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Dimension of the Torelli group for Out(F_n)

Mladen Bestvina·Kai-Uwe Bux·Dan Margalit
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Abstract

Let T_n be the kernel of the natural map from Out(F_n) to GL(n,Z). We use combinatorial Morse theory to prove that T_n has an Eilenberg-MacLane space which is (2n-4)-dimensional and that H_{2n-4}(T_n,Z) is not finitely generated (n at least 3). In particular, this recovers the result of Krstic-McCool that T_3 is not finitely presented. We also give a new proof of the fact, due to Magnus, that T_n is finitely generated.

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Mladen Bestvina, Kai-Uwe Bux, Dan Margalit. 2006-03-08. Dimension of the Torelli group for Out(F_n). https://doi.org/10.1007/s00222-007-0055-0

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