# cgeqr2 (l) - Linux Manuals

## NAME

CGEQR2 - computes a QR factorization of a complex m by n matrix A

## SYNOPSIS

SUBROUTINE CGEQR2(
M, N, A, LDA, TAU, WORK, INFO )

INTEGER INFO, LDA, M, N

COMPLEX A( LDA, * ), TAU( * ), WORK( * )

## PURPOSE

CGEQR2 computes a QR factorization of a complex 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) COMPLEX array, dimension (LDA,N)
On entry, the m by n matrix A. On exit, the elements 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, represent the unitary matrix Q as a product of elementary reflectors (see Further Details). LDA (input) INTEGER The leading dimension of the array A. LDA >= max(1,M).
TAU (output) COMPLEX array, dimension (min(M,N))
The scalar factors of the elementary reflectors (see Further Details).
WORK (workspace) COMPLEX array, dimension (N)
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value

## FURTHER DETAILS

The matrix Q is represented as a product of elementary reflectors
H(1) H(2) . . . H(k), where k min(m,n).
Each H(i) has the form

H(i) I - tau vaq
where tau is a complex scalar, and v is a complex 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).