Purpose
To compute one of the matrix products
A : = alpha*op( H ) * A, or A : = alpha*A * op( H ),
where alpha is a scalar, A is an m-by-n matrix, H is an upper
Hessenberg matrix, and op( H ) is one of
op( H ) = H or op( H ) = H', the transpose of H.
Specification
SUBROUTINE MB01UW( SIDE, TRANS, M, N, ALPHA, H, LDH, A, LDA,
$ DWORK, LDWORK, INFO )
C .. Scalar Arguments ..
CHARACTER SIDE, TRANS
INTEGER INFO, LDA, LDH, LDWORK, M, N
DOUBLE PRECISION ALPHA
C .. Array Arguments ..
DOUBLE PRECISION A(LDA,*), DWORK(*), H(LDH,*)
Arguments
Mode Parameters
SIDE CHARACTER*1
Specifies whether the Hessenberg matrix H appears on the
left or right in the matrix product as follows:
= 'L': A := alpha*op( H ) * A;
= 'R': A := alpha*A * op( H ).
TRANS CHARACTER*1
Specifies the form of op( H ) to be used in the matrix
multiplication as follows:
= 'N': op( H ) = H;
= 'T': op( H ) = H';
= 'C': op( H ) = H'.
Input/Output Parameters
M (input) INTEGER
The number of rows of the matrix A. M >= 0.
N (input) INTEGER
The number of columns of the matrix A. N >= 0.
ALPHA (input) DOUBLE PRECISION
The scalar alpha. When alpha is zero then H is not
referenced and A need not be set before entry.
H (input) DOUBLE PRECISION array, dimension (LDH,k)
where k is M when SIDE = 'L' and is N when SIDE = 'R'.
On entry with SIDE = 'L', the leading M-by-M upper
Hessenberg part of this array must contain the upper
Hessenberg matrix H.
On entry with SIDE = 'R', the leading N-by-N upper
Hessenberg part of this array must contain the upper
Hessenberg matrix H.
The elements below the subdiagonal are not referenced,
except possibly for those in the first column, which
could be overwritten, but are restored on exit.
LDH INTEGER
The leading dimension of the array H. LDH >= max(1,k),
where k is M when SIDE = 'L' and is N when SIDE = 'R'.
A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
On entry, the leading M-by-N part of this array must
contain the matrix A.
On exit, the leading M-by-N part of this array contains
the computed product.
LDA INTEGER
The leading dimension of the array A. LDA >= max(1,M).
Workspace
DWORK DOUBLE PRECISION array, dimension (LDWORK)
On exit, if INFO = 0, alpha <> 0, and LDWORK >= M*N > 0,
DWORK contains a copy of the matrix A, having the leading
dimension M.
This array is not referenced when alpha = 0.
LDWORK The length of the array DWORK.
LDWORK >= 0, if alpha = 0 or MIN(M,N) = 0;
LDWORK >= M-1, if SIDE = 'L';
LDWORK >= N-1, if SIDE = 'R'.
For maximal efficiency LDWORK should be at least M*N.
Error Indicator
INFO INTEGER
= 0: successful exit;
< 0: if INFO = -i, the i-th argument had an illegal
value.
Method
The required matrix product is computed in two steps. In the first step, the upper triangle of H is used; in the second step, the contribution of the subdiagonal is added. If the workspace can accomodate a copy of A, a fast BLAS 3 DTRMM operation is used in the first step.Further Comments
NoneExample
Program Text
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