In linear algebra, a QR decomposition (also called a QR factorization) of a matrix is a decomposition of a matrix A into a product A = QR of an orthogonal matrix Q and an upper triangular matrix R. QR decomposition is often used to solve the linear least squares problem, and is the basis for a particular eigen value algorithm, the QR algorithm. If A has n linearly independent columns, then the first n columns of Q form an orthonormal basis for the column space of A. More specifically, the first k columns of Q form an orthonormal basis for the span of the first k columns of A for any 1 ≤ k ≤ n. The fact that any column k of A only depends on the first k columns of Q is responsible for the triangular form of R.
Gnaneswar Nadh satapathi (2021). Q-R decomposition with positive diagonals of R Matrix (https://www.mathworks.com/matlabcentral/fileexchange/49807-q-r-decomposition-with-positive-diagonals-of-r-matrix), MATLAB Central File Exchange. Retrieved .
If goal is to determine a QR decomposition that enforces positive diagonals (making solution unique), I suspect the following is a much simpler approach:
[Q,R] = qr(A);
D = diag(sign(diag(R)));
Qunique = Q*D;
Runique = D*R;
(posted by Richard Brown here: https://www.mathworks.com/matlabcentral/answers/83798-sign-differences-in-qr-decomposition#answer_93385)
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