Numerical and computational aspects of direct methods for large and sparse least squares problems are considered. After a brief survey of the most often used methods, we summarize the important conclusions made from a numerical comparison in MATLAB. Significantly improved algorithms have during the last 10–15 years made sparse QR factorization attractive, and competitive to previously recommended alternatives. Of particular importance is the multifrontal approach, characterized by low fill-in, dense subproblems and naturally implemented parallelism. We describe a Householder multifrontal scheme and its implementation on sequential and parallel computers. Available software has in practice a great influence on the choice of numerical algorithms. Less appropriate algorithms are thus often used solely because of existing software packages. We briefly survey software packages for the solution of sparse linear least squares problems. Finally, we focus on various applications from optimization, leading to the solution of large and sparse linear least squares problems. In particular, we concentrate on the important case where the coefficient matrix is a fixed general sparse matrix with a variable diagonal matrix below. Inner point methods for constrained linear least squares problems, for example, give rise to such subproblems. Important gains can be made by taking advantage of structure. Closely related is also the choice of numerical method for these subproblems. We discuss why the less accurate normal equations tend to be sufficient in many applications.
