2,215 research outputs found
Linear representations of hyperelliptic mapping class groups
Let be a finite -covering of a closed surface of genus and let its branch locus. To this data, it is associated a
representation of a finite index subgroup of the mapping class group
in the centralizer of the group
in the symplectic group . 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
, which is a subgroup of
associated to a given hyperelliptic
involution on . 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 , 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 -covering , thus providing a counterexample to
the genus case of the Putman-Wieland conjecture.Comment: 22 page
Curves with prescribed symmetry and associated representations of mapping class groups
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
Let , for , be the D-M moduli stack of smooth
curves of genus labeled by unordered distinct points. The main result
of the paper is that a finite, connected \'etale cover {\cal M}^\l of , defined over a sub--adic field , 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
"-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
of . 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--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
Let , for , be the moduli stack of
-pointed, genus , stable complex curves of compact type. Various
characterizations and properties are obtained of both the algebraic and
topological fundamental groups of the stack .
Let , for , be the Teichm\"uller group associated
with a compact Riemann surface of genus with points removed ,
i.e. the group of homotopy classes of diffeomorphisms of which
preserve the orientation of and a given order of its punctures. Let
be the normal subgroup of generated by Dehn twists
along separating circles on . As a first application of the above
theory, a characterization of is given for all (for
, this was done by Johnson). Let then be the Torelli
group, i.e. the kernel of the natural representation \Gamma_{g,n}\ra
Sp_{2g}(Z). The abelianization of is determined for all
and , 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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