1. Roy Dubisch, Basic Concepts of Mathematics for Elementary Teachers, Addison-Wesley, 1977, pp. 424–426.
2. Localization in Topology
3. Peter Hilton and Jean Pedersen, Fear No More: An Adult Approach to Mathematics, Addison-Wesley, 1981, Chapter 6, Section 4.
4. Oystein Ore, Number Theory and Its History, McGraw-Hill, 1948, pp. 225–233.
5. Edward L. Spitznagel, Jr., Selected Topics in Mathematics, Holt, Rinehart and Winston, 1971, pp. 122–127.