---
name: "dgelq2"
section: "3lapack"
platform: "linux"
package: "liblapack-doc"
version: "3.2.1-8"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/dgelq2/3lapack"
---

# dgelq2(3lapack)

computes an LQ factorization of a real m by n matrix A

## NAME

DGELQ2 - computes an LQ factorization of a real m by n matrix A

## SYNOPSIS

- SUBROUTINE DGELQ2(

  M, N, A, LDA, TAU, WORK, INFO )

- INTEGER

  INFO, LDA, M, N

- DOUBLE

  PRECISION A( LDA, \* ), TAU( \* ), WORK( \* )

## PURPOSE

DGELQ2 computes an LQ factorization of a real m by n matrix A: A = L \* Q.\

## 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 elements on and below the diagonal of the array contain the m by min(m,n) lower trapezoidal matrix L (L is lower triangular if m \<= n); the elements above the diagonal, with the array TAU, represent the orthogonal 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) DOUBLE PRECISION array, dimension (min(M,N))

  The scalar factors of the elementary reflectors (see Further Details).

- WORK (workspace) DOUBLE PRECISION array, dimension (M)

  - 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\
    Q = H(k) . . . H(2) H(1), 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:n) is stored on exit in A(i,i+1:n), and tau in TAU(i).\