man sgelq2 (Fonctions bibliothèques) - compute an LQ factorization of a real m by n matrix A

NAME

SGELQ2 - compute an LQ factorization of a real m by n matrix A

SGELQ2 computes an LQ factorization of a real m by n matrix A: A = L * Q.

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 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) REAL array, dimension (min(M,N)): The scalar factors of the elementary reflectors (see Further Details).
WORK (workspace) REAL array, dimension (M)
INFO (output) INTEGER: = 0: successful exit

< 0: if INFO = -i, the i-th argument had an illegal value

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).