1. Courant, R., John, F. (1965). Introduction to Calculus and Analysis, Vol. 1. New York: Wiley, p. 31.
2. Protter, M. H., Morrey, Jr., C. B. (1964). Calculus With Analytic Geometry, 1st ed. Reading, MA: Addison-Wesley, pp. 108–111.
3. Thomas, G. B. (1972). Calculus and Analytic Geometry, alternate ed. Reading, MA: Addison-Wesley, pp. 99–105.