Ap\'ery-like sequences defined by four-term recurrence relations

Abstract

The Ap\'ery numbers may be defined by a cubic three-term recurrence relation, that is, a three-term relation where the coefficients are polynomials in the index of degree $3$. In this work, we first provide a systematic review of Ap\'ery numbers and other related sequences that satisfy quadratic or cubic three-term recurrence relations, and show how they are interrelated and how they may be classified. This leads to sequences defined by cubic $k$-term recurrence relations. The cases corresponding to $k=2$ in this framework lead to Ramanujan's theories of elliptic functions to alternative bases, while the cases corresponding to $k=3$ correspond to the Ap\'ery, Domb, Almkvist--Zudilin numbers and other sequences that are well-studied. We conduct a detailed analysis for the case $k=4$. Some of the sequences that arise are new. Of particular interest are ten sequences that are said to be self-starting in the sense that a single initial condition is enough to start the recurrence relation. Of additional interest are two sequences which take values in $\mathbb{Z}[i]$ and two others with values in $\mathbb{Z}[\sqrt{2}]$. Congruence properties and asymptotic expansions for the ten self-starting sequences are investigated and several conjectures are presented. For example, we conjecture that the integer-valued sequence defined by the recurrence relation \begin{align*} (n+1)^3T(n+1) &=2(2n+1)(5n^2+5n+2)T(n) \\ &\qquad -8n(7n^2+1)T(n-1)+22n(2n-1)(n-1)T(n-2) \end{align*} and initial condition $T(0)=1$ satisfies a Lucas congruence for every prime $p$. Moreover, the sequence is conjectured to satisfy the supercongruence $$ T(pn) \equiv T(n) \pmod{p^2} \quad\text{for all positive integers $n$} $$ if $p=2,\;59$ or $5581$, and for no other primes $p<10^4$.