n1 columns of U and W (or Q*U and Z*W) may
*  be returned in DIF(1:2), corresponding to Difu and Difl, resp.
*
*  The Difu and Difl are defined as:
*
*       Difu[(A11, B11), (A22, B22)] = sigma-min( Zu )
*  and
*       Difl[(A11, B11), (A22, B22)] = Difu[(A22, B22), (A11, B11)],
*
*  where sigma-min(Zu) is the smallest singular value of the
*  (2*n1*n2)-by-(2*n1*n2) matrix
*
*       Zu = [ kron(In2, A11)  -kron(A22', In1) ]
*            [ kron(In2, B11)  -kron(B22', In1) ].
*
*  Here, Inx is the identity matrix of size nx and A22' is the
*  transpose of A22. kron(X, Y) is the Kronecker product between
*  the matrices X and Y.
*
*  When DIF(2) is small, small changes in (A, B) can cause large changes
*  in the deflating subspace. An approximate (asymptotic) bound on the
*  maximum angular error in the computed deflating subspaces is
*
*       EPS * norm((A, B)) / DIF(2),
*
*  where EPS is the machine precision.
*
*  The reciprocal norm of the projectors on the left and right
*  eigenspaces associated with (A11, B11) may be returned in PL and PR.
*  They are computed as follows. First we compute L and R so that
*  P*(A, B)*Q is block diagonal, where
*
*       P = ( I -L ) n1           Q = ( I R ) n1
*           ( 0  I ) n2    and        ( 0 I ) n2
*             n1 n2                    n1 n2
*
*  and (L, R) is the solution to the generalized Sylvester equation
*
*       A11*R - L*A22 = -A12
*       B11*R - L*B22 = -B12
*
*  Then PL = (F-norm(L)**2+1)**(-1/2) and PR = (F-norm(R)**2+1)**(-1/2).
*  An approximate (asymptotic) bound on the average absolute error of
*  the selected eigenvalues is
*
*       EPS * norm((A, B)) / PL.
*
*  There are also global error bounds which valid for perturbations up
*  to a certain restriction:  A lower bound (x) on the smallest
*  F-norm(E,F) for which an eigenvalue of (A11, B11) may move and
*  coalesce with an eigenvalue of (A22, B22) under perturbation (E,F),
*  (i.e. (A + E, B + F), is
*
*   x = min(Difu,Difl)/((1/(PL*PL)+1/(PR*PR))**(1/2)+2*max(1/PL,1/PR)).
*
*  An approximate bound on x can be computed from DIF(1:2), PL and PR.
*
*  If y = ( F-norm(E,F) / x) <= 1, the angles between the perturbed
*  (L', R') and unperturbed (L, R) left and right deflating subspaces
*  associated with the selected cluster in the (1,1)-blocks can be
*  bounded as
*
*   max-angle(L, L') <= arctan( y * PL / (1 - y * (1 - PL * PL)**(1/2))
*   max-angle(R, R') <= arctan( y * PR / (1 - y * (1 - PR * PR)**(1/2))
*
*  See LAPACK User's Guide section 4.11 or the following references
*  for more information.
*
*  Note that if the default method for computing the Frobenius-norm-
*  based estimate DIF is not wanted (see ZLATDF), then the parameter
*  IDIFJB (see below) should be changed from 3 to 4 (routine ZLATDF
*  (IJOB = 2 will be used)). See ZTGSYL for more details.
*
*  Based on contributions by
*     Bo Kagstrom and Peter Poromaa, Department of Computing Science,
*     Umea University, S-901 87 Umea, Sweden.
*
*  References
*  ==========
*
*  [1] B. Kagstrom; A Direct Method for Reordering Eigenvalues in the
*      Generalized Real Schur Form of a Regular Matrix Pair (A, B), in
*      M.S. Moonen et al (eds), Linear Algebra for Large Scale and
*      Real-Time Applications, Kluwer Academic Publ. 1993, pp 195-218.
*
*  [2] B. Kagstrom and P. Poromaa; Computing Eigenspaces with Specified
*      Eigenvalues of a Regular Matrix Pair (A, B) and Condition
*      Estimation: Theory, Algorithms and Software, Report
*      UMINF - 94.04, Department of Computing Science, Umea University,
*      S-901 87 Umea, Sweden, 1994. Also as LAPACK Working Note 87.
*      To appear in Numerical Algorithms, 1996.
*
*  [3] B. Kagstrom and P. Poromaa, LAPACK-Style Algorithms and Software
*      for Solving the Generalized Sylvester Equation and Estimating the
*      Separation between Regular Matrix Pairs, Report UMINF - 93.23,
*      Department of Computing Science, Umea University, S-901 87 Umea,
*      Sweden, December 1993, Revised April 1994, Also as LAPACK working
*      Note 75. To appear in ACM Trans. on Math. Software, Vol 22, No 1,
*      1996.
*
*  =====================================================================
*
*     .. Parameters ..
      INTEGER            IDIFJB
      PARAMETER          ( IDIFJB = 3 )
      DOUBLE PRECISION   ZERO, ONE
      PARAMETER          ( ZERO = 0.0D+0, ONE = 1.0D+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            LQUERY, SWAP, WANTD, WANTD1, WANTD2, WANTP
      INTEGER            I, IERR, IJB, K, KASE, KS, LIWMIN, LWMIN, MN2,
     $                   N1, N2
      DOUBLE PRECISION   DSCALE, DSUM, RDSCAL, SAFMIN
*     ..
*     .. Local Arrays ..
      INTEGER            ISAVE( 3 )
*     ..
*     .. External Subroutines ..
      EXTERNAL           XERBLA, ZLACN2, ZLACPY, ZLASSQ, ZSCAL, ZTGEXC,
     $                   ZTGSYL
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          ABS, DCMPLX, DCONJG, MAX, SQRT
*     ..
*     .. External Functions ..
      DOUBLE PRECISION   DLAMCH
      EXTERNAL           DLAMCH
*     ..
*     .. Executable Statements ..
*
*     Decode and test the input parameters
*
      INFO = 0
      LQUERY = ( LWORK.EQ.-1 .OR. LIWORK.EQ.-1 )
*
      IF( IJOB.LT.0 .OR. IJOB.GT.5 ) THEN
         INFO = -1
      ELSE IF( N.LT.0 ) THEN
         INFO = -5
      ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
         INFO = -7
      ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
         INFO = -9
      ELSE IF( LDQ.LT.1 .OR. ( WANTQ .AND. LDQ.LT.N ) ) THEN
         INFO = -13
      ELSE IF( LDZ.LT.1 .OR. ( WANTZ .AND. LDZ.LT.N ) ) THEN
         INFO = -15
      END IF
*
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'ZTGSEN', -INFO )
         RETURN
      END IF
*
      IERR = 0
*
      WANTP = IJOB.EQ.1 .OR. IJOB.GE.4
      WANTD1 = IJOB.EQ.2 .OR. IJOB.EQ.4
      WANTD2 = IJOB.EQ.3 .OR. IJOB.EQ.5
      WANTD = WANTD1 .OR. WANTD2
*
*     Set M to the dimension of the specified pair of deflating
*     subspaces.
*
      M = 0
      DO 10 K = 1, N
         ALPHA( K ) = A( K, K )
         BETA( K ) = B( K, K )
         IF( K.LT.N ) THEN
            IF( SELECT( K ) )
     $         M = M + 1
         ELSE
            IF( SELECT( N ) )
     $         M = M + 1
         END IF
   10 CONTINUE
*
      IF( IJOB.EQ.1 .OR. IJOB.EQ.2 .OR. IJOB.EQ.4 ) THEN
         LWMIN = MAX( 1, 2*M*( N-M ) )
         LIWMIN = MAX( 1, N+2 )
      ELSE IF( IJOB.EQ.3 .OR. IJOB.EQ.5 ) THEN
         LWMIN = MAX( 1, 4*M*( N-M ) )
         LIWMIN = MAX( 1, 2*M*( N-M ), N+2 )
      ELSE
         LWMIN = 1
         LIWMIN = 1
      END IF
*
      WORK( 1 ) = LWMIN
      IWORK( 1 ) = LIWMIN
*
      IF( LWORK.LT.LWMIN .AND. .NOT.LQUERY ) THEN
         INFO = -21
      ELSE IF( LIWORK.LT.LIWMIN .AND. .NOT.LQUERY ) THEN
         INFO = -23
      END IF
*
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'ZTGSEN', -INFO )
         RETURN
      ELSE IF( LQUERY ) THEN
         RETURN
      END IF
*
*     Quick return if possible.
*
      IF( M.EQ.N .OR. M.EQ.0 ) THEN
         IF( WANTP ) THEN
            PL = ONE
            PR = ONE
         END IF
         IF( WANTD ) THEN
            DSCALE = ZERO
            DSUM = ONE
            DO 20 I = 1, N
               CALL ZLASSQ( N, A( 1, I ), 1, DSCALE, DSUM )
               CALL ZLASSQ( N, B( 1, I ), 1, DSCALE, DSUM )
   20       CONTINUE
            DIF( 1 ) = DSCALE*SQRT( DSUM )
            DIF( 2 ) = DIF( 1 )
         END IF
         GO TO 70
      END IF
*
*     Get machine constant
*
      SAFMIN = DLAMCH( 'S' )
*
*     Collect the selected blocks at the top-left corner of (A, B).
*
      KS = 0
      DO 30 K = 1, N
         SWAP = SELECT( K )
         IF( SWAP ) THEN
            KS = KS + 1
*
*           Swap the K-th block to position KS. Compute unitary Q
*           and Z that will swap adjacent diagonal blocks in (A, B).
*
            IF( K.NE.KS )
     $         CALL ZTGEXC( WANTQ, WANTZ, N, A, LDA, B, LDB, Q, LDQ, Z,
     $                      LDZ, K, KS, IERR )
*
            IF( IERR.GT.0 ) THEN
*
*              Swap is rejected: exit.
*
               INFO = 1
               IF( WANTP ) THEN
                  PL = ZERO
                  PR = ZERO
               END IF
               IF( WANTD ) THEN
                  DIF( 1 ) = ZERO
                  DIF( 2 ) = ZERO
               END IF
               GO TO 70
            END IF
         END IF
   30 CONTINUE
      IF( WANTP ) THEN
*
*        Solve generalized Sylvester equation for R and L:
*                   A11 * R - L * A22 = A12
*                   B11 * R - L * B22 = B12
*
         N1 = M
         N2 = N - M
         I = N1 + 1
         CALL ZLACPY( 'Full', N1, N2, A( 1, I ), LDA, WORK, N1 )
         CALL ZLACPY( 'Full', N1, N2, B( 1, I ), LDB, WORK( N1*N2+1 ),
     $                N1 )
         IJB = 0
         CALL ZTGSYL( 'N', IJB, N1, N2, A, LDA, A( I, I ), LDA, WORK,
     $                N1, B, LDB, B( I, I ), LDB, WORK( N1*N2+1 ), N1,
     $                DSCALE, DIF( 1 ), WORK( N1*N2*2+1 ),
     $                LWORK-2*N1*N2, IWORK, IERR )
*
*        Estimate the reciprocal of norms of "projections" onto
*        left and right eigenspaces
*
         RDSCAL = ZERO
         DSUM = ONE
         CALL ZLASSQ( N1*N2, WORK, 1, RDSCAL, DSUM )
         PL = RDSCAL*SQRT( DSUM )
         IF( PL.EQ.ZERO ) THEN
            PL = ONE
         ELSE
            PL = DSCALE / ( SQRT( DSCALE*DSCALE / PL+PL )*SQRT( PL ) )
         END IF
         RDSCAL = ZERO
         DSUM = ONE
         CALL ZLASSQ( N1*N2, WORK( N1*N2+1 ), 1, RDSCAL, DSUM )
         PR = RDSCAL*SQRT( DSUM )
         IF( PR.EQ.ZERO ) THEN
            PR = ONE
         ELSE
            PR = DSCALE / ( SQRT( DSCALE*DSCALE / PR+PR )*SQRT( PR ) )
         END IF
      END IF
      IF( WANTD ) THEN
*
*        Compute estimates Difu and Difl.
*
         IF( WANTD1 ) THEN
            N1 = M
            N2 = N - M
            I = N1 + 1
            IJB = IDIFJB
*
*           Frobenius norm-based Difu estimate.
*
            CALL ZTGSYL( 'N', IJB, N1, N2, A, LDA, A( I, I ), LDA, WORK,
     $                   N1, B, LDB, B( I, I ), LDB, WORK( N1*N2+1 ),
     $                   N1, DSCALE, DIF( 1 ), WORK( N1*N2*2+1 ),
     $                   LWORK-2*N1*N2, IWORK, IERR )
*
*           Frobenius norm-based Difl estimate.
*
            CALL ZTGSYL( 'N', IJB, N2, N1, A( I, I ), LDA, A, LDA, WORK,
     $                   N2, B( I, I ), LDB, B, LDB, WORK( N1*N2+1 ),
     $                   N2, DSCALE, DIF( 2 ), WORK( N1*N2*2+1 ),
     $                   LWORK-2*N1*N2, IWORK, IERR )
         ELSE
*
*           Compute 1-norm-based estimates of Difu and Difl using
*           reversed communication with ZLACN2. In each step a
*           generalized Sylvester equation or a transposed variant
*           is solved.
*
            KASE = 0
            N1 = M
            N2 = N - M
            I = N1 + 1
            IJB = 0
            MN2 = 2*N1*N2
*
*           1-norm-based estimate of Difu.
*
   40       CONTINUE
            CALL ZLACN2( MN2, WORK( MN2+1 ), WORK, DIF( 1 ), KASE,
     $                   ISAVE )
            IF( KASE.NE.0 ) THEN
               IF( KASE.EQ.1 ) THEN
*
*                 Solve generalized Sylvester equation
*
                  CALL ZTGSYL( 'N', IJB, N1, N2, A, LDA, A( I, I ), LDA,
     $                         WORK, N1, B, LDB, B( I, I ), LDB,
     $                         WORK( N1*N2+1 ), N1, DSCALE, DIF( 1 ),
     $                         WORK( N1*N2*2+1 ), LWORK-2*N1*N2, IWORK,
     $                         IERR )
               ELSE
*
*                 Solve the transposed variant.
*
                  CALL ZTGSYL( 'C', IJB, N1, N2, A, LDA, A( I, I ), LDA,
     $                         WORK, N1, B, LDB, B( I, I ), LDB,
     $                         WORK( N1*N2+1 ), N1, DSCALE, DIF( 1 ),
     $                         WORK( N1*N2*2+1 ), LWORK-2*N1*N2, IWORK,
     $                         IERR )
               END IF
               GO TO 40
            END IF
            DIF( 1 ) = DSCALE / DIF( 1 )
*
*           1-norm-based estimate of Difl.
*
   50       CONTINUE
            CALL ZLACN2( MN2, WORK( MN2+1 ), WORK, DIF( 2 ), KASE,
     $                   ISAVE )
            IF( KASE.NE.0 ) THEN
               IF( KASE.EQ.1 ) THEN
*
*                 Solve generalized Sylvester equation
*
                  CALL ZTGSYL( 'N', IJB, N2, N1, A( I, I ), LDA, A, LDA,
     $                         WORK, N2, B( I, I ), LDB, B, LDB,
     $                         WORK( N1*N2+1 ), N2, DSCALE, DIF( 2 ),
     $                         WORK( N1*N2*2+1 ), LWORK-2*N1*N2, IWORK,
     $                         IERR )
               ELSE
*
*                 Solve the transposed variant.
*
                  CALL ZTGSYL( 'C', IJB, N2, N1, A( I, I ), LDA, A, LDA,
     $                         WORK, N2, B, LDB, B( I, I ), LDB,
     $                         WORK( N1*N2+1 ), N2, DSCALE, DIF( 2 ),
     $                         WORK( N1*N2*2+1 ), LWORK-2*N1*N2, IWORK,
     $                         IERR )
               END IF
               GO TO 50
            END IF
            DIF( 2 ) = DSCALE / DIF( 2 )
         END IF
      END IF
*
*     If B(K,K) is complex, make it real and positive (normalization
*     of the generalized Schur form) and Store the generalized
*     eigenvalues of reordered pair (A, B)
*
      DO 60 K = 1, N
         DSCALE = ABS( B( K, K ) )
         IF( DSCALE.GT.SAFMIN ) THEN
            WORK( 1 ) = DCONJG( B( K, K ) / DSCALE )
            WORK( 2 ) = B( K, K ) / DSCALE
            B( K, K ) = DSCALE
            CALL ZSCAL( N-K, WORK( 1 ), B( K, K+1 ), LDB )
            CALL ZSCAL( N-K+1, WORK( 1 ), A( K, K ), LDA )
            IF( WANTQ )
     $         CALL ZSCAL( N, WORK( 2 ), Q( 1, K ), 1 )
         ELSE
            B( K, K ) = DCMPLX( ZERO, ZERO )
         END IF
*
         ALPHA( K ) = A( K, K )
         BETA( K ) = B( K, K )
*
   60 CONTINUE
*
   70 CONTINUE
*
      WORK( 1 ) = LWMIN
      IWORK( 1 ) = LIWMIN
*
      RETURN
*
*     End of ZTGSEN
*
      END