stgsna.f

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*> \brief \b STGSNA
*
*  =========== DOCUMENTATION ===========
*
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*
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*
*  Definition:
*  ===========
*
*       SUBROUTINE STGSNA( JOB, HOWMNY, SELECT, N, A, LDA, B, LDB, VL,
*                          LDVL, VR, LDVR, S, DIF, MM, M, WORK, LWORK,
*                          IWORK, INFO )
*
*       .. Scalar Arguments ..
*       CHARACTER          HOWMNY, JOB
*       INTEGER            INFO, LDA, LDB, LDVL, LDVR, LWORK, M, MM, N
*       ..
*       .. Array Arguments ..
*       LOGICAL            SELECT( * )
*       INTEGER            IWORK( * )
*       REAL               A( LDA, * ), B( LDB, * ), DIF( * ), S( * ),
*      $                   VL( LDVL, * ), VR( LDVR, * ), WORK( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> STGSNA estimates reciprocal condition numbers for specified
*> eigenvalues and/or eigenvectors of a matrix pair (A, B) in
*> generalized real Schur canonical form (or of any matrix pair
*> (Q*A*Z**T, Q*B*Z**T) with orthogonal matrices Q and Z, where
*> Z**T denotes the transpose of Z.
*>
*> (A, B) must be in generalized real Schur form (as returned by SGGES),
*> i.e. A is block upper triangular with 1-by-1 and 2-by-2 diagonal
*> blocks. B is upper triangular.
*>
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] JOB
*> \verbatim
*>          JOB is CHARACTER*1
*>          Specifies whether condition numbers are required for
*>          eigenvalues (S) or eigenvectors (DIF):
*>          = 'E': for eigenvalues only (S);
*>          = 'V': for eigenvectors only (DIF);
*>          = 'B': for both eigenvalues and eigenvectors (S and DIF).
*> \endverbatim
*>
*> \param[in] HOWMNY
*> \verbatim
*>          HOWMNY is CHARACTER*1
*>          = 'A': compute condition numbers for all eigenpairs;
*>          = 'S': compute condition numbers for selected eigenpairs
*>                 specified by the array SELECT.
*> \endverbatim
*>
*> \param[in] SELECT
*> \verbatim
*>          SELECT is LOGICAL array, dimension (N)
*>          If HOWMNY = 'S', SELECT specifies the eigenpairs for which
*>          condition numbers are required. To select condition numbers
*>          for the eigenpair corresponding to a real eigenvalue w(j),
*>          SELECT(j) must be set to .TRUE.. To select condition numbers
*>          corresponding to a complex conjugate pair of eigenvalues w(j)
*>          and w(j+1), either SELECT(j) or SELECT(j+1) or both, must be
*>          set to .TRUE..
*>          If HOWMNY = 'A', SELECT is not referenced.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The order of the square matrix pair (A, B). N >= 0.
*> \endverbatim
*>
*> \param[in] A
*> \verbatim
*>          A is REAL array, dimension (LDA,N)
*>          The upper quasi-triangular matrix A in the pair (A,B).
*> \endverbatim
*>
*> \param[in] LDA
*> \verbatim
*>          LDA is INTEGER
*>          The leading dimension of the array A. LDA >= max(1,N).
*> \endverbatim
*>
*> \param[in] B
*> \verbatim
*>          B is REAL array, dimension (LDB,N)
*>          The upper triangular matrix B in the pair (A,B).
*> \endverbatim
*>
*> \param[in] LDB
*> \verbatim
*>          LDB is INTEGER
*>          The leading dimension of the array B. LDB >= max(1,N).
*> \endverbatim
*>
*> \param[in] VL
*> \verbatim
*>          VL is REAL array, dimension (LDVL,M)
*>          If JOB = 'E' or 'B', VL must contain left eigenvectors of
*>          (A, B), corresponding to the eigenpairs specified by HOWMNY
*>          and SELECT. The eigenvectors must be stored in consecutive
*>          columns of VL, as returned by STGEVC.
*>          If JOB = 'V', VL is not referenced.
*> \endverbatim
*>
*> \param[in] LDVL
*> \verbatim
*>          LDVL is INTEGER
*>          The leading dimension of the array VL. LDVL >= 1.
*>          If JOB = 'E' or 'B', LDVL >= N.
*> \endverbatim
*>
*> \param[in] VR
*> \verbatim
*>          VR is REAL array, dimension (LDVR,M)
*>          If JOB = 'E' or 'B', VR must contain right eigenvectors of
*>          (A, B), corresponding to the eigenpairs specified by HOWMNY
*>          and SELECT. The eigenvectors must be stored in consecutive
*>          columns ov VR, as returned by STGEVC.
*>          If JOB = 'V', VR is not referenced.
*> \endverbatim
*>
*> \param[in] LDVR
*> \verbatim
*>          LDVR is INTEGER
*>          The leading dimension of the array VR. LDVR >= 1.
*>          If JOB = 'E' or 'B', LDVR >= N.
*> \endverbatim
*>
*> \param[out] S
*> \verbatim
*>          S is REAL array, dimension (MM)
*>          If JOB = 'E' or 'B', the reciprocal condition numbers of the
*>          selected eigenvalues, stored in consecutive elements of the
*>          array. For a complex conjugate pair of eigenvalues two
*>          consecutive elements of S are set to the same value. Thus
*>          S(j), DIF(j), and the j-th columns of VL and VR all
*>          correspond to the same eigenpair (but not in general the
*>          j-th eigenpair, unless all eigenpairs are selected).
*>          If JOB = 'V', S is not referenced.
*> \endverbatim
*>
*> \param[out] DIF
*> \verbatim
*>          DIF is REAL array, dimension (MM)
*>          If JOB = 'V' or 'B', the estimated reciprocal condition
*>          numbers of the selected eigenvectors, stored in consecutive
*>          elements of the array. For a complex eigenvector two
*>          consecutive elements of DIF are set to the same value. If
*>          the eigenvalues cannot be reordered to compute DIF(j), DIF(j)
*>          is set to 0; this can only occur when the true value would be
*>          very small anyway.
*>          If JOB = 'E', DIF is not referenced.
*> \endverbatim
*>
*> \param[in] MM
*> \verbatim
*>          MM is INTEGER
*>          The number of elements in the arrays S and DIF. MM >= M.
*> \endverbatim
*>
*> \param[out] M
*> \verbatim
*>          M is INTEGER
*>          The number of elements of the arrays S and DIF used to store
*>          the specified condition numbers; for each selected real
*>          eigenvalue one element is used, and for each selected complex
*>          conjugate pair of eigenvalues, two elements are used.
*>          If HOWMNY = 'A', M is set to N.
*> \endverbatim
*>
*> \param[out] WORK
*> \verbatim
*>          WORK is REAL array, dimension (MAX(1,LWORK))
*>          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
*> \endverbatim
*>
*> \param[in] LWORK
*> \verbatim
*>          LWORK is INTEGER
*>          The dimension of the array WORK. LWORK >= max(1,N).
*>          If JOB = 'V' or 'B' LWORK >= 2*N*(N+2)+16.
*>
*>          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.
*> \endverbatim
*>
*> \param[out] IWORK
*> \verbatim
*>          IWORK is INTEGER array, dimension (N + 6)
*>          If JOB = 'E', IWORK is not referenced.
*> \endverbatim
*>
*> \param[out] INFO
*> \verbatim
*>          INFO is INTEGER
*>          =0: Successful exit
*>          <0: If INFO = -i, the i-th argument had an illegal value
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup realOTHERcomputational
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>  The reciprocal of the condition number of a generalized eigenvalue
*>  w = (a, b) is defined as
*>
*>       S(w) = (|u**TAv|**2 + |u**TBv|**2)**(1/2) / (norm(u)*norm(v))
*>
*>  where u and v are the left and right eigenvectors of (A, B)
*>  corresponding to w; |z| denotes the absolute value of the complex
*>  number, and norm(u) denotes the 2-norm of the vector u.
*>  The pair (a, b) corresponds to an eigenvalue w = a/b (= u**TAv/u**TBv)
*>  of the matrix pair (A, B). If both a and b equal zero, then (A B) is
*>  singular and S(I) = -1 is returned.
*>
*>  An approximate error bound on the chordal distance between the i-th
*>  computed generalized eigenvalue w and the corresponding exact
*>  eigenvalue lambda is
*>
*>       chord(w, lambda) <= EPS * norm(A, B) / S(I)
*>
*>  where EPS is the machine precision.
*>
*>  The reciprocal of the condition number DIF(i) of right eigenvector u
*>  and left eigenvector v corresponding to the generalized eigenvalue w
*>  is defined as follows:
*>
*>  a) If the i-th eigenvalue w = (a,b) is real
*>
*>     Suppose U and V are orthogonal transformations such that
*>
*>              U**T*(A, B)*V  = (S, T) = ( a   *  ) ( b  *  )  1
*>                                        ( 0  S22 ),( 0 T22 )  n-1
*>                                          1  n-1     1 n-1
*>
*>     Then the reciprocal condition number DIF(i) is
*>
*>                Difl((a, b), (S22, T22)) = sigma-min( Zl ),
*>
*>     where sigma-min(Zl) denotes the smallest singular value of the
*>     2(n-1)-by-2(n-1) matrix
*>
*>         Zl = [ kron(a, In-1)  -kron(1, S22) ]
*>              [ kron(b, In-1)  -kron(1, T22) ] .
*>
*>     Here In-1 is the identity matrix of size n-1. kron(X, Y) is the
*>     Kronecker product between the matrices X and Y.
*>
*>     Note that if the default method for computing DIF(i) is wanted
*>     (see SLATDF), then the parameter DIFDRI (see below) should be
*>     changed from 3 to 4 (routine SLATDF(IJOB = 2 will be used)).
*>     See STGSYL for more details.
*>
*>  b) If the i-th and (i+1)-th eigenvalues are complex conjugate pair,
*>
*>     Suppose U and V are orthogonal transformations such that
*>
*>              U**T*(A, B)*V = (S, T) = ( S11  *   ) ( T11  *  )  2
*>                                       ( 0    S22 ),( 0    T22) n-2
*>                                         2    n-2     2    n-2
*>
*>     and (S11, T11) corresponds to the complex conjugate eigenvalue
*>     pair (w, conjg(w)). There exist unitary matrices U1 and V1 such
*>     that
*>
*>       U1**T*S11*V1 = ( s11 s12 ) and U1**T*T11*V1 = ( t11 t12 )
*>                      (  0  s22 )                    (  0  t22 )
*>
*>     where the generalized eigenvalues w = s11/t11 and
*>     conjg(w) = s22/t22.
*>
*>     Then the reciprocal condition number DIF(i) is bounded by
*>
*>         min( d1, max( 1, |real(s11)/real(s22)| )*d2 )
*>
*>     where, d1 = Difl((s11, t11), (s22, t22)) = sigma-min(Z1), where
*>     Z1 is the complex 2-by-2 matrix
*>
*>              Z1 =  [ s11  -s22 ]
*>                    [ t11  -t22 ],
*>
*>     This is done by computing (using real arithmetic) the
*>     roots of the characteristical polynomial det(Z1**T * Z1 - lambda I),
*>     where Z1**T denotes the transpose of Z1 and det(X) denotes
*>     the determinant of X.
*>
*>     and d2 is an upper bound on Difl((S11, T11), (S22, T22)), i.e. an
*>     upper bound on sigma-min(Z2), where Z2 is (2n-2)-by-(2n-2)
*>
*>              Z2 = [ kron(S11**T, In-2)  -kron(I2, S22) ]
*>                   [ kron(T11**T, In-2)  -kron(I2, T22) ]
*>
*>     Note that if the default method for computing DIF is wanted (see
*>     SLATDF), then the parameter DIFDRI (see below) should be changed
*>     from 3 to 4 (routine SLATDF(IJOB = 2 will be used)). See STGSYL
*>     for more details.
*>
*>  For each eigenvalue/vector specified by SELECT, DIF stores a
*>  Frobenius norm-based estimate of Difl.
*>
*>  An approximate error bound for the i-th computed eigenvector VL(i) or
*>  VR(i) is given by
*>
*>             EPS * norm(A, B) / DIF(i).
*>
*>  See ref. [2-3] for more details and further references.
*> \endverbatim
*
*> \par Contributors:
*  ==================
*>
*>     Bo Kagstrom and Peter Poromaa, Department of Computing Science,
*>     Umea University, S-901 87 Umea, Sweden.
*
*> \par References:
*  ================
*>
*> \verbatim
*>
*>  [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.
*> \endverbatim
*>
*  =====================================================================
      SUBROUTINE stgsna( JOB, HOWMNY, SELECT, N, A, LDA, B, LDB, VL,
     $                   LDVL, VR, LDVR, S, DIF, MM, M, WORK, LWORK,
     $                   IWORK, INFO )
*
*  -- LAPACK computational routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      CHARACTER          HOWMNY, JOB
      INTEGER            INFO, LDA, LDB, LDVL, LDVR, LWORK, M, MM, N
*     ..
*     .. Array Arguments ..
      LOGICAL            SELECT( * )
      INTEGER            IWORK( * )
      REAL               A( LDA, * ), B( LDB, * ), DIF( * ), S( * ),
     $                   vl( ldvl, * ), vr( ldvr, * ), work( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      INTEGER            DIFDRI
      PARAMETER          ( DIFDRI = 3 )
      REAL               ZERO, ONE, TWO, FOUR
      parameter( zero = 0.0e+0, one = 1.0e+0, two = 2.0e+0,
     $                   four = 4.0e+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            LQUERY, PAIR, SOMCON, WANTBH, WANTDF, WANTS
      INTEGER            I, IERR, IFST, ILST, IZ, K, KS, LWMIN, N1, N2
      REAL               ALPHAI, ALPHAR, ALPRQT, BETA, C1, C2, COND,
     $                   eps, lnrm, rnrm, root1, root2, scale, smlnum,
     $                   tmpii, tmpir, tmpri, tmprr, uhav, uhavi, uhbv,
     $                   uhbvi
*     ..
*     .. Local Arrays ..
      REAL               DUMMY( 1 ), DUMMY1( 1 )
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      REAL               SDOT, SLAMCH, SLAPY2, SNRM2
      EXTERNAL           lsame, sdot, slamch, slapy2, snrm2
*     ..
*     .. External Subroutines ..
      EXTERNAL           sgemv, slacpy, slag2, stgexc, stgsyl, xerbla
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          max, min, sqrt
*     ..
*     .. Executable Statements ..
*
*     Decode and test the input parameters
*
      wantbh = lsame( job, 'B' )
      wants = lsame( job, 'E' ) .OR. wantbh
      wantdf = lsame( job, 'V' ) .OR. wantbh
*
      somcon = lsame( howmny, 'S' )
*
      info = 0
      lquery = ( lwork.EQ.-1 )
*
      IF( .NOT.wants .AND. .NOT.wantdf ) THEN
         info = -1
      ELSE IF( .NOT.lsame( howmny, 'A' ) .AND. .NOT.somcon ) THEN
         info = -2
      ELSE IF( n.LT.0 ) THEN
         info = -4
      ELSE IF( lda.LT.max( 1, n ) ) THEN
         info = -6
      ELSE IF( ldb.LT.max( 1, n ) ) THEN
         info = -8
      ELSE IF( wants .AND. ldvl.LT.n ) THEN
         info = -10
      ELSE IF( wants .AND. ldvr.LT.n ) THEN
         info = -12
      ELSE
*
*        Set M to the number of eigenpairs for which condition numbers
*        are required, and test MM.
*
         IF( somcon ) THEN
            m = 0
            pair = .false.
            DO 10 k = 1, n
               IF( pair ) THEN
                  pair = .false.
               ELSE
                  IF( k.LT.n ) THEN
                     IF( a( k+1, k ).EQ.zero ) THEN
                        IF( SELECT( k ) )
     $                     m = m + 1
                     ELSE
                        pair = .true.
                        IF( SELECT( k ) .OR. SELECT( k+1 ) )
     $                     m = m + 2
                     END IF
                  ELSE
                     IF( SELECT( n ) )
     $                  m = m + 1
                  END IF
               END IF
   10       CONTINUE
         ELSE
            m = n
         END IF
*
         IF( n.EQ.0 ) THEN
            lwmin = 1
         ELSE IF( lsame( job, 'V' ) .OR. lsame( job, 'B' ) ) THEN
            lwmin = 2*n*( n + 2 ) + 16
         ELSE
            lwmin = n
         END IF
         work( 1 ) = lwmin
*
         IF( mm.LT.m ) THEN
            info = -15
         ELSE IF( lwork.LT.lwmin .AND. .NOT.lquery ) THEN
            info = -18
         END IF
      END IF
*
      IF( info.NE.0 ) THEN
         CALL xerbla( 'STGSNA', -info )
         RETURN
      ELSE IF( lquery ) THEN
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( n.EQ.0 )
     $   RETURN
*
*     Get machine constants
*
      eps = slamch( 'p' )
      SMLNUM = SLAMCH( 's' ) / EPS
      KS = 0
      PAIR = .FALSE.
*
      DO 20 K = 1, N
*
*        Determine whether A(k,k) begins a 1-by-1 or 2-by-2 block.
*
         IF( PAIR ) THEN
            PAIR = .FALSE.
            GO TO 20
         ELSE
.LT.            IF( KN )
.NE.     $         PAIR = A( K+1, K )ZERO
         END IF
*
*        Determine whether condition numbers are required for the k-th
*        eigenpair.
*
         IF( SOMCON ) THEN
            IF( PAIR ) THEN
.NOT..AND..NOT.               IF( SELECT( K )  SELECT( K+1 ) )
     $            GO TO 20
            ELSE
.NOT.               IF( SELECT( K ) )
     $            GO TO 20
            END IF
         END IF
*
         KS = KS + 1
*
         IF( WANTS ) THEN
*
*           Compute the reciprocal condition number of the k-th
*           eigenvalue.
*
            IF( PAIR ) THEN
*
*              Complex eigenvalue pair.
*
               RNRM = SLAPY2( SNRM2( N, VR( 1, KS ), 1 ),
     $                SNRM2( N, VR( 1, KS+1 ), 1 ) )
               LNRM = SLAPY2( SNRM2( N, VL( 1, KS ), 1 ),
     $                SNRM2( N, VL( 1, KS+1 ), 1 ) )
               CALL SGEMV( 'n', N, N, ONE, A, LDA, VR( 1, KS ), 1, ZERO,
     $                     WORK, 1 )
               TMPRR = SDOT( N, WORK, 1, VL( 1, KS ), 1 )
               TMPRI = SDOT( N, WORK, 1, VL( 1, KS+1 ), 1 )
               CALL SGEMV( 'n', N, N, ONE, A, LDA, VR( 1, KS+1 ), 1,
     $                     ZERO, WORK, 1 )
               TMPII = SDOT( N, WORK, 1, VL( 1, KS+1 ), 1 )
               TMPIR = SDOT( N, WORK, 1, VL( 1, KS ), 1 )
               UHAV = TMPRR + TMPII
               UHAVI = TMPIR - TMPRI
               CALL SGEMV( 'n', N, N, ONE, B, LDB, VR( 1, KS ), 1, ZERO,
     $                     WORK, 1 )
               TMPRR = SDOT( N, WORK, 1, VL( 1, KS ), 1 )
               TMPRI = SDOT( N, WORK, 1, VL( 1, KS+1 ), 1 )
               CALL SGEMV( 'n', N, N, ONE, B, LDB, VR( 1, KS+1 ), 1,
     $                     ZERO, WORK, 1 )
               TMPII = SDOT( N, WORK, 1, VL( 1, KS+1 ), 1 )
               TMPIR = SDOT( N, WORK, 1, VL( 1, KS ), 1 )
               UHBV = TMPRR + TMPII
               UHBVI = TMPIR - TMPRI
               UHAV = SLAPY2( UHAV, UHAVI )
               UHBV = SLAPY2( UHBV, UHBVI )
               COND = SLAPY2( UHAV, UHBV )
               S( KS ) = COND / ( RNRM*LNRM )
               S( KS+1 ) = S( KS )
*
            ELSE
*
*              Real eigenvalue.
*
               RNRM = SNRM2( N, VR( 1, KS ), 1 )
               LNRM = SNRM2( N, VL( 1, KS ), 1 )
               CALL SGEMV( 'n', n, n, one, a, lda, vr( 1, ks ), 1, zero,
     $                     work, 1 )
               uhav = sdot( n, work, 1, vl( 1, ks ), 1 )
               CALL sgemv( 'N', n, n, one, b, ldb, vr( 1, ks ), 1, zero,
     $                     work, 1 )
               uhbv = sdot( n, work, 1, vl( 1, ks ), 1 )
               cond = slapy2( uhav, uhbv )
               IF( cond.EQ.zero ) THEN
                  s( ks ) = -one
               ELSE
                  s( ks ) = cond / ( rnrm*lnrm )
               END IF
            END IF
         END IF
*
         IF( wantdf ) THEN
            IF( n.EQ.1 ) THEN
               dif( ks ) = slapy2( a( 1, 1 ), b( 1, 1 ) )
               GO TO 20
            END IF
*
*           Estimate the reciprocal condition number of the k-th
*           eigenvectors.
            IF( pair ) THEN
*
*              Copy the  2-by 2 pencil beginning at (A(k,k), B(k, k)).
*              Compute the eigenvalue(s) at position K.
*
               work( 1 ) = a( k, k )
               work( 2 ) = a( k+1, k )
               work( 3 ) = a( k, k+1 )
               work( 4 ) = a( k+1, k+1 )
               work( 5 ) = b( k, k )
               work( 6 ) = b( k+1, k )
               work( 7 ) = b( k, k+1 )
               work( 8 ) = b( k+1, k+1 )
               CALL slag2( work, 2, work( 5 ), 2, smlnum*eps, beta,
     $                     dummy1( 1 ), alphar, dummy( 1 ), alphai )
               alprqt = one
               c1 = two*( alphar*alphar+alphai*alphai+beta*beta )
               c2 = four*beta*beta*alphai*alphai
               root1 = c1 + sqrt( c1*c1-4.0*c2 )
               root2 = c2 / root1
               root1 = root1 / two
               cond = min( sqrt( root1 ), sqrt( root2 ) )
            END IF
*
*           Copy the matrix (A, B) to the array WORK and swap the
*           diagonal block beginning at A(k,k) to the (1,1) position.
*
            CALL slacpy( 'Full', n, n, a, lda, work, n )
            CALL slacpy( 'Full', n, n, b, ldb, work( n*n+1 ), n )
            ifst = k
            ilst = 1
*
            CALL stgexc( .false., .false., n, work, n, work( n*n+1 ), n,
     $                   dummy, 1, dummy1, 1, ifst, ilst,
     $                   work( n*n*2+1 ), lwork-2*n*n, ierr )
*
            IF( ierr.GT.0 ) THEN
*
*              Ill-conditioned problem - swap rejected.
*
               dif( ks ) = zero
            ELSE
*
*              Reordering successful, solve generalized Sylvester
*              equation for R and L,
*                         A22 * R - L * A11 = A12
*                         B22 * R - L * B11 = B12,
*              and compute estimate of Difl((A11,B11), (A22, B22)).
*
               n1 = 1
               IF( work( 2 ).NE.zero )
     $            n1 = 2
               n2 = n - n1
               IF( n2.EQ.0 ) THEN
                  dif( ks ) = cond
               ELSE
                  i = n*n + 1
                  iz = 2*n*n + 1
                  CALL stgsyl( 'N', difdri, n2, n1, work( n*n1+n1+1 ),
     $                         n, work, n, work( n1+1 ), n,
     $                         work( n*n1+n1+i ), n, work( i ), n,
     $                         work( n1+i ), n, scale, dif( ks ),
     $                         work( iz+1 ), lwork-2*n*n, iwork, ierr )
*
                  IF( pair )
     $               dif( ks ) = min( max( one, alprqt )*dif( ks ),
     $                           cond )
               END IF
            END IF
            IF( pair )
     $         dif( ks+1 ) = dif( ks )
         END IF
         IF( pair )
     $      ks = ks + 1
*
   20 CONTINUE
      work( 1 ) = lwmin
      RETURN
*
*     End of STGSNA
*
      END