          SUBROUTINE SLAHQR( WANTT, WANTZ, N, ILO, IHI, H, LDH, WR, WI,
         $                   ILOZ, IHIZ, Z, LDZ, INFO )
    *
    *  -- LAPACK auxiliary routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          INTEGER            IHI, IHIZ, ILO, ILOZ, INFO, LDH, LDZ, N
          LOGICAL            WANTT, WANTZ
    *     ..
    *     .. Array Arguments ..
          REAL               H( LDH, * ), WI( * ), WR( * ), Z( LDZ, * )
    *     ..
    *
    *     Purpose
    *     =======
    *
    *     SLAHQR is an auxiliary routine called by SHSEQR to update the
    *     eigenvalues and Schur decomposition already computed by SHSEQR, by
    *     dealing with the Hessenberg submatrix in rows and columns ILO to
    *     IHI.
    *
    *     Arguments
    *     =========
    *
    *     WANTT   (input) LOGICAL
    *          = .TRUE. : the full Schur form T is required;
    *          = .FALSE.: only eigenvalues are required.
    *
    *     WANTZ   (input) LOGICAL
    *          = .TRUE. : the matrix of Schur vectors Z is required;
    *          = .FALSE.: Schur vectors are not required.
    *
    *     N       (input) INTEGER
    *          The order of the matrix H.  N >= 0.
    *
    *     ILO     (input) INTEGER
    *     IHI     (input) INTEGER
    *          It is assumed that H is already upper quasi-triangular in
    *          rows and columns IHI+1:N, and that H(ILO,ILO-1) = 0 (unless
    *          ILO = 1). SLAHQR works primarily with the Hessenberg
    *          submatrix in rows and columns ILO to IHI, but applies
    *          transformations to all of H if WANTT is .TRUE..
    *          1 <= ILO <= max(1,IHI); IHI <= N.
    *
    *     H       (input/output) REAL array, dimension (LDH,N)
    *          On entry, the upper Hessenberg matrix H.
    *          On exit, if INFO is zero and if WANTT is .TRUE., H is upper
    *          quasi-triangular in rows and columns ILO:IHI, with any
    *          2-by-2 diagonal blocks in standard form. If INFO is zero
    *          and WANTT is .FALSE., the contents of H are unspecified on
    *          exit.  The output state of H if INFO is nonzero is given
    *          below under the description of INFO.
    *
    *     LDH     (input) INTEGER
    *          The leading dimension of the array H. LDH >= max(1,N).
    *
    *     WR      (output) REAL array, dimension (N)
    *     WI      (output) REAL array, dimension (N)
    *          The real and imaginary parts, respectively, of the computed
    *          eigenvalues ILO to IHI are stored in the corresponding
    *          elements of WR and WI. If two eigenvalues are computed as a
    *          complex conjugate pair, they are stored in consecutive
    *          elements of WR and WI, say the i-th and (i+1)th, with
    *          WI(i) > 0 and WI(i+1) < 0. If WANTT is .TRUE., the
    *          eigenvalues are stored in the same order as on the diagonal
    *          of the Schur form returned in H, with WR(i) = H(i,i), and, if
    *          H(i:i+1,i:i+1) is a 2-by-2 diagonal block,
    *          WI(i) = sqrt(H(i+1,i)*H(i,i+1)) and WI(i+1) = -WI(i).
    *
    *     ILOZ    (input) INTEGER
    *     IHIZ    (input) INTEGER
    *          Specify the rows of Z to which transformations must be
    *          applied if WANTZ is .TRUE..
    *          1 <= ILOZ <= ILO; IHI <= IHIZ <= N.
    *
    *     Z       (input/output) REAL array, dimension (LDZ,N)
    *          If WANTZ is .TRUE., on entry Z must contain the current
    *          matrix Z of transformations accumulated by SHSEQR, and on
    *          exit Z has been updated; transformations are applied only to
    *          the submatrix Z(ILOZ:IHIZ,ILO:IHI).
    *          If WANTZ is .FALSE., Z is not referenced.
    *
    *     LDZ     (input) INTEGER
    *          The leading dimension of the array Z. LDZ >= max(1,N).
    *
    *     INFO    (output) INTEGER
    *           =   0: successful exit
    *          .GT. 0: If INFO = i, SLAHQR failed to compute all the
    *                  eigenvalues ILO to IHI in a total of 30 iterations
    *                  per eigenvalue; elements i+1:ihi of WR and WI
    *                  contain those eigenvalues which have been
    *                  successfully computed.
    *
    *                  If INFO .GT. 0 and WANTT is .FALSE., then on exit,
    *                  the remaining unconverged eigenvalues are the
    *                  eigenvalues of the upper Hessenberg matrix rows
    *                  and columns ILO thorugh INFO of the final, output
    *                  value of H.
    *
    *                  If INFO .GT. 0 and WANTT is .TRUE., then on exit
    *          (*)       (initial value of H)*U  = U*(final value of H)
    *                  where U is an orthognal matrix.    The final
    *                  value of H is upper Hessenberg and triangular in
    *                  rows and columns INFO+1 through IHI.
    *
    *                  If INFO .GT. 0 and WANTZ is .TRUE., then on exit
    *                      (final value of Z)  = (initial value of Z)*U
    *                  where U is the orthogonal matrix in (*)
    *                  (regardless of the value of WANTT.)
    *
    *     Further Details
    *     ===============
    *
    *     02-96 Based on modifications by
    *     David Day, Sandia National Laboratory, USA
    *
    *     12-04 Further modifications by
    *     Ralph Byers, University of Kansas, USA
    *
    *       This is a modified version of SLAHQR from LAPACK version 3.0.
    *       It is (1) more robust against overflow and underflow and
    *       (2) adopts the more conservative Ahues & Tisseur stopping
    *       criterion (LAWN 122, 1997).
    *
    *     =========================================================
    *
    *     .. Parameters ..
          INTEGER            ITMAX
          PARAMETER          ( ITMAX = 30 )
          REAL               ZERO, ONE, TWO
          PARAMETER          ( ZERO = 0.0e0, ONE = 1.0e0, TWO = 2.0e0 )
          REAL               DAT1, DAT2
          PARAMETER          ( DAT1 = 3.0e0 / 4.0e0, DAT2 = -0.4375e0 )
    *     ..
    *     .. Local Scalars ..
          REAL               AA, AB, BA, BB, CS, DET, H11, H12, H21, H21S,
         $                   H22, RT1I, RT1R, RT2I, RT2R, RTDISC, S, SAFMAX,
         $                   SAFMIN, SMLNUM, SN, SUM, T1, T2, T3, TR, TST,
         $                   ULP, V2, V3
          INTEGER            I, I1, I2, ITS, J, K, L, M, NH, NR, NZ
    *     ..
    *     .. Local Arrays ..
          REAL               V( 3 )
    *     ..
    *     .. External Functions ..
          REAL               SLAMCH
          EXTERNAL           SLAMCH
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           SCOPY, SLABAD, SLANV2, SLARFG, SROT
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          ABS, MAX, MIN, REAL, SQRT
    *     ..
    *     .. Executable Statements ..
    *
          INFO = 0
    *
    *     Quick return if possible
    *
          IF( N.EQ.0 )
         $   RETURN
          IF( ILO.EQ.IHI ) THEN
             WR( ILO ) = H( ILO, ILO )
             WI( ILO ) = ZERO
             RETURN
          END IF
    *
    *     ==== clear out the trash ====
          DO 10 J = ILO, IHI - 3
             H( J+2, J ) = ZERO
             H( J+3, J ) = ZERO
       10 CONTINUE
          IF( ILO.LE.IHI-2 )
         $   H( IHI, IHI-2 ) = ZERO
    *
          NH = IHI - ILO + 1
          NZ = IHIZ - ILOZ + 1
    *
    *     Set machine-dependent constants for the stopping criterion.
    *
          SAFMIN = SLAMCH( 'SAFE MINIMUM' )
          SAFMAX = ONE / SAFMIN
          CALL SLABAD( SAFMIN, SAFMAX )
          ULP = SLAMCH( 'PRECISION' )
          SMLNUM = SAFMIN*( REAL( NH ) / ULP )
    *
    *     I1 and I2 are the indices of the first row and last column of H
    *     to which transformations must be applied. If eigenvalues only are
    *     being computed, I1 and I2 are set inside the main loop.
    *
          IF( WANTT ) THEN
             I1 = 1
             I2 = N
          END IF
    *
    *     The main loop begins here. I is the loop index and decreases from
    *     IHI to ILO in steps of 1 or 2. Each iteration of the loop works
    *     with the active submatrix in rows and columns L to I.
    *     Eigenvalues I+1 to IHI have already converged. Either L = ILO or
    *     H(L,L-1) is negligible so that the matrix splits.
    *
          I = IHI
       20 CONTINUE
          L = ILO
          IF( I.LT.ILO )
         $   GO TO 160
    *
    *     Perform QR iterations on rows and columns ILO to I until a
    *     submatrix of order 1 or 2 splits off at the bottom because a
    *     subdiagonal element has become negligible.
    *
          DO 140 ITS = 0, ITMAX
    *
    *        Look for a single small subdiagonal element.
    *
             DO 30 K = I, L + 1, -1
                IF( ABS( H( K, K-1 ) ).LE.SMLNUM )
         $         GO TO 40
                TST = ABS( H( K-1, K-1 ) ) + ABS( H( K, K ) )
                IF( TST.EQ.ZERO ) THEN
                   IF( K-2.GE.ILO )
         $            TST = TST + ABS( H( K-1, K-2 ) )
                   IF( K+1.LE.IHI )
         $            TST = TST + ABS( H( K+1, K ) )
                END IF
    *           ==== The following is a conservative small subdiagonal
    *           .    deflation  criterion due to Ahues & Tisseur (LAWN 122,
    *           .    1997). It has better mathematical foundation and
    *           .    improves accuracy in some cases.  ====
                IF( ABS( H( K, K-1 ) ).LE.ULP*TST ) THEN
                   AB = MAX( ABS( H( K, K-1 ) ), ABS( H( K-1, K ) ) )
                   BA = MIN( ABS( H( K, K-1 ) ), ABS( H( K-1, K ) ) )
                   AA = MAX( ABS( H( K, K ) ),
         $              ABS( H( K-1, K-1 )-H( K, K ) ) )
                   BB = MIN( ABS( H( K, K ) ),
         $              ABS( H( K-1, K-1 )-H( K, K ) ) )
                   S = AA + AB
                   IF( BA*( AB / S ).LE.MAX( SMLNUM,
         $             ULP*( BB*( AA / S ) ) ) )GO TO 40
                END IF
       30    CONTINUE
       40    CONTINUE
             L = K
             IF( L.GT.ILO ) THEN
    *
    *           H(L,L-1) is negligible
    *
                H( L, L-1 ) = ZERO
             END IF
    *
    *        Exit from loop if a submatrix of order 1 or 2 has split off.
    *
             IF( L.GE.I-1 )
         $      GO TO 150
    *
    *        Now the active submatrix is in rows and columns L to I. If
    *        eigenvalues only are being computed, only the active submatrix
    *        need be transformed.
    *
             IF( .NOT.WANTT ) THEN
                I1 = L
                I2 = I
             END IF
    *
             IF( ITS.EQ.10 .OR. ITS.EQ.20 ) THEN
    *
    *           Exceptional shift.
    *
                H11 = DAT1*S + H( I, I )
                H12 = DAT2*S
                H21 = S
                H22 = H11
             ELSE
    *
    *           Prepare to use Francis' double shift
    *           (i.e. 2nd degree generalized Rayleigh quotient)
    *
                H11 = H( I-1, I-1 )
                H21 = H( I, I-1 )
                H12 = H( I-1, I )
                H22 = H( I, I )
             END IF
             S = ABS( H11 ) + ABS( H12 ) + ABS( H21 ) + ABS( H22 )
             IF( S.EQ.ZERO ) THEN
                RT1R = ZERO
                RT1I = ZERO
                RT2R = ZERO
                RT2I = ZERO
             ELSE
                H11 = H11 / S
                H21 = H21 / S
                H12 = H12 / S
                H22 = H22 / S
                TR = ( H11+H22 ) / TWO
                DET = ( H11-TR )*( H22-TR ) - H12*H21
                RTDISC = SQRT( ABS( DET ) )
                IF( DET.GE.ZERO ) THEN
    *
    *              ==== complex conjugate shifts ====
    *
                   RT1R = TR*S
                   RT2R = RT1R
                   RT1I = RTDISC*S
                   RT2I = -RT1I
                ELSE
    *
    *              ==== real shifts (use only one of them)  ====
    *
                   RT1R = TR + RTDISC
                   RT2R = TR - RTDISC
                   IF( ABS( RT1R-H22 ).LE.ABS( RT2R-H22 ) ) THEN
                      RT1R = RT1R*S
                      RT2R = RT1R
                   ELSE
                      RT2R = RT2R*S
                      RT1R = RT2R
                   END IF
                   RT1I = ZERO
                   RT2I = ZERO
                END IF
             END IF
    *
    *        Look for two consecutive small subdiagonal elements.
    *
             DO 50 M = I - 2, L, -1
    *           Determine the effect of starting the double-shift QR
    *           iteration at row M, and see if this would make H(M,M-1)
    *           negligible.  (The following uses scaling to avoid
    *           overflows and most underflows.)
    *
                H21S = H( M+1, M )
                S = ABS( H( M, M )-RT2R ) + ABS( RT2I ) + ABS( H21S )
                H21S = H( M+1, M ) / S
                V( 1 ) = H21S*H( M, M+1 ) + ( H( M, M )-RT1R )*
         $               ( ( H( M, M )-RT2R ) / S ) - RT1I*( RT2I / S )
                V( 2 ) = H21S*( H( M, M )+H( M+1, M+1 )-RT1R-RT2R )
                V( 3 ) = H21S*H( M+2, M+1 )
                S = ABS( V( 1 ) ) + ABS( V( 2 ) ) + ABS( V( 3 ) )
                V( 1 ) = V( 1 ) / S
                V( 2 ) = V( 2 ) / S
                V( 3 ) = V( 3 ) / S
                IF( M.EQ.L )
         $         GO TO 60
                IF( ABS( H( M, M-1 ) )*( ABS( V( 2 ) )+ABS( V( 3 ) ) ).LE.
         $          ULP*ABS( V( 1 ) )*( ABS( H( M-1, M-1 ) )+ABS( H( M,
         $          M ) )+ABS( H( M+1, M+1 ) ) ) )GO TO 60
       50    CONTINUE
       60    CONTINUE
    *
    *        Double-shift QR step
    *
             DO 130 K = M, I - 1
    *
    *           The first iteration of this loop determines a reflection G
    *           from the vector V and applies it from left and right to H,
    *           thus creating a nonzero bulge below the subdiagonal.
    *
    *           Each subsequent iteration determines a reflection G to
    *           restore the Hessenberg form in the (K-1)th column, and thus
    *           chases the bulge one step toward the bottom of the active
    *           submatrix. NR is the order of G.
    *
                NR = MIN( 3, I-K+1 )
                IF( K.GT.M )
         $         CALL SCOPY( NR, H( K, K-1 ), 1, V, 1 )
                CALL SLARFG( NR, V( 1 ), V( 2 ), 1, T1 )
                IF( K.GT.M ) THEN
                   H( K, K-1 ) = V( 1 )
                   H( K+1, K-1 ) = ZERO
                   IF( K.LT.I-1 )
         $            H( K+2, K-1 ) = ZERO
                ELSE IF( M.GT.L ) THEN
                   H( K, K-1 ) = -H( K, K-1 )
                END IF
                V2 = V( 2 )
                T2 = T1*V2
                IF( NR.EQ.3 ) THEN
                   V3 = V( 3 )
                   T3 = T1*V3
    *
    *              Apply G from the left to transform the rows of the matrix
    *              in columns K to I2.
    *
                   DO 70 J = K, I2
                      SUM = H( K, J ) + V2*H( K+1, J ) + V3*H( K+2, J )
                      H( K, J ) = H( K, J ) - SUM*T1
                      H( K+1, J ) = H( K+1, J ) - SUM*T2
                      H( K+2, J ) = H( K+2, J ) - SUM*T3
       70          CONTINUE
    *
    *              Apply G from the right to transform the columns of the
    *              matrix in rows I1 to min(K+3,I).
    *
                   DO 80 J = I1, MIN( K+3, I )
                      SUM = H( J, K ) + V2*H( J, K+1 ) + V3*H( J, K+2 )
                      H( J, K ) = H( J, K ) - SUM*T1
                      H( J, K+1 ) = H( J, K+1 ) - SUM*T2
                      H( J, K+2 ) = H( J, K+2 ) - SUM*T3
       80          CONTINUE
    *
                   IF( WANTZ ) THEN
    *
    *                 Accumulate transformations in the matrix Z
    *
                      DO 90 J = ILOZ, IHIZ
                         SUM = Z( J, K ) + V2*Z( J, K+1 ) + V3*Z( J, K+2 )
                         Z( J, K ) = Z( J, K ) - SUM*T1
                         Z( J, K+1 ) = Z( J, K+1 ) - SUM*T2
                         Z( J, K+2 ) = Z( J, K+2 ) - SUM*T3
       90             CONTINUE
                   END IF
                ELSE IF( NR.EQ.2 ) THEN
    *
    *              Apply G from the left to transform the rows of the matrix
    *              in columns K to I2.
    *
                   DO 100 J = K, I2
                      SUM = H( K, J ) + V2*H( K+1, J )
                      H( K, J ) = H( K, J ) - SUM*T1
                      H( K+1, J ) = H( K+1, J ) - SUM*T2
      100          CONTINUE
    *
    *              Apply G from the right to transform the columns of the
    *              matrix in rows I1 to min(K+3,I).
    *
                   DO 110 J = I1, I
                      SUM = H( J, K ) + V2*H( J, K+1 )
                      H( J, K ) = H( J, K ) - SUM*T1
                      H( J, K+1 ) = H( J, K+1 ) - SUM*T2
      110          CONTINUE
    *
                   IF( WANTZ ) THEN
    *
    *                 Accumulate transformations in the matrix Z
    *
                      DO 120 J = ILOZ, IHIZ
                         SUM = Z( J, K ) + V2*Z( J, K+1 )
                         Z( J, K ) = Z( J, K ) - SUM*T1
                         Z( J, K+1 ) = Z( J, K+1 ) - SUM*T2
      120             CONTINUE
                   END IF
                END IF
      130    CONTINUE
    *
      140 CONTINUE
    *
    *     Failure to converge in remaining number of iterations
    *
          INFO = I
          RETURN
    *
      150 CONTINUE
    *
          IF( L.EQ.I ) THEN
    *
    *        H(I,I-1) is negligible: one eigenvalue has converged.
    *
             WR( I ) = H( I, I )
             WI( I ) = ZERO
          ELSE IF( L.EQ.I-1 ) THEN
    *
    *        H(I-1,I-2) is negligible: a pair of eigenvalues have converged.
    *
    *        Transform the 2-by-2 submatrix to standard Schur form,
    *        and compute and store the eigenvalues.
    *
             CALL SLANV2( H( I-1, I-1 ), H( I-1, I ), H( I, I-1 ),
         $                H( I, I ), WR( I-1 ), WI( I-1 ), WR( I ), WI( I ),
         $                CS, SN )
    *
             IF( WANTT ) THEN
    *
    *           Apply the transformation to the rest of H.
    *
                IF( I2.GT.I )
         $         CALL SROT( I2-I, H( I-1, I+1 ), LDH, H( I, I+1 ), LDH,
         $                    CS, SN )
                CALL SROT( I-I1-1, H( I1, I-1 ), 1, H( I1, I ), 1, CS, SN )
             END IF
             IF( WANTZ ) THEN
    *
    *           Apply the transformation to Z.
    *
                CALL SROT( NZ, Z( ILOZ, I-1 ), 1, Z( ILOZ, I ), 1, CS, SN )
             END IF
          END IF
    *
    *     return to start of the main loop with new value of I.
    *
          I = L - 1
          GO TO 20
    *
      160 CONTINUE
          RETURN
    *
    *     End of SLAHQR
    *
          END