Apéry-Style Irrationality Proofs via Linear Recurrences

UnsolvedNumber theory
Construction - Finite
Breakthrough

About the problem

In 1979, Apéry proved that \(\zeta(3)\) is irrational. At its core, the proof uses a particular recurrence relation, shown below. When initialized two different ways, this gives a pair of series that can be configured to converge “quickly” to \(\zeta(3)\). This fast convergence suffices for a proof of irrationality.

\[n^3 u_n = (34n^3- 51n^2 + 27n - 5) u_{n-1} - (n-1)^3 u_{n-2}\]

The aim of this problem is to find analogous recurrences and initializations that can be used to prove the irrationality of other “famous” constants.

Prompt

The prompt is posed independently for each of the following target constants: ζ(5), ζ(7), Catalan's constant, the Calegari-Dimitrov-Tang constant, and the Euler-Mascheroni constant γ.

The goal of this problem is to prove the irrationality of a particular real number in a specific way.

Let $P_0(x), \ldots, P_k(x)$ be polynomials with integer coefficients, where $P_k(n) \neq 0$ for $n \geq k$. Let $\alpha = (\alpha_0, \ldots, \alpha_{k-1})$ and $\beta = (\beta_0, \ldots, \beta_{k-1})$ be tuples of integers.

Let $(a_n)$ and $(b_n)$ denote the sequences defined by the recurrence
$$P_k(n)u_n = P_{k-1}(n)u_{n-1} + \cdots + P_0(n)u_{n-k}$$
for $n \geq k$, when initialized with $\alpha$ and $\beta$ respectively.

Let $L_n$ denote $\operatorname{lcm}(1,2,\ldots,n)$ and let $B_n = (L_n)^m b_n$, where $m$ is a fixed positive integer.

Let $X$ be a real number. It is known that if the following hold, then $X$ is irrational.

1. $a_n$ is an integer for all $n \geq 0$.

2. $B_n$ is an integer for all $n \geq 0$.

3. $X \neq b_n / a_n$ for all $n \geq 0$.

4. $\lvert b_n - a_n X\rvert \leq C\rho^n e^{-mn}$ for all $n \geq 0$, where $C$ and $\rho$ are positive real numbers and $\rho < 1$.

Your task is to find $P_i$, $\alpha$, $\beta$, and $m$ that establish the irrationality of ___ in this way.