This paper appeared in: Princeton Companion to Applied Mathematics, (N.J.Higham, ed.), Princeton University Press 2015, 804-807

This paper appeared in: Princeton Companion to Applied Mathematics, (N.J.Higham, ed.), Princeton University Press 2015, 804-807

This paper appeared in: Princeton Companion to Applied Mathematics, (N.J.Higham, ed.), Princeton University Press 2015, 804-807

This paper appeared in: Princeton Companion to Applied Mathematics, (N.J.Higham, ed.), Princeton University Press 2015, 804-807

This paper appeared in: Princeton Companion to Applied Mathematics, (N.J.Higham, ed.), Princeton University Press 2015, 804-807

This paper appeared in: Princeton Companion to Applied Mathematics, (N.J.Higham, ed.), Princeton University Press 2015, 804-807