A transfer trick yields non-vanishing top ℓ2-Betti numbers for Out(Fn), Aut(Fn) and Torelli groups, vanishing results for subgroups of 3-manifold groups, and ergodic dimension d+1 for F2^d × Z.
The Euler characteristic of $\operatorname{Out}(F_n)$
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abstract
We prove that the rational Euler characteristic of $\operatorname{Out}(F_n)$ is always negative and its asymptotic growth rate is $\Gamma(n- \frac32)/\sqrt{2\pi} \log^2 n$. This settles a 1987 conjecture of J. Smillie and the second author. We establish connections with the Lambert $W$-function and the zeta function.
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2019 1verdicts
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On the top-dimensional $\ell^2$-Betti numbers
A transfer trick yields non-vanishing top ℓ2-Betti numbers for Out(Fn), Aut(Fn) and Torelli groups, vanishing results for subgroups of 3-manifold groups, and ergodic dimension d+1 for F2^d × Z.