71 research outputs found
Abstract commensurators of braid groups
Let B_n be the braid group on n strands, with n at least 4, and let Mod(S) be
the extended mapping class group of the sphere with n+1 punctures. We show that
the abstract commensurator of B_n is isomorphic to a semidirect product of
Mod(S) with a group we refer to as the transvection subgroup, Tv(B_n). We also
show that Tv(B_n) is itself isomorphic to a semidirect product of an infinite
dimensional rational vector space with the multiplicative group of nonzero
rational numbers.Comment: 10 page
Abelian quotients of subgroups of the mapping class group and higher Prym representations
A well-known conjecture asserts that the mapping class group of a surface
(possibly with punctures/boundary) does not virtually surject onto if the
genus of the surface is large. We prove that if this conjecture holds for some
genus, then it also holds for all larger genera. We also prove that if there is
a counterexample to this conjecture, then there must be a counterexample of a
particularly simple form. We prove these results by relating the conjecture to
a family of linear representations of the mapping class group that we call the
higher Prym representations. They generalize the classical symplectic
representation.Comment: 20 pages, 3 figures; appendix added containing a new counterexample
in genus 1; to appear in J. London Math. So
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