Genericity conjecture for pseudo-Anosov mapping classes in metric balls

Let Σ\Sigma be a complete hyperbolic surface of finite type that is not a three-punctured sphere, and let Mod(Σ)\operatorname{Mod}(\Sigma) be its mapping class group. Let SS be a specific or arbitrary finite generating set of Mod(Σ)\operatorname{Mod}(\Sigma), and let BS(R)B_S(R) be the collection of mapping classes whose word norm with respect to SS is at most RR. Genericity conjecture.

limR#{gBS(R):g is pseudo-Anosov}#BS(R)=1.\lim_{R \rightarrow \infty} \frac{\# \{g \in B_S(R): g\text{ is pseudo-Anosov}\}}{\#B_S(R)}=1.

This conjecture expresses the belief that pseudo-Anosov mapping classes are generic in metric balls of the mapping class group, independently of the chosen finite generating set. It is presented as one of two metric interpretations of the expectation that most mapping classes are pseudo-Anosov; the supplied text does not state whether it has been resolved.

Sources & referencesView supporting material

Primary source

Inhyeok Choi, “Counting pseudo-Anosovs as weakly contracting isometries”, arXiv:2408.00603 (2025).

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