
slahr2.f(3) LAPACK slahr2.f(3)
NAME
slahr2.f 
SYNOPSIS
Functions/Subroutines
subroutine slahr2 (N, K, NB, A, LDA, TAU, T, LDT, Y, LDY)
SLAHR2 reduces the specified number of first columns of a general rectangular matrix A
so that elements below the specified subdiagonal are zero, and returns auxiliary
matrices which are needed to apply the transformation to the unreduced part of A.
Function/Subroutine Documentation
subroutine slahr2 (integerN, integerK, integerNB, real, dimension( lda, * )A, integerLDA,
real, dimension( nb )TAU, real, dimension( ldt, nb )T, integerLDT, real, dimension( ldy,
nb )Y, integerLDY)
SLAHR2 reduces the specified number of first columns of a general rectangular matrix A so
that elements below the specified subdiagonal are zero, and returns auxiliary matrices
which are needed to apply the transformation to the unreduced part of A.
Purpose:
SLAHR2 reduces the first NB columns of A real general nBY(nk+1)
matrix A so that elements below the kth 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 auxiliary routine called by SGEHRD.
Parameters:
N
N is INTEGER
The order of the matrix A.
K
K is INTEGER
The offset for the reduction. Elements below the kth
subdiagonal in the first NB columns are reduced to zero.
K < N.
NB
NB is INTEGER
The number of columns to be reduced.
A
A is REAL array, dimension (LDA,NK+1)
On entry, the nby(nk+1) general matrix A.
On exit, the elements on and above the kth subdiagonal in
the first NB columns are overwritten with the corresponding
elements of the reduced matrix; the elements below the kth
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
LDA is INTEGER
The leading dimension of the array A. LDA >= max(1,N).
TAU
TAU is REAL array, dimension (NB)
The scalar factors of the elementary reflectors. See Further
Details.
T
T is REAL array, dimension (LDT,NB)
The upper triangular matrix T.
LDT
LDT is INTEGER
The leading dimension of the array T. LDT >= NB.
Y
Y is REAL array, dimension (LDY,NB)
The nbynb matrix Y.
LDY
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+k1) = 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 (nk+1)bynb 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 a a a a )
( a a a a a )
( a a 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).
This subroutine is a slight modification of LAPACK3.0's DLAHRD
incorporating improvements proposed by QuintanaOrti and Van de
Gejin. Note that the entries of A(1:K,2:NB) differ from those
returned by the original LAPACK3.0's DLAHRD routine. (This
subroutine is not backward compatible with LAPACK3.0's DLAHRD.)
References:
Gregorio QuintanaOrti and Robert van de Geijn, 'Improving the
performance of reduction to Hessenberg form,' ACM Transactions on Mathematical
Software, 32(2):180194, June 2006.
Definition at line 182 of file slahr2.f.
Author
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Version 3.4.2 Tue Sep 25 2012 slahr2.f(3) 
