1. G. H.Hardy and E. M.Wright, An Introduction to the Theory of Numbers, Fourth Ed., Oxford, 1954.
2. L. E.Dickson, History of the Theory of Numbers, Vol. 1, (Divisibility and Primality), Chelsea, 1952, (Chapter 3).
3. J. E. Rubin, Set Theory for the Mathematician, Holden-Day, 1967.
4. C. T. Long, Elementary Introduction to Number Theory, D. C. Heath, 1965.
5. J. F.Scott, A History of Mathematics, Taylor and Francis, London, 1958, (p. 185).