dgelqf (l) - Linux Man Pages
dgelqf: computes an LQ factorization of a real M-by-N matrix A
NAMEDGELQF - computes an LQ factorization of a real M-by-N matrix A
- SUBROUTINE DGELQF(
- M, N, A, LDA, TAU, WORK, LWORK, INFO )
- INTEGER INFO, LDA, LWORK, M, N
- DOUBLE PRECISION A( LDA, * ), TAU( * ), WORK( * )
PURPOSEDGELQF 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) 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/output) DOUBLE PRECISION array, dimension (MAX(1,LWORK))
- On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
- LWORK (input) INTEGER
- The dimension of the array WORK. LWORK >= max(1,M). For optimum performance LWORK >= M*NB, where NB is the optimal blocksize. If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA.
- INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
FURTHER DETAILSThe matrix Q is represented as a product of elementary reflectors
Each H(i) has the form
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).