| // Copyright ©2017 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" |
| |
| // Dgerq2 computes an RQ factorization of the m×n matrix A, |
| // A = R * Q. |
| // On exit, if m <= n, the upper triangle of the subarray |
| // A[0:m, n-m:n] contains the m×m upper triangular matrix R. |
| // If m >= n, the elements on and above the (m-n)-th subdiagonal |
| // contain the m×n upper trapezoidal matrix R. |
| // The remaining elements, with tau, represent the |
| // orthogonal matrix Q as a product of min(m,n) elementary |
| // reflectors. |
| // |
| // The matrix Q is represented as a product of elementary reflectors |
| // Q = H_0 H_1 . . . H_{min(m,n)-1}. |
| // Each H(i) has the form |
| // H_i = I - tau_i * v * v^T |
| // where v is a vector with v[0:n-k+i-1] stored in A[m-k+i, 0:n-k+i-1], |
| // v[n-k+i:n] = 0 and v[n-k+i] = 1. |
| // |
| // tau must have length min(m,n) and work must have length m, otherwise |
| // Dgerq2 will panic. |
| // |
| // Dgerq2 is an internal routine. It is exported for testing purposes. |
| func (impl Implementation) Dgerq2(m, n int, a []float64, lda int, tau, work []float64) { |
| checkMatrix(m, n, a, lda) |
| k := min(m, n) |
| if len(tau) < k { |
| panic(badTau) |
| } |
| if len(work) < m { |
| panic(badWork) |
| } |
| |
| for i := k - 1; i >= 0; i-- { |
| // Generate elementary reflector H[i] to annihilate |
| // A[m-k+i, 0:n-k+i-1]. |
| mki := m - k + i |
| nki := n - k + i |
| var aii float64 |
| aii, tau[i] = impl.Dlarfg(nki+1, a[mki*lda+nki], a[mki*lda:], 1) |
| |
| // Apply H[i] to A[0:m-k+i-1, 0:n-k+i] from the right. |
| a[mki*lda+nki] = 1 |
| impl.Dlarf(blas.Right, mki, nki+1, a[mki*lda:], 1, tau[i], a, lda, work) |
| a[mki*lda+nki] = aii |
| } |
| } |