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SUBROUTINE SB04NW( ABSCHR, UL, N, M, C, LDC, INDX, AB, LDAB, D )
C
C RELEASE 4.0, WGS COPYRIGHT 1999.
C
C PURPOSE
C
C To construct the right-hand side D for a system of equations in
C Hessenberg form solved via SB04NY (case with 1 right-hand side).
C
C ARGUMENTS
C
C Mode Parameters
C
C ABSCHR CHARACTER*1
C Indicates whether AB contains A or B, as follows:
C = 'A': AB contains A;
C = 'B': AB contains B.
C
C UL CHARACTER*1
C Indicates whether AB is upper or lower Hessenberg matrix,
C as follows:
C = 'U': AB is upper Hessenberg;
C = 'L': AB is lower Hessenberg.
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 C (input) DOUBLE PRECISION array, dimension (LDC,M)
C The leading N-by-M part of this array must contain both
C the not yet modified part of the coefficient matrix C of
C the Sylvester equation AX + XB = C, and both the currently
C computed part of the solution of the Sylvester equation.
C
C LDC INTEGER
C The leading dimension of array C. LDC >= MAX(1,N).
C
C INDX (input) INTEGER
C The position of the column/row of C to be used in the
C construction of the right-hand side D.
C
C AB (input) DOUBLE PRECISION array, dimension (LDAB,*)
C The leading N-by-N or M-by-M part of this array must
C contain either A or B of the Sylvester equation
C AX + XB = C.
C
C LDAB INTEGER
C The leading dimension of array AB.
C LDAB >= MAX(1,N) or LDAB >= MAX(1,M) (depending on
C ABSCHR = 'A' or ABSCHR = 'B', respectively).
C
C D (output) DOUBLE PRECISION array, dimension (*)
C The leading N or M part of this array (depending on
C ABSCHR = 'B' or ABSCHR = 'A', respectively) contains the
C right-hand side.
C
C NUMERICAL ASPECTS
C
C None.
C
C CONTRIBUTORS
C
C Release 3.0: V. Sima, Katholieke Univ. Leuven, Belgium, Aug. 1997.
C Supersedes Release 2.0 routine SB04BW by M. Vanbegin, and
C P. Van Dooren, Philips Research Laboratory, Brussels, Belgium.
C
C REVISIONS
C
C -
C
C KEYWORDS
C
C Hessenberg form, orthogonal transformation, real Schur form,
C Sylvester equation.
C
C ******************************************************************
C
C .. Parameters ..
DOUBLE PRECISION ONE
PARAMETER ( ONE = 1.0D0 )
C .. Scalar Arguments ..
CHARACTER ABSCHR, UL
INTEGER INDX, LDAB, LDC, M, N
C .. Array Arguments ..
DOUBLE PRECISION AB(LDAB,*), C(LDC,*), D(*)
C .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
C .. External Subroutines ..
EXTERNAL DCOPY, DGEMV
C .. Executable Statements ..
C
C For speed, no tests on the input scalar arguments are made.
C Quick return if possible.
C
IF ( N.EQ.0 .OR. M.EQ.0 )
$ RETURN
C
IF ( LSAME( ABSCHR, 'B' ) ) THEN
C
C Construct the column of the right-hand side.
C
CALL DCOPY( N, C(1,INDX), 1, D, 1 )
IF ( LSAME( UL, 'U' ) ) THEN
IF ( INDX.GT.1 ) THEN
CALL DGEMV( 'N', N, INDX-1, -ONE, C, LDC, AB(1,INDX), 1,
$ ONE, D, 1 )
END IF
ELSE
IF ( INDX.LT.M ) THEN
CALL DGEMV( 'N', N, M-INDX, -ONE, C(1,INDX+1), LDC,
$ AB(INDX+1,INDX), 1, ONE, D, 1 )
END IF
END IF
ELSE
C
C Construct the row of the right-hand side.
C
CALL DCOPY( M, C(INDX,1), LDC, D, 1 )
IF ( LSAME( UL, 'U' ) ) THEN
IF ( INDX.LT.N ) THEN
CALL DGEMV( 'T', N-INDX, M, -ONE, C(INDX+1,1), LDC,
$ AB(INDX,INDX+1), LDAB, ONE, D, 1 )
END IF
ELSE
IF ( INDX.GT.1 ) THEN
CALL DGEMV( 'T', INDX-1, M, -ONE, C, LDC, AB(INDX,1),
$ LDAB, ONE, D, 1 )
END IF
END IF
END IF
C
RETURN
C *** Last line of SB04NW ***
END
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