Brendle–Margalit conjectures on complexes of regions and geometric normal subgroups

Let Σ\Sigma be a surface, let CR(Σ)\mathcal{C}_R(\Sigma) be a complex of regions, and let NN be a normal subgroup of Map±(Σ)\operatorname{Map}^{\pm}(\Sigma). Brendle–Margalit conjectures for complexes of regions.

  1. The natural map
Map±(Σ)AutCR(Σ)\operatorname{Map}^{\pm}(\Sigma)\to\operatorname{Aut}\mathcal{C}_R(\Sigma)

is an isomorphism if and only if CR(Σ)\mathcal{C}_R(\Sigma) is connected and admits no exchange automorphisms.

  1. The natural map
Map±(Σ)AutN\operatorname{Map}^{\pm}(\Sigma)\to\operatorname{Aut}N

is an isomorphism if and only if NN contains elements whose supports L,RΣL,R\subset\Sigma are disjoint and separated by a pair of pants. The first assertion concerns automorphisms of complexes of regions, while the second characterizes geometric normal subgroups through elements with separated supports. The supplied text presents this as a rephrasing and generalization of a Brendle–Margalit conjecture and does not give a resolution of both assertions.

Sources & referencesView supporting material

Primary source

Alan McLeay, “The mapping class group of the Cantor tree has only geometric normal subgroups”, arXiv:2002.06970 (2020).

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