1. Wilhelm Blaschke, Vorlesungen über Differentialgeometrie, vol. 2: Affine Differentialgeometrie, 2nd ed., Springer, Berlin, 1923; reprinted by Chelsea, New York, 1967, pp. 15–16.
2. Joseph Edwards, An Elementary Treatise on the Differential Calculus, 3rd ed., Macmillan, London, 1900, pp. 110, 280–283, 435–436.
3. Francisco Gomes Teixeira, Traité des courbes spéciales remarquables planes et gauches, vol. 1, Paris, 1908–15; reprinted by Chelsea, New York, 1971, pp. 30–37.
4. William C. Graustein, Introduction to Higher Geometry, Macmillan, New York, 1930, p. 248.
5. Heinrich W. Guggenheimer, Differential Geometry, McGraw-Hill, New York, 1963; reprinted by Dover, New York, 1977, pp. 151–153.