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SUBROUTINE SB04MD( N, M, A, LDA, B, LDB, C, LDC, Z, LDZ, IWORK,
$ DWORK, LDWORK, INFO )
C
C RELEASE 4.0, WGS COPYRIGHT 1999.
C
C PURPOSE
C
C To solve for X the continuous-time Sylvester equation
C
C AX + XB = C
C
C where A, B, C and X are general N-by-N, M-by-M, N-by-M and
C N-by-M matrices respectively.
C
C ARGUMENTS
C
C Input/Output Parameters
C
C N (input) INTEGER
C The order of the matrix A. N >= 0.
C
C M (input) INTEGER
C The order of the matrix B. M >= 0.
C
C A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
C On entry, the leading N-by-N part of this array must
C contain the coefficient matrix A of the equation.
C On exit, the leading N-by-N upper Hessenberg part of this
C array contains the matrix H, and the remainder of the
C leading N-by-N part, together with the elements 2,3,...,N
C of array DWORK, contain the orthogonal transformation
C matrix U (stored in factored form).
C
C LDA INTEGER
C The leading dimension of array A. LDA >= MAX(1,N).
C
C B (input/output) DOUBLE PRECISION array, dimension (LDB,M)
C On entry, the leading M-by-M part of this array must
C contain the coefficient matrix B of the equation.
C On exit, the leading M-by-M part of this array contains
C the quasi-triangular Schur factor S of the matrix B'.
C
C LDB INTEGER
C The leading dimension of array B. LDB >= MAX(1,M).
C
C C (input/output) DOUBLE PRECISION array, dimension (LDC,M)
C On entry, the leading N-by-M part of this array must
C contain the coefficient matrix C of the equation.
C On exit, the leading N-by-M part of this array contains
C the solution matrix X of the problem.
C
C LDC INTEGER
C The leading dimension of array C. LDC >= MAX(1,N).
C
C Z (output) DOUBLE PRECISION array, dimension (LDZ,M)
C The leading M-by-M part of this array contains the
C orthogonal matrix Z used to transform B' to real upper
C Schur form.
C
C LDZ INTEGER
C The leading dimension of array Z. LDZ >= MAX(1,M).
C
C Workspace
C
C IWORK INTEGER array, dimension (4*N)
C
C DWORK DOUBLE PRECISION array, dimension (LDWORK)
C On exit, if INFO = 0, DWORK(1) returns the optimal value
C of LDWORK, and DWORK(2), DWORK(3),..., DWORK(N) contain
C the scalar factors of the elementary reflectors used to
C reduce A to upper Hessenberg form, as returned by LAPACK
C Library routine DGEHRD.
C
C LDWORK INTEGER
C The length of the array DWORK.
C LDWORK = MAX(1, 2*N*N + 8*N, 5*M, N + M).
C For optimum performance LDWORK should be larger.
C
C Error Indicator
C
C INFO INTEGER
C = 0: successful exit;
C < 0: if INFO = -i, the i-th argument had an illegal
C value;
C > 0: if INFO = i, 1 <= i <= M, the QR algorithm failed to
C compute all the eigenvalues (see LAPACK Library
C routine DGEES);
C > M: if a singular matrix was encountered whilst solving
C for the (INFO-M)-th column of matrix X.
C
C METHOD
C
C The matrix A is transformed to upper Hessenberg form H = U'AU by
C the orthogonal transformation matrix U; matrix B' is transformed
C to real upper Schur form S = Z'B'Z using the orthogonal
C transformation matrix Z. The matrix C is also multiplied by the
C transformations, F = U'CZ, and the solution matrix Y of the
C transformed system
C
C HY + YS' = F
C
C is computed by back substitution. Finally, the matrix Y is then
C multiplied by the orthogonal transformation matrices, X = UYZ', in
C order to obtain the solution matrix X to the original problem.
C
C REFERENCES
C
C [1] Golub, G.H., Nash, S. and Van Loan, C.F.
C A Hessenberg-Schur method for the problem AX + XB = C.
C IEEE Trans. Auto. Contr., AC-24, pp. 909-913, 1979.
C
C NUMERICAL ASPECTS
C 3 3 2 2
C The algorithm requires about (5/3) N + 10 M + 5 N M + 2.5 M N
C operations and is backward stable.
C
C CONTRIBUTORS
C
C Release 3.0: V. Sima, Katholieke Univ. Leuven, Belgium, Aug. 1997.
C Supersedes Release 2.0 routine SB04AD by G. Golub, S. Nash, and
C C. Van Loan, Stanford University, California, United States of
C America, January 1982.
C
C REVISIONS
C
C V. Sima, Katholieke Univ. Leuven, Belgium, June 2000, Aug. 2000.
C
C KEYWORDS
C
C Hessenberg form, orthogonal transformation, real Schur form,
C Sylvester equation.
C
C ******************************************************************
C
C .. Parameters ..
DOUBLE PRECISION ZERO, ONE
PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0 )
C .. Scalar Arguments ..
INTEGER INFO, LDA, LDB, LDC, LDWORK, LDZ, M, N
C .. Array Arguments ..
INTEGER IWORK(*)
DOUBLE PRECISION A(LDA,*), B(LDB,*), C(LDC,*), DWORK(*), Z(LDZ,*)
C .. Local Scalars ..
INTEGER I, IEIG, IFAIL, IHI, ILO, IND, ITAU, JWORK,
$ SDIM, WRKOPT
C .. Local Scalars ..
LOGICAL SELECT
C .. Local Arrays ..
LOGICAL BWORK(1)
C .. External Subroutines ..
EXTERNAL DCOPY, DGEES, DGEHRD, DGEMM, DGEMV, DLACPY,
$ DORMHR, DSWAP, SB04MU, SB04MY, XERBLA
C .. Intrinsic Functions ..
INTRINSIC INT, MAX
C .. Executable Statements ..
C
INFO = 0
C
C Test the input scalar arguments.
C
IF( N.LT.0 ) THEN
INFO = -1
ELSE IF( M.LT.0 ) THEN
INFO = -2
ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
INFO = -4
ELSE IF( LDB.LT.MAX( 1, M ) ) THEN
INFO = -6
ELSE IF( LDC.LT.MAX( 1, N ) ) THEN
INFO = -8
ELSE IF( LDZ.LT.MAX( 1, M ) ) THEN
INFO = -10
ELSE IF( LDWORK.LT.MAX( 1, 2*N*N + 8*N, 5*M, N + M ) ) THEN
INFO = -13
END IF
C
IF ( INFO.NE.0 ) THEN
C
C Error return.
C
CALL XERBLA( 'SB04MD', -INFO )
RETURN
END IF
C
C Quick return if possible.
C
IF ( N.EQ.0 .OR. M.EQ.0 ) THEN
DWORK(1) = ONE
RETURN
END IF
C
ILO = 1
IHI = N
WRKOPT = 1
C
C Step 1 : Reduce A to upper Hessenberg and B' to quasi-upper
C triangular. That is, H = U' * A * U (store U in factored
C form) and S = Z' * B' * Z (save Z).
C
C (Note: Comments in the code beginning "Workspace:" describe the
C minimal amount of real workspace needed at that point in the
C code, as well as the preferred amount for good performance.
C NB refers to the optimal block size for the immediately
C following subroutine, as returned by ILAENV.)
C
DO 20 I = 2, M
CALL DSWAP( I-1, B(1,I), 1, B(I,1), LDB )
20 CONTINUE
C
C Workspace: need 5*M;
C prefer larger.
C
IEIG = M + 1
JWORK = IEIG + M
CALL DGEES( 'Vectors', 'Not ordered', SELECT, M, B, LDB,
$ SDIM, DWORK, DWORK(IEIG), Z, LDZ, DWORK(JWORK),
$ LDWORK-JWORK+1, BWORK, INFO )
IF ( INFO.NE.0 )
$ RETURN
WRKOPT = MAX( WRKOPT, INT( DWORK(JWORK) )+JWORK-1 )
C
C Workspace: need 2*N;
C prefer N + N*NB.
C
ITAU = 2
JWORK = ITAU + N - 1
CALL DGEHRD( N, ILO, IHI, A, LDA, DWORK(ITAU), DWORK(JWORK),
$ LDWORK-JWORK+1, IFAIL )
WRKOPT = MAX( WRKOPT, INT( DWORK(JWORK) )+JWORK-1 )
C
C Step 2 : Form F = ( U' * C ) * Z. Use BLAS 3, if enough space.
C
C Workspace: need N + M;
C prefer N + M*NB.
C
CALL DORMHR( 'Left', 'Transpose', N, M, ILO, IHI, A, LDA,
$ DWORK(ITAU), C, LDC, DWORK(JWORK), LDWORK-JWORK+1,
$ IFAIL )
WRKOPT = MAX( WRKOPT, INT( DWORK(JWORK) )+JWORK-1 )
C
IF ( LDWORK.GE.JWORK - 1 + N*M ) THEN
CALL DGEMM( 'No transpose', 'No transpose', N, M, M, ONE, C,
$ LDC, Z, LDZ, ZERO, DWORK(JWORK), N )
CALL DLACPY( 'Full', N, M, DWORK(JWORK), N, C, LDC )
WRKOPT = MAX( WRKOPT, JWORK - 1 + N*M )
ELSE
C
DO 40 I = 1, N
CALL DGEMV( 'Transpose', M, M, ONE, Z, LDZ, C(I,1), LDC,
$ ZERO, DWORK(JWORK), 1 )
CALL DCOPY( M, DWORK(JWORK), 1, C(I,1), LDC )
40 CONTINUE
C
END IF
C
IND = M
60 CONTINUE
IF ( IND.GT.1 ) THEN
C
C Step 3 : Solve H * Y + Y * S' = F for Y.
C
IF ( B(IND,IND-1).EQ.ZERO ) THEN
C
C Solve a special linear algebraic system of order N.
C Workspace: N*(N+1)/2 + 3*N.
C
CALL SB04MY( M, N, IND, A, LDA, B, LDB, C, LDC,
$ DWORK(JWORK), IWORK, INFO )
C
IF ( INFO.NE.0 ) THEN
INFO = INFO + M
RETURN
END IF
WRKOPT = MAX( WRKOPT, JWORK + N*( N + 1 )/2 + 2*N - 1 )
IND = IND - 1
ELSE
C
C Solve a special linear algebraic system of order 2*N.
C Workspace: 2*N*N + 8*N;
C
CALL SB04MU( M, N, IND, A, LDA, B, LDB, C, LDC,
$ DWORK(JWORK), IWORK, INFO )
C
IF ( INFO.NE.0 ) THEN
INFO = INFO + M
RETURN
END IF
WRKOPT = MAX( WRKOPT, JWORK + 2*N*N + 7*N - 1 )
IND = IND - 2
END IF
GO TO 60
ELSE IF ( IND.EQ.1 ) THEN
C
C Solve a special linear algebraic system of order N.
C Workspace: N*(N+1)/2 + 3*N;
C
CALL SB04MY( M, N, IND, A, LDA, B, LDB, C, LDC,
$ DWORK(JWORK), IWORK, INFO )
IF ( INFO.NE.0 ) THEN
INFO = INFO + M
RETURN
END IF
WRKOPT = MAX( WRKOPT, JWORK + N*( N + 1 )/2 + 2*N - 1 )
END IF
C
C Step 4 : Form C = ( U * Y ) * Z'. Use BLAS 3, if enough space.
C
C Workspace: need N + M;
C prefer N + M*NB.
C
CALL DORMHR( 'Left', 'No transpose', N, M, ILO, IHI, A, LDA,
$ DWORK(ITAU), C, LDC, DWORK(JWORK), LDWORK-JWORK+1,
$ IFAIL )
WRKOPT = MAX( WRKOPT, INT( DWORK(JWORK) )+JWORK-1 )
C
IF ( LDWORK.GE.JWORK - 1 + N*M ) THEN
CALL DGEMM( 'No transpose', 'Transpose', N, M, M, ONE, C, LDC,
$ Z, LDZ, ZERO, DWORK(JWORK), N )
CALL DLACPY( 'Full', N, M, DWORK(JWORK), N, C, LDC )
ELSE
C
DO 80 I = 1, N
CALL DGEMV( 'No transpose', M, M, ONE, Z, LDZ, C(I,1), LDC,
$ ZERO, DWORK(JWORK), 1 )
CALL DCOPY( M, DWORK(JWORK), 1, C(I,1), LDC )
80 CONTINUE
END IF
C
RETURN
C *** Last line of SB04MD ***
END
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