dgeqr2(3)

NAME

DGEQR2 - compute a QR factorization of a real m by n ma

trix A

SYNOPSIS

SUBROUTINE DGEQR2( M, N, A, LDA, TAU, WORK, INFO )
    INTEGER        INFO, LDA, M, N
    DOUBLE          PRECISION A( LDA, * ), TAU( * ), WORK(
* )

PURPOSE

DGEQR2 computes a QR factorization of a real m by n matrix

A: A = Q * R.

ARGUMENTS

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.

A (input/output) DOUBLE PRECISION array, dimension

(LDA,N)

On entry, the m by n matrix A. On exit, the ele

ments on and above the diagonal of the array contain the min(m,n)

by n upper trapezoidal matrix R (R is upper triangular if m >=

n); the elements below the diagonal, with the array TAU, repre

sent the orthogonal matrix Q as a product of elementary reflec

tors (see Further Details). LDA (input) INTEGER The leading

dimension of the array A. LDA >= max(1,M).

TAU (output) DOUBLE PRECISION array, dimension

(min(M,N))

The scalar factors of the elementary reflectors

(see Further Details).

WORK (workspace) DOUBLE PRECISION array, dimension (N)

INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = -i, the i-th argument had an ille

gal value

FURTHER DETAILS

The matrix Q is represented as a product of elementary re

flectors

Q = H(1) H(2) . . . H(k), where k = min(m,n).

Each H(i) has the form

H(i) = I - tau * v * v'

where tau is a real scalar, and v is a real vector with
v(1:i-1) = 0 and v(i) = 1; v(i+1:m) is stored on exit in

A(i+1:m,i), and tau in TAU(i).

LAPACK version 3.0 15 June 2000

DGEQR2(3)