English

Automatic continuity of pure mapping class groups

Geometric Topology 2024-06-17 v2 Group Theory

Abstract

We completely classify the orientable infinite-type surfaces SS such that PMap(S)\operatorname{PMap}(S), the pure mapping class group, has automatic continuity. This classification includes surfaces with noncompact boundary. In the case of surfaces with finitely many ends and no noncompact boundary components, we prove the mapping class group Map(S)\operatorname{Map}(S) does not have automatic continuity. We also completely classify the surfaces such that PMapc(S)\overline{\operatorname{PMap}_c(S)}, the subgroup of the pure mapping class group composed of elements with representatives that can be approximated by compactly supported homeomorphisms, has automatic continuity. In some cases when PMapc(S)\overline{\operatorname{PMap}_c(S)} has automatic continuity, we show any homomorphism from PMapc(S)\overline{\operatorname{PMap}_c(S)} to a countable group is trivial.

Keywords

Cite

@article{arxiv.2306.02599,
  title  = {Automatic continuity of pure mapping class groups},
  author = {Ryan Dickmann},
  journal= {arXiv preprint arXiv:2306.02599},
  year   = {2024}
}

Comments

20 pages, 4 figures. v2: Incorporated referee comments. Accepted to New York Journal of Mathematics

R2 v1 2026-06-28T10:56:08.162Z