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SUBROUTINE MB01UD( SIDE, TRANS, M, N, ALPHA, H, LDH, A, LDA, B,
$ LDB, INFO )
C
C RELEASE 4.0, WGS COPYRIGHT 1999.
C
C PURPOSE
C
C To compute one of the matrix products
C
C B = alpha*op( H ) * A, or B = alpha*A * op( H ),
C
C where alpha is a scalar, A and B are m-by-n matrices, H is an
C upper Hessenberg matrix, and op( H ) is one of
C
C op( H ) = H or op( H ) = H', the transpose of H.
C
C ARGUMENTS
C
C Mode Parameters
C
C SIDE CHARACTER*1
C Specifies whether the Hessenberg matrix H appears on the
C left or right in the matrix product as follows:
C = 'L': B = alpha*op( H ) * A;
C = 'R': B = alpha*A * op( H ).
C
C TRANS CHARACTER*1
C Specifies the form of op( H ) to be used in the matrix
C multiplication as follows:
C = 'N': op( H ) = H;
C = 'T': op( H ) = H';
C = 'C': op( H ) = H'.
C
C Input/Output Parameters
C
C M (input) INTEGER
C The number of rows of the matrices A and B. M >= 0.
C
C N (input) INTEGER
C The number of columns of the matrices A and B. N >= 0.
C
C ALPHA (input) DOUBLE PRECISION
C The scalar alpha. When alpha is zero then H is not
C referenced and A need not be set before entry.
C
C H (input) DOUBLE PRECISION array, dimension (LDH,k)
C where k is M when SIDE = 'L' and is N when SIDE = 'R'.
C On entry with SIDE = 'L', the leading M-by-M upper
C Hessenberg part of this array must contain the upper
C Hessenberg matrix H.
C On entry with SIDE = 'R', the leading N-by-N upper
C Hessenberg part of this array must contain the upper
C Hessenberg matrix H.
C The elements below the subdiagonal are not referenced,
C except possibly for those in the first column, which
C could be overwritten, but are restored on exit.
C
C LDH INTEGER
C The leading dimension of the array H. LDH >= max(1,k),
C where k is M when SIDE = 'L' and is N when SIDE = 'R'.
C
C A (input) DOUBLE PRECISION array, dimension (LDA,N)
C The leading M-by-N part of this array must contain the
C matrix A.
C
C LDA INTEGER
C The leading dimension of the array A. LDA >= max(1,M).
C
C B (output) DOUBLE PRECISION array, dimension (LDB,N)
C The leading M-by-N part of this array contains the
C computed product.
C
C LDB INTEGER
C The leading dimension of the array B. LDB >= max(1,M).
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
C METHOD
C
C The required matrix product is computed in two steps. In the first
C step, the upper triangle of H is used; in the second step, the
C contribution of the subdiagonal is added. A fast BLAS 3 DTRMM
C operation is used in the first step.
C
C CONTRIBUTOR
C
C V. Sima, Katholieke Univ. Leuven, Belgium, January 1999.
C
C REVISIONS
C
C -
C
C KEYWORDS
C
C Elementary matrix operations, matrix operations.
C
C ******************************************************************
C
C .. Parameters ..
DOUBLE PRECISION ZERO, ONE
PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0 )
C .. Scalar Arguments ..
CHARACTER SIDE, TRANS
INTEGER INFO, LDA, LDB, LDH, M, N
DOUBLE PRECISION ALPHA
C .. Array Arguments ..
DOUBLE PRECISION A(LDA,*), B(LDB,*), H(LDH,*)
C .. Local Scalars ..
LOGICAL LSIDE, LTRANS
INTEGER I, J
C .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
C .. External Subroutines ..
EXTERNAL DAXPY, DLACPY, DLASET, DSWAP, DTRMM, XERBLA
C .. Intrinsic Functions ..
INTRINSIC MAX, MIN
C
C .. Executable Statements ..
C
C Test the input scalar arguments.
C
INFO = 0
LSIDE = LSAME( SIDE, 'L' )
LTRANS = LSAME( TRANS, 'T' ) .OR. LSAME( TRANS, 'C' )
C
IF( ( .NOT.LSIDE ).AND.( .NOT.LSAME( SIDE, 'R' ) ) )THEN
INFO = -1
ELSE IF( ( .NOT.LTRANS ).AND.( .NOT.LSAME( TRANS, 'N' ) ) )THEN
INFO = -2
ELSE IF( M.LT.0 ) THEN
INFO = -3
ELSE IF( N.LT.0 ) THEN
INFO = -4
ELSE IF( LDH.LT.1 .OR. ( LSIDE .AND. LDH.LT.M ) .OR.
$ ( .NOT.LSIDE .AND. LDH.LT.N ) ) THEN
INFO = -7
ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
INFO = -9
ELSE IF( LDB.LT.MAX( 1, M ) ) THEN
INFO = -11
END IF
C
IF ( INFO.NE.0 ) THEN
C
C Error return.
C
CALL XERBLA( 'MB01UD', -INFO )
RETURN
END IF
C
C Quick return, if possible.
C
IF ( MIN( M, N ).EQ.0 )
$ RETURN
C
IF( ALPHA.EQ.ZERO ) THEN
C
C Set B to zero and return.
C
CALL DLASET( 'Full', M, N, ZERO, ZERO, B, LDB )
RETURN
END IF
C
C Copy A in B and compute one of the matrix products
C B = alpha*op( triu( H ) ) * A, or
C B = alpha*A * op( triu( H ) ),
C involving the upper triangle of H.
C
CALL DLACPY( 'Full', M, N, A, LDA, B, LDB )
CALL DTRMM( SIDE, 'Upper', TRANS, 'Non-unit', M, N, ALPHA, H,
$ LDH, B, LDB )
C
C Add the contribution of the subdiagonal of H.
C If SIDE = 'L', the subdiagonal of H is swapped with the
C corresponding elements in the first column of H, and the
C calculations are organized for column operations.
C
IF( LSIDE ) THEN
IF( M.GT.2 )
$ CALL DSWAP( M-2, H( 3, 2 ), LDH+1, H( 3, 1 ), 1 )
IF( LTRANS ) THEN
DO 20 J = 1, N
DO 10 I = 1, M - 1
B( I, J ) = B( I, J ) + ALPHA*H( I+1, 1 )*A( I+1, J )
10 CONTINUE
20 CONTINUE
ELSE
DO 40 J = 1, N
DO 30 I = 2, M
B( I, J ) = B( I, J ) + ALPHA*H( I, 1 )*A( I-1, J )
30 CONTINUE
40 CONTINUE
END IF
IF( M.GT.2 )
$ CALL DSWAP( M-2, H( 3, 2 ), LDH+1, H( 3, 1 ), 1 )
C
ELSE
C
IF( LTRANS ) THEN
DO 50 J = 1, N - 1
IF ( H( J+1, J ).NE.ZERO )
$ CALL DAXPY( M, ALPHA*H( J+1, J ), A( 1, J ), 1,
$ B( 1, J+1 ), 1 )
50 CONTINUE
ELSE
DO 60 J = 1, N - 1
IF ( H( J+1, J ).NE.ZERO )
$ CALL DAXPY( M, ALPHA*H( J+1, J ), A( 1, J+1 ), 1,
$ B( 1, J ), 1 )
60 CONTINUE
END IF
END IF
C
RETURN
C *** Last line of MB01UD ***
END
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