Lockwood, E. H. (1961). A Book of Curves(PDF). Cambridge University Press. p. 162. Archived(PDF) from the original on 21 February 2017. Retrieved 6 April 2017.
Lockwood, E. H. (1961). A Book of Curves(PDF). Cambridge University Press. p. 162. Archived(PDF) from the original on 21 February 2017. Retrieved 6 April 2017.