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Register a new userSubgroups generated by two pseudo-Anosov elements in a mapping class group. II. Uniform bound on exponents
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Koji Fujiwara
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Let $S$ be a compact orientable surface, and $Mod(S)$ its mapping class group. Then there exists a constant $M(S)$, which depends on $S$, with the following property. Suppose $a,b in Mod(S)$ are independent (i.e., $[a^n,b^m] ot=1$ for any $n,m ot=0$) pseudo-Anosov elements. Then for any $n,m ge M$, the subgroup $<a^n,b^m>$ is free of rank two, and convex-cocompact in the sense of Farb-Mosher. In particular all non-trivial elements in $<a^n,b^m>$ are pseudo-Anosov. We also show that there exists a constant $N$, which depends on $a,b$, such that $<a^n,b^m>$ is free of rank two and convex-cocompact if $|n|+|m| ge N$ and $nm ot=0$.
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For some $g geq 3$, let $Gamma$ be a finite index subgroup of the mapping class group of a genus $g$ surface (possibly with boundary components and punctures). An old conjecture of Ivanov says that the abelianization of $Gamma$ should be finite. In this note, we prove two theorems supporting this conjecture. For the first, let $T_x$ denote the Dehn twist about a simple closed curve $x$. For some $n geq 1$, we have $T_x^n in Gamma$. We prove that $T_x^n$ is torsion in the abelianization of $Gamma$. Our second result shows that the abelianization of $Gamma$ is finite if $Gamma$ contains a large chunk (in a certain technical sense) of the Johnson kernel, that is, the subgroup of the mapping class group generated by twists about separating curves. This generalizes work of Hain and Boggi.
In this paper, we prove a combination theorem for indicable subgroups of infinite-type (or big) mapping class groups. Importantly, all subgroups from the combination theorem, as well as those from the other results of the paper, can be constructed so that they do not lie in the closure of the compactly supported mapping class group and do not lie in the isometry group for any hyperbolic metric on the relevant infinite-type surface. Along the way, we prove an embedding theorem for indicable subgroups of mapping class groups, a corollary of which gives embeddings of pure big mapping class groups into other big mapping class groups that are not induced by embeddings of the underlying surfaces. We also give new constructions of free groups, wreath products with $mathbb Z$, and Baumslag-Solitar groups in big mapping class groups that can be used as an input for the combination theorem. One application of our combination theorem is a new construction of right-angled Artin groups in big mapping class groups.
These are the lecture notes for my course at the 2011 Park City Mathematics Graduate Summer School. The first two lectures covered the basics of the Torelli group and the Johnson homomorphism, and the third and fourth lectures discussed the second cohomology group of the level p congruence subgroup of the mapping class group, following my papers The second rational homology group of the moduli space of curves with level structures and The Picard group of the moduli space of curves with level structures.
We calculate the abelianizations of the level $L$ subgroup of the genus $g$ mapping class group and the level $L$ congruence subgroup of the $2g times 2g$ symplectic group for $L$ odd and $g geq 3$.
We prove that various subgroups of the mapping class group $Mod(Sigma)$ of a surface $Sigma$ are at least exponentially distorted. Examples include the Torelli group (answering a question of Hamenstadt), the point-pushing and surface braid subgroups, and the Lagrangian subgroup. Our techniques include a method to compute lower bounds on distortion via representation theory and an extension of Johnson theory to arbitrary subgroups of $H_1(Sigma;mathbb{Z})$.
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