# On a conjecture for $\ell$-torsion in class groups of number fields: from the perspective of moments

## TL;DR

This paper explores the conjecture that the $ ext{l}$-torsion subgroup in class groups of number fields is very small, analyzing its connections with various counting problems and emphasizing the utility of the method of moments.

## Contribution

It explicitly relates the $ ext{l}$-torsion conjecture to other major conjectures and counting problems, highlighting the role of moments in understanding these relationships.

## Key findings

- Reinforces the belief that the $ ext{l}$-torsion conjecture is true.
- Establishes quantitative links between $ ext{l}$-torsion bounds and counting problems.
- Demonstrates the broad utility of the method of moments in number theory.

## Abstract

It is conjectured that within the class group of any number field, for every integer $\ell \geq 1$, the $\ell$-torsion subgroup is very small (in an appropriate sense, relative to the discriminant of the field). In nearly all settings, the full strength of this conjecture remains open, and even partial progress is limited. Significant recent progress toward average versions of the $\ell$-torsion conjecture has crucially relied on counts for number fields, raising interest in how these two types of question relate. In this paper we make explicit the quantitative relationships between the $\ell$-torsion conjecture and other well-known conjectures: the Cohen-Lenstra heuristics, counts for number fields of fixed discriminant, counts for number fields of bounded discriminant (or related invariants), and counts for elliptic curves with fixed conductor. All of these considerations reinforce that we expect the $\ell$-torsion conjecture is true, despite limited progress toward it. Our perspective focuses on the relation between pointwise bounds, averages, and higher moments, and demonstrates the broad utility of the "method of moments."