Canonicalizing zeta generators: genus zero and genus one

TL;DR

This paper develops a canonical framework for zeta generators in genus zero and one, linking motivic multizeta values, elliptic associators, and modular forms, with explicit high-order computations and resolution of ambiguities.

Contribution

It introduces a canonical choice of polynomials for zeta generators, establishing a systematic connection between genus-zero and genus-one cases and resolving previous ambiguities.

Findings

Canonical isomorphism mapping motivic multizeta values to the f-alphabet

Explicit high-order expansions of genus-one zeta generators

Resolution of ambiguities in non-geometric genus-one zeta generators

Abstract

Zeta generators are derivations associated with odd Riemann zeta values that act freely on the Lie algebra of the fundamental group of Riemann surfaces with marked points. The genus-zero incarnation of zeta generators are Ihara derivations of certain Lie polynomials in two generators that can be obtained from the Drinfeld associator. We characterize a canonical choice of these polynomials, together with their non-Lie counterparts at even degrees $w\geq 2$, through the action of the dual space of formal and motivic multizeta values. Based on these canonical polynomials, we propose a canonical isomorphism that maps motivic multizeta values into the $f$-alphabet. The canonical Lie polynomials from the genus-zero setup determine canonical zeta generators in genus one that act on the two generators of Enriquez' elliptic associators. Up to a single contribution at fixed degree, the zeta generators in genus one are systematically expanded in terms of Tsunogai's geometric derivations dual to holomorphic Eisenstein series, leading to a wealth of explicit high-order computations. Earlier ambiguities in defining the non-geometric part of genus-one zeta generators are resolved by imposing a new representation-theoretic condition. The tight interplay between zeta generators in genus zero and genus one unravelled in this work connects the construction of single-valued multiple polylogarithms on the sphere with iterated-Eisenstein-integral representations of modular graph forms.