The Euler characteristic of $\operatorname{Out}(F_n)$

We prove that the rational Euler characteristic of $\operatorname{Out}(F_n)$ is always negative and its asymptotic growth rate is $Γ(n- \frac32)/\sqrt{2π} \log^2 n$. 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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