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# sgeqr2

```
NAME
SGEQR2 - compute a QR factorization of a real m by n matrix
A

SYNOPSIS
SUBROUTINE SGEQR2( M, N, A, LDA, TAU, WORK, INFO )

INTEGER        INFO, LDA, M, N

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

PURPOSE
SGEQR2 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) REAL 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, represent the orthogo-
nal 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) REAL array, dimension (min(M,N))
The scalar factors of the elementary reflectors (see
Further Details).

WORK    (workspace) REAL 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

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).
```