1. T. M. Apostol, Mathematical Analysis, 2nd edition, Addison-Wesley Publishing Co., Reading, MA, 1974, p. 7.
2. G. H. Hardy and F. M. Wright, An Introduction to the Theory of Numbers, 4th edition, Oxford University Press, Fair Lawn, NJ, 1971, p. 46.
3. 69.27 The irrationality of e and others
4. I. Niven, Irrational Numbers, Carus Mathematical Monographs 11, MAA, 1967, pp. 7–11.
5. G. Pólya and G. Szegő, Problems and Theorems in Analysis, Vol. 2, Springer-Verlag, New York, 1976, p. 154 (problems 258, 259), pp. 361–2.