# slahr2 (l) - Linux Manuals

## slahr2: 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

## NAME

SLAHR2 - 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## SYNOPSIS

- SUBROUTINE SLAHR2(
- N, K, NB, A, LDA, TAU, T, LDT, Y, LDY )

- INTEGER K, LDA, LDT, LDY, N, NB

- REAL A( LDA, * ), T( LDT, NB ), TAU( NB ), Y( LDY, NB )

## PURPOSE

SLAHR2 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 Qaq * A * Q. The routine returns the matrices V and T which determine Q as a block reflector I - V*T*Vaq, and also the matrix Y = A * V * T. This is an auxiliary routine called by SGEHRD.## ARGUMENTS

- N (input) INTEGER
- The order of the matrix A.
- K (input) INTEGER
- The offset for the reduction. Elements below the k-th subdiagonal in the first NB columns are reduced to zero. K < N.
- NB (input) INTEGER
- The number of columns to be reduced.
- A (input/output) REAL array, dimension (LDA,N-K+1)
- 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 (input) INTEGER The leading dimension of the array A. LDA >= max(1,N).
- TAU (output) REAL array, dimension (NB)
- The scalar factors of the elementary reflectors. See Further Details.
- T (output) REAL array, dimension (LDT,NB)
- The upper triangular matrix T.
- LDT (input) INTEGER
- The leading dimension of the array T. LDT >= NB.
- Y (output) REAL array, dimension (LDY,NB)
- The n-by-nb matrix Y.
- LDY (input) INTEGER
- The leading dimension of the array Y. LDY >= N.

## FURTHER DETAILS

The matrix Q is represented as a product of nb elementary reflectorsQ

Each H(i) has the form

H(i)

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*Vaq) * (A - Y*Vaq).

The contents of A on exit are illustrated by the following example with n = 7, k = 3 and nb = 2:

(

(

(

(

(

(

(

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 file is a slight modification of LAPACK-3.0aqs SLAHRD incorporating improvements proposed by Quintana-Orti and Van de Gejin. Note that the entries of A(1:K,2:NB) differ from those returned by the original LAPACK routine. This function is not backward compatible with LAPACK3.0.