Irrationality sequence

In mathematics, a sequence of positive integers a_n is called an irrationality sequence if it has the property that, for every sequence x_n of positive integers, the sum of the series

$\sum_{n=1}^\infty \frac{1}{a_n x_n}$

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".

Examples

The powers of two whose exponents are powers of two, $2^{2^n}$, form an irrationality sequence. However, although Sylvester's sequence

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 $x_n=1$ gives

$\frac{1}{2}+\frac{1}{3}+\frac{1}{7}+\frac{1}{43}+\cdots=1,$

a series converging to a rational number. Likewise, the factorials $n!$ do not form an irrationality sequence, because the sequence $x_n=n+2$ leads to a series with a rational sum,

$\sum_{n=0}^{\infty}\frac{1}{(n+2)n!}=\frac{1}{2}+\frac{1}{3}+\frac{1}{8}+\frac{1}{30}+\frac{1}{144}+\cdots=1.$

Growth rate

Any sequence a_n that grows at a rate such that

$\limsup_n \frac{\log\log a_n}{n} > \log 2$

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

$\lim_{n\to\infty} a_n^{1/n}=\infty.$

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

$\lim_{n\to\infty} a_n^{1/2^n}<\infty.$

Related properties

Analogously to irrationality sequences, Hančl (1996) has defined a transcendental sequence to be an integer sequence a_n such that, for every sequence x_n of positive integers, the sum of the series

$\sum_{n=1}^\infty \frac{1}{a_n x_n}$

exists and is an transcendental number.