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Free subgroups of Kleinian groups

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Misha Belolipetsky
(IMPA Rio de Janeiro/MPIM Bonn)
Tue, 03/03/2020 - 10:15 - 12:00
MPIM Lecture Hall

We show that any cocompact Kleinian group $\Gamma$ has an exhaustive filtration by normal subgroups $\Gamma_i$ such that any subgroup of $\Gamma_i$ generated by $k_i$ elements is free, where $k_i \ge [\Gamma:\Gamma_i]^C$ and $C = C(\Gamma) > 0$. Together with this result we prove that $\log k_i \ge C_1 \mathrm{sys}_1(M_i)$, where $\mathrm{sys}_1(M_i)$ denotes the systole of $M_i$, thus providing a large set of new examples for a conjecture of Gromov. In the second theorem $C_1> 0$ is an absolute constant. We also consider a generalization of these results to non-cocompact Kleinian groups.

In the talk I am going to discuss the proofs of these theorems and some related open problems. This is a joint work with Cayo Dória.

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