O'Connor and Robertson, J. J. and E. F. Florian Cajori. The MacTutor History of Mathematics archive, University of S. Andrews, Scotland. [5 July 2012]. (原始内容存档于2015-02-02).
O'Connor and Robertson, J. J. and E. F. Florian Cajori. The MacTutor History of Mathematics archive, University of S. Andrews, Scotland. [5 July 2012]. (原始内容存档于2015-02-02).