LAPACK 3.3.1
Linear Algebra PACKage

ctgsy2.f

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00001       SUBROUTINE CTGSY2( TRANS, IJOB, M, N, A, LDA, B, LDB, C, LDC, D,
00002      $                   LDD, E, LDE, F, LDF, SCALE, RDSUM, RDSCAL,
00003      $                   INFO )
00004 *
00005 *  -- LAPACK auxiliary routine (version 3.3.1) --
00006 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
00007 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
00008 *  -- April 2011                                                      --
00009 *
00010 *     .. Scalar Arguments ..
00011       CHARACTER          TRANS
00012       INTEGER            IJOB, INFO, LDA, LDB, LDC, LDD, LDE, LDF, M, N
00013       REAL               RDSCAL, RDSUM, SCALE
00014 *     ..
00015 *     .. Array Arguments ..
00016       COMPLEX            A( LDA, * ), B( LDB, * ), C( LDC, * ),
00017      $                   D( LDD, * ), E( LDE, * ), F( LDF, * )
00018 *     ..
00019 *
00020 *  Purpose
00021 *  =======
00022 *
00023 *  CTGSY2 solves the generalized Sylvester equation
00024 *
00025 *              A * R - L * B = scale *   C               (1)
00026 *              D * R - L * E = scale * F
00027 *
00028 *  using Level 1 and 2 BLAS, where R and L are unknown M-by-N matrices,
00029 *  (A, D), (B, E) and (C, F) are given matrix pairs of size M-by-M,
00030 *  N-by-N and M-by-N, respectively. A, B, D and E are upper triangular
00031 *  (i.e., (A,D) and (B,E) in generalized Schur form).
00032 *
00033 *  The solution (R, L) overwrites (C, F). 0 <= SCALE <= 1 is an output
00034 *  scaling factor chosen to avoid overflow.
00035 *
00036 *  In matrix notation solving equation (1) corresponds to solve
00037 *  Zx = scale * b, where Z is defined as
00038 *
00039 *         Z = [ kron(In, A)  -kron(B**H, Im) ]             (2)
00040 *             [ kron(In, D)  -kron(E**H, Im) ],
00041 *
00042 *  Ik is the identity matrix of size k and X**H is the transpose of X.
00043 *  kron(X, Y) is the Kronecker product between the matrices X and Y.
00044 *
00045 *  If TRANS = 'C', y in the conjugate transposed system Z**H*y = scale*b
00046 *  is solved for, which is equivalent to solve for R and L in
00047 *
00048 *              A**H * R  + D**H * L   = scale *  C           (3)
00049 *              R  * B**H + L  * E**H  = scale * -F
00050 *
00051 *  This case is used to compute an estimate of Dif[(A, D), (B, E)] =
00052 *  = sigma_min(Z) using reverse communicaton with CLACON.
00053 *
00054 *  CTGSY2 also (IJOB >= 1) contributes to the computation in CTGSYL
00055 *  of an upper bound on the separation between to matrix pairs. Then
00056 *  the input (A, D), (B, E) are sub-pencils of two matrix pairs in
00057 *  CTGSYL.
00058 *
00059 *  Arguments
00060 *  =========
00061 *
00062 *  TRANS   (input) CHARACTER*1
00063 *          = 'N', solve the generalized Sylvester equation (1).
00064 *          = 'T': solve the 'transposed' system (3).
00065 *
00066 *  IJOB    (input) INTEGER
00067 *          Specifies what kind of functionality to be performed.
00068 *          =0: solve (1) only.
00069 *          =1: A contribution from this subsystem to a Frobenius
00070 *              norm-based estimate of the separation between two matrix
00071 *              pairs is computed. (look ahead strategy is used).
00072 *          =2: A contribution from this subsystem to a Frobenius
00073 *              norm-based estimate of the separation between two matrix
00074 *              pairs is computed. (SGECON on sub-systems is used.)
00075 *          Not referenced if TRANS = 'T'.
00076 *
00077 *  M       (input) INTEGER
00078 *          On entry, M specifies the order of A and D, and the row
00079 *          dimension of C, F, R and L.
00080 *
00081 *  N       (input) INTEGER
00082 *          On entry, N specifies the order of B and E, and the column
00083 *          dimension of C, F, R and L.
00084 *
00085 *  A       (input) COMPLEX array, dimension (LDA, M)
00086 *          On entry, A contains an upper triangular matrix.
00087 *
00088 *  LDA     (input) INTEGER
00089 *          The leading dimension of the matrix A. LDA >= max(1, M).
00090 *
00091 *  B       (input) COMPLEX array, dimension (LDB, N)
00092 *          On entry, B contains an upper triangular matrix.
00093 *
00094 *  LDB     (input) INTEGER
00095 *          The leading dimension of the matrix B. LDB >= max(1, N).
00096 *
00097 *  C       (input/output) COMPLEX array, dimension (LDC, N)
00098 *          On entry, C contains the right-hand-side of the first matrix
00099 *          equation in (1).
00100 *          On exit, if IJOB = 0, C has been overwritten by the solution
00101 *          R.
00102 *
00103 *  LDC     (input) INTEGER
00104 *          The leading dimension of the matrix C. LDC >= max(1, M).
00105 *
00106 *  D       (input) COMPLEX array, dimension (LDD, M)
00107 *          On entry, D contains an upper triangular matrix.
00108 *
00109 *  LDD     (input) INTEGER
00110 *          The leading dimension of the matrix D. LDD >= max(1, M).
00111 *
00112 *  E       (input) COMPLEX array, dimension (LDE, N)
00113 *          On entry, E contains an upper triangular matrix.
00114 *
00115 *  LDE     (input) INTEGER
00116 *          The leading dimension of the matrix E. LDE >= max(1, N).
00117 *
00118 *  F       (input/output) COMPLEX array, dimension (LDF, N)
00119 *          On entry, F contains the right-hand-side of the second matrix
00120 *          equation in (1).
00121 *          On exit, if IJOB = 0, F has been overwritten by the solution
00122 *          L.
00123 *
00124 *  LDF     (input) INTEGER
00125 *          The leading dimension of the matrix F. LDF >= max(1, M).
00126 *
00127 *  SCALE   (output) REAL
00128 *          On exit, 0 <= SCALE <= 1. If 0 < SCALE < 1, the solutions
00129 *          R and L (C and F on entry) will hold the solutions to a
00130 *          slightly perturbed system but the input matrices A, B, D and
00131 *          E have not been changed. If SCALE = 0, R and L will hold the
00132 *          solutions to the homogeneous system with C = F = 0.
00133 *          Normally, SCALE = 1.
00134 *
00135 *  RDSUM   (input/output) REAL
00136 *          On entry, the sum of squares of computed contributions to
00137 *          the Dif-estimate under computation by CTGSYL, where the
00138 *          scaling factor RDSCAL (see below) has been factored out.
00139 *          On exit, the corresponding sum of squares updated with the
00140 *          contributions from the current sub-system.
00141 *          If TRANS = 'T' RDSUM is not touched.
00142 *          NOTE: RDSUM only makes sense when CTGSY2 is called by
00143 *          CTGSYL.
00144 *
00145 *  RDSCAL  (input/output) REAL
00146 *          On entry, scaling factor used to prevent overflow in RDSUM.
00147 *          On exit, RDSCAL is updated w.r.t. the current contributions
00148 *          in RDSUM.
00149 *          If TRANS = 'T', RDSCAL is not touched.
00150 *          NOTE: RDSCAL only makes sense when CTGSY2 is called by
00151 *          CTGSYL.
00152 *
00153 *  INFO    (output) INTEGER
00154 *          On exit, if INFO is set to
00155 *            =0: Successful exit
00156 *            <0: If INFO = -i, input argument number i is illegal.
00157 *            >0: The matrix pairs (A, D) and (B, E) have common or very
00158 *                close eigenvalues.
00159 *
00160 *  Further Details
00161 *  ===============
00162 *
00163 *  Based on contributions by
00164 *     Bo Kagstrom and Peter Poromaa, Department of Computing Science,
00165 *     Umea University, S-901 87 Umea, Sweden.
00166 *
00167 *  =====================================================================
00168 *
00169 *     .. Parameters ..
00170       REAL               ZERO, ONE
00171       INTEGER            LDZ
00172       PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0, LDZ = 2 )
00173 *     ..
00174 *     .. Local Scalars ..
00175       LOGICAL            NOTRAN
00176       INTEGER            I, IERR, J, K
00177       REAL               SCALOC
00178       COMPLEX            ALPHA
00179 *     ..
00180 *     .. Local Arrays ..
00181       INTEGER            IPIV( LDZ ), JPIV( LDZ )
00182       COMPLEX            RHS( LDZ ), Z( LDZ, LDZ )
00183 *     ..
00184 *     .. External Functions ..
00185       LOGICAL            LSAME
00186       EXTERNAL           LSAME
00187 *     ..
00188 *     .. External Subroutines ..
00189       EXTERNAL           CAXPY, CGESC2, CGETC2, CSCAL, CLATDF, XERBLA
00190 *     ..
00191 *     .. Intrinsic Functions ..
00192       INTRINSIC          CMPLX, CONJG, MAX
00193 *     ..
00194 *     .. Executable Statements ..
00195 *
00196 *     Decode and test input parameters
00197 *
00198       INFO = 0
00199       IERR = 0
00200       NOTRAN = LSAME( TRANS, 'N' )
00201       IF( .NOT.NOTRAN .AND. .NOT.LSAME( TRANS, 'C' ) ) THEN
00202          INFO = -1
00203       ELSE IF( NOTRAN ) THEN
00204          IF( ( IJOB.LT.0 ) .OR. ( IJOB.GT.2 ) ) THEN
00205             INFO = -2
00206          END IF
00207       END IF
00208       IF( INFO.EQ.0 ) THEN
00209          IF( M.LE.0 ) THEN
00210             INFO = -3
00211          ELSE IF( N.LE.0 ) THEN
00212             INFO = -4
00213          ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
00214             INFO = -5
00215          ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
00216             INFO = -8
00217          ELSE IF( LDC.LT.MAX( 1, M ) ) THEN
00218             INFO = -10
00219          ELSE IF( LDD.LT.MAX( 1, M ) ) THEN
00220             INFO = -12
00221          ELSE IF( LDE.LT.MAX( 1, N ) ) THEN
00222             INFO = -14
00223          ELSE IF( LDF.LT.MAX( 1, M ) ) THEN
00224             INFO = -16
00225          END IF
00226       END IF
00227       IF( INFO.NE.0 ) THEN
00228          CALL XERBLA( 'CTGSY2', -INFO )
00229          RETURN
00230       END IF
00231 *
00232       IF( NOTRAN ) THEN
00233 *
00234 *        Solve (I, J) - system
00235 *           A(I, I) * R(I, J) - L(I, J) * B(J, J) = C(I, J)
00236 *           D(I, I) * R(I, J) - L(I, J) * E(J, J) = F(I, J)
00237 *        for I = M, M - 1, ..., 1; J = 1, 2, ..., N
00238 *
00239          SCALE = ONE
00240          SCALOC = ONE
00241          DO 30 J = 1, N
00242             DO 20 I = M, 1, -1
00243 *
00244 *              Build 2 by 2 system
00245 *
00246                Z( 1, 1 ) = A( I, I )
00247                Z( 2, 1 ) = D( I, I )
00248                Z( 1, 2 ) = -B( J, J )
00249                Z( 2, 2 ) = -E( J, J )
00250 *
00251 *              Set up right hand side(s)
00252 *
00253                RHS( 1 ) = C( I, J )
00254                RHS( 2 ) = F( I, J )
00255 *
00256 *              Solve Z * x = RHS
00257 *
00258                CALL CGETC2( LDZ, Z, LDZ, IPIV, JPIV, IERR )
00259                IF( IERR.GT.0 )
00260      $            INFO = IERR
00261                IF( IJOB.EQ.0 ) THEN
00262                   CALL CGESC2( LDZ, Z, LDZ, RHS, IPIV, JPIV, SCALOC )
00263                   IF( SCALOC.NE.ONE ) THEN
00264                      DO 10 K = 1, N
00265                         CALL CSCAL( M, CMPLX( SCALOC, ZERO ), C( 1, K ),
00266      $                              1 )
00267                         CALL CSCAL( M, CMPLX( SCALOC, ZERO ), F( 1, K ),
00268      $                              1 )
00269    10                CONTINUE
00270                      SCALE = SCALE*SCALOC
00271                   END IF
00272                ELSE
00273                   CALL CLATDF( IJOB, LDZ, Z, LDZ, RHS, RDSUM, RDSCAL,
00274      $                         IPIV, JPIV )
00275                END IF
00276 *
00277 *              Unpack solution vector(s)
00278 *
00279                C( I, J ) = RHS( 1 )
00280                F( I, J ) = RHS( 2 )
00281 *
00282 *              Substitute R(I, J) and L(I, J) into remaining equation.
00283 *
00284                IF( I.GT.1 ) THEN
00285                   ALPHA = -RHS( 1 )
00286                   CALL CAXPY( I-1, ALPHA, A( 1, I ), 1, C( 1, J ), 1 )
00287                   CALL CAXPY( I-1, ALPHA, D( 1, I ), 1, F( 1, J ), 1 )
00288                END IF
00289                IF( J.LT.N ) THEN
00290                   CALL CAXPY( N-J, RHS( 2 ), B( J, J+1 ), LDB,
00291      $                        C( I, J+1 ), LDC )
00292                   CALL CAXPY( N-J, RHS( 2 ), E( J, J+1 ), LDE,
00293      $                        F( I, J+1 ), LDF )
00294                END IF
00295 *
00296    20       CONTINUE
00297    30    CONTINUE
00298       ELSE
00299 *
00300 *        Solve transposed (I, J) - system:
00301 *           A(I, I)**H * R(I, J) + D(I, I)**H * L(J, J) = C(I, J)
00302 *           R(I, I) * B(J, J) + L(I, J) * E(J, J)   = -F(I, J)
00303 *        for I = 1, 2, ..., M, J = N, N - 1, ..., 1
00304 *
00305          SCALE = ONE
00306          SCALOC = ONE
00307          DO 80 I = 1, M
00308             DO 70 J = N, 1, -1
00309 *
00310 *              Build 2 by 2 system Z**H
00311 *
00312                Z( 1, 1 ) = CONJG( A( I, I ) )
00313                Z( 2, 1 ) = -CONJG( B( J, J ) )
00314                Z( 1, 2 ) = CONJG( D( I, I ) )
00315                Z( 2, 2 ) = -CONJG( E( J, J ) )
00316 *
00317 *
00318 *              Set up right hand side(s)
00319 *
00320                RHS( 1 ) = C( I, J )
00321                RHS( 2 ) = F( I, J )
00322 *
00323 *              Solve Z**H * x = RHS
00324 *
00325                CALL CGETC2( LDZ, Z, LDZ, IPIV, JPIV, IERR )
00326                IF( IERR.GT.0 )
00327      $            INFO = IERR
00328                CALL CGESC2( LDZ, Z, LDZ, RHS, IPIV, JPIV, SCALOC )
00329                IF( SCALOC.NE.ONE ) THEN
00330                   DO 40 K = 1, N
00331                      CALL CSCAL( M, CMPLX( SCALOC, ZERO ), C( 1, K ),
00332      $                           1 )
00333                      CALL CSCAL( M, CMPLX( SCALOC, ZERO ), F( 1, K ),
00334      $                           1 )
00335    40             CONTINUE
00336                   SCALE = SCALE*SCALOC
00337                END IF
00338 *
00339 *              Unpack solution vector(s)
00340 *
00341                C( I, J ) = RHS( 1 )
00342                F( I, J ) = RHS( 2 )
00343 *
00344 *              Substitute R(I, J) and L(I, J) into remaining equation.
00345 *
00346                DO 50 K = 1, J - 1
00347                   F( I, K ) = F( I, K ) + RHS( 1 )*CONJG( B( K, J ) ) +
00348      $                        RHS( 2 )*CONJG( E( K, J ) )
00349    50          CONTINUE
00350                DO 60 K = I + 1, M
00351                   C( K, J ) = C( K, J ) - CONJG( A( I, K ) )*RHS( 1 ) -
00352      $                        CONJG( D( I, K ) )*RHS( 2 )
00353    60          CONTINUE
00354 *
00355    70       CONTINUE
00356    80    CONTINUE
00357       END IF
00358       RETURN
00359 *
00360 *     End of CTGSY2
00361 *
00362       END
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