Margaret E. Baron: (1969) , Pergamon Press, republished 2014 Moritz Cantor: . I: (ADB). Band 9, Duncker & Humblot, Leipzig 1879, S. 631–633. Christiaan Huygens: (1651) Examen de la Cyclométrie du três savant Grégoire de Saint-Vincent, , Tome XI, link from Internet Archive. David Eugene Smith: (1923) , Ginn & Co., v.1, p. 425. Hans Wussing: (2008) , S. 433, Springer, .