Approximate Generalized Inverse Preconditioning Methods for Least Squares Problems

iv erative methods to solve least squares problems more efficiently. We especially focused on one kind of preconditioners, in which preconditioners are the approximate generalized inverses of the coefficient matrices of the least squares problems. We proposed two different approaches for how to construct the approximate generalized inverses of the coefficient matrices: one is based on the Minimal Residual method with the steepest descend direction, and the other is based on the Greville’s Method which is an old method developed for computing the generalized inverse based on the rank-one update. And for these two preconditioners, we also discuss how to apply them to least squares problems. Both theoretical issues and practical implementation issues about the preconditioning are discussed in this thesis. Our numerical tests showed that our methods performed competitively rank deficient ill-conditioned problems. As an example of problems from the real world, we apply our preconditioners to the linear programming problems, where many large-scale sparse least squares problems with rank deficient coefficient matrices arise. Our numerical tests showed that our methods showed more robustness than the Cholesky decomposition method.