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Semisimple actions of mapping class groups on CAT(0) spaces

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arxiv 0908.0685 v1 pith:56YVGTEQ submitted 2009-08-05 math.GT math.GR

classification math.GTmath.GR
keywords classisometriesmappingsemisimpleactionscompletegenusgroup
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Let S be an orientable surface of finite type and let Mod(S) be its mapping class group. We consider actions of Mod(S) by semisimple isometries on complete CAT(0) spaces. If the genus of S is at least 3, then in any such action all Dehn twists act as elliptic isometries. The action of Mod(S) on the completion of Teichm\"uller space with the Weil-Petersson metric shows that there are interesting actions of this type. Whenever the mapping class group of a closed orientable surface of genus g acts by semisimple isometries on a complete CAT(0) space of dimension less than g it must fix a point. The mapping class group of a closed surface of genus 2 acts properly by semisimple isometries on a complete CAT(0) space of dimension 18.

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  1. Abelianizations of finite-index subgroups of the handlebody group

    math.GT 2026-07 accept novelty 6.0 of 10

    For genus ≥ 4, meridian multitwists vanish in H_1 of any finite-index subgroup of the handlebody group, and subgroups containing the Torelli group, twist group, or Johnson kernel have trivial rational abelianization.

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