In mathematics, a sequence of positive integers an is called an irrationality sequence if it has the property that for every sequence xn of positive integers, the sum of the series
exists (that is, it converges) and is an irrational number. The problem of characterizing irrationality sequences was posed by Paul Erdős and Ernst G. Straus, who originally called the property of being an irrationality sequence "Property P".
- 2, 3, 7, 43, 1807, 3263443, ...
(in which each term is one more than the product of all previous terms) also grows doubly exponentially, it does not form an irrationality sequence. For, letting gives
a series converging to a rational number. Likewise, the factorials, , do not form an irrationality sequence because the sequence leads to a series with a rational sum,
Any sequence an that grows at a rate such that
is an irrationality sequence. This includes sequences that grow at a more than doubly exponential rate as well as some doubly exponential sequences that grow more quickly than the powers of powers of two.
Every irrationality sequence must grow quickly enough that
However, it is not known whether there exists such a sequence in which the greatest common divisor of each pair of terms is 1 (unlike the powers of powers of two) and for which
Analogously to irrationality sequences, Hančl (1996) has defined a transcendental sequence to be an integer sequence an such that, for every sequence xn of positive integers, the sum of the series
- Guy, Richard K. (2004), "E24 Irrationality sequences", Unsolved problems in number theory (3rd ed.), Springer-Verlag, p. 346, ISBN 0-387-20860-7, Zbl 1058.11001.
- Erdős, P.; Graham, R. L. (1980), Old and new problems and results in combinatorial number theory, Monographies de L'Enseignement Mathématique 28, Geneva: Université de Genève L'Enseignement Mathématique, p. 128, MR 592420.
- Erdős, P. (1975), "Some problems and results on the irrationality of the sum of infinite series" (PDF), Journal of Mathematical Sciences 10: 1–7 (1976), MR 539489.
- Erdős, P. (1988), "On the irrationality of certain series: problems and results", New advances in transcendence theory (Durham, 1986) (PDF), Cambridge: Cambridge Univ. Press, pp. 102–109, MR 971997.
- Hančl, Jaroslav (1996), "Transcendental sequences", Mathematica Slovaca 46 (2-3): 177–179, MR 1427003.