James Demmel Yozo Hida E. Jason Riedy Xiaoye S. Li Extra-Precise Iterative Refinement for Overdetermined Least Squares Problems. 2009 35 ACM Trans. Math. Softw. 4 http://doi.acm.org/10.1145/1462173.1462177 db/journals/toms/toms35.html#DemmelHRL09