Hypergeometric $L$-functions in average polynomial time, II

TL;DR

This paper presents an efficient algorithm to compute the mod-$p^e$ reductions of Frobenius traces for hypergeometric motives across all primes up to X, extending previous work to higher powers of p with practical implementation.

Contribution

It introduces a novel algorithm that generalizes previous methods to compute higher power reductions of Frobenius traces in average polynomial time, incorporating $p$-adic transcendental functions.

Findings

Algorithm achieves quasilinear time complexity in X

Includes implementation in SageMath

Extends previous mod-$p$ methods to mod-$p^e$ computations

Abstract

For a fixed positive integer $e$, we describe an algorithm for computing, for all primes $p \leq X$, the mod-$p^e$ reduction of the trace of Frobenius at $p$ of a fixed hypergeometric motive over $\mathbb{Q}$ in time quasilinear in $X$. This extends our previous work for the mod-$p$ reduction, again combining the Beukers--Cohen--Mellit trace formula with average polynomial time techniques of Harvey and Harvey--Sutherland; the key new ingredient is an expanded version of Harvey's "generic prime" construction, making it possible to incorporate certain $p$-adic transcendental functions into the computation. One of these is the $p$-adic Gamma function, whose average polynomial time computation is an intermediate step which may be of independent interest. We also provide an implementation in Sage and discuss the remaining computational issues around computing hypergeometric $L$-series.