### Rational Homology of Bianchi Groups.

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For $n$ at least 3, the Dehn functions of $Out\left({F}_{n}\right)$ and $Aut\left({F}_{n}\right)$ are exponential. Hatcher and Vogtmann proved that they are at most exponential, and the complementary lower bound in the case $n=3$ was established by Bridson and Vogtmann. Handel and Mosher completed the proof by reducing the lower bound for $n$ bigger than 3 to the case $n=3$. In this note we give a shorter, more direct proof of this last reduction.

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