The asymptotic-dimension conjecture for the mapping class group

Let SS be the finite-type surface under consideration, of genus gg, and let Mod(S){\rm Mod}(S) denote its mapping class group. Write asdim(Mod(S)){\rm asdim}({\rm Mod}(S)) for its asymptotic dimension and vd(Mod(S)){\rm vd}({\rm Mod}(S)) for its virtual cohomological dimension.

Asymptotic-dimension conjecture.

asdim(Mod(S))=vd(Mod(S))=4g5.{\rm asdim}({\rm Mod}(S))={\rm vd}({\rm Mod}(S))=4g-5.

This conjecture, attributed to Bestvina and Bromberg, predicts that the asymptotic dimension of the mapping class group equals its virtual cohomological dimension, with both given by 4g54g-5. The surrounding results provide support for the conjecture, but the supplied text does not state that it has been resolved.

Sources & referencesView supporting material

Primary source

Ursula Hamenstädt, “An EZ-structure for the mapping class group”, arXiv:2505.18808 (2026).

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