Abstract
We describe homomorphisms \(\varphi :H\rightarrow G\) for which G is acylindrically hyperbolic and H is a topological group which is either completely metrizable or locally countably compact Hausdorff. It is shown that, in a certain sense, either the image of \(\varphi \) is small or \(\varphi \) is almost continuous. We also describe homomorphisms from the Hawaiian earring group to G as above. We prove a more precise result for homomorphisms \(\varphi :H\rightarrow {\text {Mod}}(\Sigma )\), where H is as above and \({\text {Mod}}(\Sigma )\) is the mapping class group of a connected compact surface \(\Sigma \). In this case there exists an open normal subgroup \(V\leqslant H\) such that \(\varphi (V)\) is finite. We also prove the analogous statement for homomorphisms \(\varphi :H\rightarrow {\text {Out}}(G)\), where G is a one-ended hyperbolic group. Some automatic continuity results for relatively hyperbolic groups and fundamental groups of graphs of groups are also deduced. As a by-product, we prove that the Hawaiian earring group is acylindrically hyperbolic, but does not admit any universal acylindrical action on a hyperbolic space.
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Communicated by Christian Rosendal.
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The work of the second author is supported by European Research Council grant PCG-336983 and by the Severo Ochoa Programme for Centres of Excellence in R&D SEV-20150554.
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Bogopolski, O., Corson, S.M. Abstract homomorphisms from some topological groups to acylindrically hyperbolic groups. Math. Ann. 384, 1017–1055 (2022). https://doi.org/10.1007/s00208-021-02278-4
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DOI: https://doi.org/10.1007/s00208-021-02278-4