↑Korn,Theresa M.;Korn,Granino Arthur(2000),Mathematical Handbook for Scientists and Engineers: Definitions, Theorems, and Formulas for Reference and Review,New York:Dover Publications,pp.157–160,ISBN0-486-41147-8,OCLC43864234
↑Dubrovin,B.A.;A.T. Fomenko, S.P. Novikov(1991),Modern Geometry--Methods and Applications: Part I: The Geometry of Surfaces, Transformation Groups, and Fields (Graduate Texts in Mathematics)(2nd ed.),Springer,pp.14–17,ISBN978-0-387-97663-1