2,215 research outputs found

    Linear representations of hyperelliptic mapping class groups

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    Let p:Sβ†’Sgp:S\to S_g be a finite GG-covering of a closed surface of genus gβ‰₯1g\geq 1 and let BB its branch locus. To this data, it is associated a representation of a finite index subgroup of the mapping class group Mod⁑(Sgβˆ–B)\operatorname{Mod}(S_g\smallsetminus B) in the centralizer of the group GG in the symplectic group Sp⁑(H1(S,Q))\operatorname{Sp}(H_1(S,{\mathbb Q})). They are called \emph{virtual linear representations} of the mapping class group and are related, via a conjecture of Putman and Wieland, to a question of Kirby and Ivanov on the abelianization of finite index subgroup of the mapping class group. The purpose of this paper is to study the restriction of such representations to the hyperelliptic mapping class group Mod⁑(Sg,B)ΞΉ\operatorname{Mod}(S_g,B)^\iota, which is a subgroup of Mod⁑(Sgβˆ–B)\operatorname{Mod}(S_g\smallsetminus B) associated to a given hyperelliptic involution ΞΉ\iota on SgS_g. We extend to hyperelliptic mapping class groups some previous results on virtual linear representations of the mapping class group. We then show that, for all gβ‰₯1g\geq 1, there are virtual linear representations of hyperelliptic mapping class groups with nontrivial finite orbits. In particular, we show that there is such a representation associated to an unramified GG-covering Sβ†’S2S\to S_2, thus providing a counterexample to the genus 22 case of the Putman-Wieland conjecture.Comment: 22 page

    Curves with prescribed symmetry and associated representations of mapping class groups

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    Let C be a complex smooth projective algebraic curve endowed with an action of a finite group G such that the quotient curve has genus at least 3. We prove that if the G-curve C is very general for these properties, then the natural map from the group algebra QG to the algebra of Q-endomorphisms of its Jacobian is an isomorphism. We use this to obtain (topological) properties regarding certain virtual linear representations of a mapping class group. For example, we show that the connected component of the Zariski closure of such a representation acts Q-irreducibly in a G-isogeny space of H^1(C; Q)and with image often a Q-almost simple group

    Profinite complexes of curves, their automorphisms and anabelian properties of moduli stacks of curves

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    Let Mg,[n]{\cal M}_{g,[n]}, for 2gβˆ’2+n>02g-2+n>0, be the D-M moduli stack of smooth curves of genus gg labeled by nn unordered distinct points. The main result of the paper is that a finite, connected \'etale cover {\cal M}^\l of Mg,[n]{\cal M}_{g,[n]}, defined over a sub-pp-adic field kk, is "almost" anabelian in the sense conjectured by Grothendieck for curves and their moduli spaces. The precise result is the following. Let \pi_1({\cal M}^\l_{\ol{k}}) be the geometric algebraic fundamental group of {\cal M}^\l and let {Out}^*(\pi_1({\cal M}^\l_{\ol{k}})) be the group of its exterior automorphisms which preserve the conjugacy classes of elements corresponding to simple loops around the Deligne-Mumford boundary of {\cal M}^\l (this is the "βˆ—\ast-condition" motivating the "almost" above). Let us denote by {Out}^*_{G_k}(\pi_1({\cal M}^\l_{\ol{k}})) the subgroup consisting of elements which commute with the natural action of the absolute Galois group GkG_k of kk. Let us assume, moreover, that the generic point of the D-M stack {\cal M}^\l has a trivial automorphisms group. Then, there is a natural isomorphism: {Aut}_k({\cal M}^\l)\cong{Out}^*_{G_k}(\pi_1({\cal M}^\l_{\ol{k}})). This partially extends to moduli spaces of curves the anabelian properties proved by Mochizuki for hyperbolic curves over sub-pp-adic fields.Comment: This paper has been withdrawn because of a flaw in the paper "Profinite Teichm\"uller theory" of the first author, on which this paper built o

    Fundamental groups of moduli stacks of stable curves of compact type

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    Let M~g,n\widetilde{\cal M}_{g,n}, for 2gβˆ’2+n>02g-2+n>0, be the moduli stack of nn-pointed, genus gg, stable complex curves of compact type. Various characterizations and properties are obtained of both the algebraic and topological fundamental groups of the stack M~g,n\widetilde{\cal M}_{g,n}. Let Ξ“g,n\Gamma_{g,n}, for 2gβˆ’2+n>02g-2+n>0, be the Teichm\"uller group associated with a compact Riemann surface of genus gg with nn points removed Sg,nS_{g,n}, i.e. the group of homotopy classes of diffeomorphisms of Sg,nS_{g,n} which preserve the orientation of Sg,nS_{g,n} and a given order of its punctures. Let Kg,nK_{g,n} be the normal subgroup of Ξ“g,n\Gamma_{g,n} generated by Dehn twists along separating circles on Sg,nS_{g,n}. As a first application of the above theory, a characterization of Kg,nK_{g,n} is given for all nβ‰₯0n\geq 0 (for n=0,1n=0,1, this was done by Johnson). Let then Tg,n{\cal T}_{g,n} be the Torelli group, i.e. the kernel of the natural representation \Gamma_{g,n}\ra Sp_{2g}(Z). The abelianization of Tg,n{\cal T}_{g,n} is determined for all gβ‰₯1g\geq 1 and nβ‰₯1n\geq 1, thus completing classical results by Johnson and Mess.Comment: 25 pages; minor corrections in some proofs; typos corrected; theorem numbering change
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