Sparse QR factorization in MATLAB

Abstract

In the recently presented sparse matrix extension of MATLAB, there is no routine for sparse QR factorization. Sparse linear least-squares problems are instead solved by the augmented system method. The accuracy in computed solutions is strongly dependent on a scaling parameter δ. Its optimal value is expensive to compute, and it must therefore be approximated by a simple heuristic. We describe a multifrontal method for sparse QR factorization and its implementation in MATLAB. It is well known that the multifrontal approach is suitable for vector machines. We show that it is also attractive in MATLAB. In both cases, scalar operations are expensive, and the reformulation of the sparse problem into dense subproblems is advantageous. Using the new routine, we implement two methods for the solution of sparse linear least-squares problems and compare these with the built-in MATLAB function. We show that the QR-based methods normally are much faster and more accurate than the MATLAB implementation of the augmented system method. A better choice of the parameter δ or iterative refinement must be used to make the augmented system method as accurate as the methods based on QR factorization.