Computing Igusa's local zeta function of univariates in deterministic polynomial-time

TL;DR

This paper provides a constructive, elementary proof that Igusa's local zeta function for univariate polynomials is rational, and introduces a deterministic polynomial-time algorithm to compute it, improving on previous methods.

Contribution

It offers the first deterministic polynomial-time algorithm for computing Igusa's local zeta function for univariate polynomials, based on a new elementary proof of its rationality.

Findings

Elementary proof of rationality for univariate Igusa's zeta function

Constructive closed-form expression for root counts $N_k(f)$

Deterministic poly-time algorithm for computing $Z_{f,p}(s)"

Abstract

Igusa's local zeta function $Z_{f,p}(s)$ is the generating function that counts the number of integral roots, $N_{k}(f)$, of $f(\mathbf x) \bmod p^k$, for all $k$. It is a famous result, in analytic number theory, that $Z_{f,p}$ is a rational function in $\mathbb{Q}(p^s)$. We give an elementary proof of this fact for a univariate polynomial $f$. Our proof is constructive as it gives a closed-form expression for the number of roots $N_{k}(f)$. Our proof, when combined with the recent root-counting algorithm of (Dwivedi, Mittal, Saxena, CCC, 2019), yields the first deterministic poly($|f|, \log p$) time algorithm to compute $Z_{f,p}(s)$. Previously, an algorithm was known only in the case when $f$ completely splits over $\mathbb{Q}_p$; it required the rational roots to use the concept of generating function of a tree (Z\'u\~niga-Galindo, J.Int.Seq., 2003).