| // Copyright ©2016 The gonum Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| package gonum |
| |
| import "gonum.org/v1/gonum/blas" |
| |
| // Dgeql2 computes the QL factorization of the m×n matrix A. That is, Dgeql2 |
| // computes Q and L such that |
| // A = Q * L |
| // where Q is an m×m orthonormal matrix and L is a lower trapezoidal matrix. |
| // |
| // Q is represented as a product of elementary reflectors, |
| // Q = H_{k-1} * ... * H_1 * H_0 |
| // where k = min(m,n) and each H_i has the form |
| // H_i = I - tau[i] * v_i * v_i^T |
| // Vector v_i has v[m-k+i+1:m] = 0, v[m-k+i] = 1, and v[:m-k+i+1] is stored on |
| // exit in A[0:m-k+i-1, n-k+i]. |
| // |
| // tau must have length at least min(m,n), and Dgeql2 will panic otherwise. |
| // |
| // work is temporary memory storage and must have length at least n. |
| // |
| // Dgeql2 is an internal routine. It is exported for testing purposes. |
| func (impl Implementation) Dgeql2(m, n int, a []float64, lda int, tau, work []float64) { |
| checkMatrix(m, n, a, lda) |
| if len(tau) < min(m, n) { |
| panic(badTau) |
| } |
| if len(work) < n { |
| panic(badWork) |
| } |
| k := min(m, n) |
| var aii float64 |
| for i := k - 1; i >= 0; i-- { |
| // Generate elementary reflector H_i to annihilate A[0:m-k+i-1, n-k+i]. |
| aii, tau[i] = impl.Dlarfg(m-k+i+1, a[(m-k+i)*lda+n-k+i], a[n-k+i:], lda) |
| |
| // Apply H_i to A[0:m-k+i, 0:n-k+i-1] from the left. |
| a[(m-k+i)*lda+n-k+i] = 1 |
| impl.Dlarf(blas.Left, m-k+i+1, n-k+i, a[n-k+i:], lda, tau[i], a, lda, work) |
| a[(m-k+i)*lda+n-k+i] = aii |
| } |
| } |