Thomas Koberda

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Consider the mapping class group Modg,p of a surface Σg,p of genus g with p punctures, and a finite collection {f1,. .. , f k } of mapping classes, each of which is either a Dehn twist about a simple closed curve or a pseudo-Anosov homeomorphism supported on a connected subsurface. In this paper we prove that for all sufficiently large N , the mapping(More)
We prove that finitely generated purely loxodromic subgroups of a right-angled Artin group A(Γ) fulfill equivalent conditions that parallel characterizations of convex cocompactness in mapping class groups Mod(S). In particular, such subgroups are quasiconvex in A(Γ). In addition, we identify a milder condition for a finitely generated subgroup of A(Γ) that(More)
We study the action of the mapping class group on the integral homology of finite covers of a topological surface. We use the homological representation of the mapping class to construct a faithful infinite–dimensional representation of the mapping class group. We show that this representation detects the Nielsen–Thurston classification of each mapping(More)
We prove that for each sufficiently complicated orientable surface S , there exists an infinite image linear representation ρ of π 1 (S) such that if γ ∈ π 1 (S) is freely homotopic to a simple closed curve on S , then ρ(γ) has finite order. Furthermore, we prove that given a sufficiently complicated orientable surface S , there exists a regular finite(More)
Let p : Σ ′ → Σ be a finite Galois cover, possibly branched, with Galois group G. We are interested in the structure of the cohomology of Σ ′ as a module over G. We treat the cases of branched and unbranched covers separately. In the case of branched covers, we give a complete classification of possible module structures of H1(Σ ′ , C). In the unbranched(More)
We show that no finite index subgroup of a sufficiently complicated mapping class group or braid group can act faithfully by C 1`bv diffeomorphisms on the circle, which generalizes a result of Farb–Franks, and which parallels a result of Ghys and Burger–Monod concerning differentiable actions of higher rank lattices on the circle. This answers a question of(More)