dlahrd.f (3) Linux Manual Page
dlahrd.f –
Synopsis
Functions/Subroutines
subroutine dlahrd (N, K, NB, A, LDA, TAU, T, LDT, Y, LDY)DLAHRD reduces the first nb columns of a general rectangular matrix A so that elements below the k-th subdiagonal are zero, and returns auxiliary matrices which are needed to apply the transformation to the unreduced part of A.
Function/Subroutine Documentation
subroutine dlahrd (integerN, integerK, integerNB, double precision, dimension( lda, * )A, integerLDA, double precision, dimension( nb )TAU, double precision, dimension( ldt, nb )T, integerLDT, double precision, dimension( ldy, nb )Y, integerLDY)
DLAHRD reduces the first nb columns of a general rectangular matrix A so that elements below the k-th subdiagonal are zero, and returns auxiliary matrices which are needed to apply the transformation to the unreduced part of A. Purpose:
DLAHRD reduces the first NB columns of a real general n-by-(n-k+1)
matrix A so that elements below the k-th subdiagonal are zero. The
reduction is performed by an orthogonal similarity transformation
Q**T * A * Q. The routine returns the matrices V and T which determine
Q as a block reflector I – V*T*V**T, and also the matrix Y = A * V * T.
This is an OBSOLETE auxiliary routine.
This routine will be ‘deprecated’ in a future release.
Please use the new routine DLAHR2 instead.
Parameters:
- N
N is INTEGER
K
The order of the matrix A.K is INTEGER
NB
The offset for the reduction. Elements below the k-th
subdiagonal in the first NB columns are reduced to zero.NB is INTEGER
A
The number of columns to be reduced.A is DOUBLE PRECISION array, dimension (LDA,N-K+1)
LDA
On entry, the n-by-(n-k+1) general matrix A.
On exit, the elements on and above the k-th subdiagonal in
the first NB columns are overwritten with the corresponding
elements of the reduced matrix; the elements below the k-th
subdiagonal, with the array TAU, represent the matrix Q as a
product of elementary reflectors. The other columns of A are
unchanged. See Further Details.LDA is INTEGER
TAU
The leading dimension of the array A. LDA >= max(1,N).TAU is DOUBLE PRECISION array, dimension (NB)
T
The scalar factors of the elementary reflectors. See Further
Details.T is DOUBLE PRECISION array, dimension (LDT,NB)
LDT
The upper triangular matrix T.LDT is INTEGER
Y
The leading dimension of the array T. LDT >= NB.Y is DOUBLE PRECISION array, dimension (LDY,NB)
LDY
The n-by-nb matrix Y.LDY is INTEGER
The leading dimension of the array Y. LDY >= N.
Author:
- Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
- September 2012
Further Details:
The matrix Q is represented as a product of nb elementary reflectors
Q = H(1) H(2) . . . H(nb).
Each H(i) has the form
H(i) = I – tau * v * v**T
where tau is a real scalar, and v is a real vector with
v(1:i+k-1) = 0, v(i+k) = 1; v(i+k+1:n) is stored on exit in
A(i+k+1:n,i), and tau in TAU(i).
The elements of the vectors v together form the (n-k+1)-by-nb matrix
V which is needed, with T and Y, to apply the transformation to the
unreduced part of the matrix, using an update of the form:
A := (I – V*T*V**T) * (A – Y*V**T).
The contents of A on exit are illustrated by the following example
with n = 7, k = 3 and nb = 2:
( a h a a a )
( a h a a a )
( a h a a a )
( h h a a a )
( v1 h a a a )
( v1 v2 a a a )
( v1 v2 a a a )
where a denotes an element of the original matrix A, h denotes a
modified element of the upper Hessenberg matrix H, and vi denotes an
element of the vector defining H(i).
Definition at line 170 of file dlahrd.f.