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Commentarii Mathematici Helvetici

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Volume 91, Issue 4, 2016, pp. 751–806
DOI: 10.4171/CMH/402

Published online: 2016-10-24

Assembling homology classes in automorphism groups of free groups

James Conant[1], Allen Hatcher[2], Martin Kassabov[3] and Karen Vogtmann[4]

(1) University of Tennessee, Knoxville, United States
(2) Cornell University, Ithaca, USA
(3) Cornell University, Ithaca, United States
(4) University of Warwick, Coventry, United Kingdom

The observation that a graph of rank $n$ can be assembled from graphs of smaller rank $k$ with $s$ leaves by pairing the leaves together leads to a process for assembling homology classes for Out$(F_n)$ and Aut$(F_n)$ from classes for groups $\Gamma_{k,s}$, where the $\Gamma_{k,s}$ generalize Out$(F_k)=\Gamma_{k,0}$ and Aut$(F_k)=\Gamma_{k,1}$. The symmetric group $\mathfrak G_s$ acts on $H_*(\Gamma_{k,s})$ by permuting leaves, and for trivial rational coefficients we compute the $\mathfrak G_s$-module structure on $H_*(\Gamma_{k,s})$ completely for $k \leq 2$. Assembling these classes then produces all the known nontrivial rational homology classes for Aut$(F_n)$ and Out$(F_n)$ with the possible exception of classes for $n=7$ recently discovered by L. Bartholdi. It also produces an enormous number of candidates for other nontrivial classes, some old and some new, but we limit the number of these which can be nontrivial using the representation theory of symmetric groups. We gain new insight into some of the most promising candidates by finding small subgroups of Aut$(F_n)$ and Out$(F_n)$ which support them and by finding geometric representations for the candidate classes as maps of closed manifolds into the moduli space of graphs. Finally, our results have implications for the homology of the Lie algebra of symplectic derivations.

Keywords: Automorphisms of free groups, moduli spaces of graphs, cohomology

Conant James, Hatcher Allen, Kassabov Martin, Vogtmann Karen: Assembling homology classes in automorphism groups of free groups. Comment. Math. Helv. 91 (2016), 751-806. doi: 10.4171/CMH/402