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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."