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arXiv · 2304.10980

Counting embeddings of free groups into $\mathrm{SL}_2(\mathbb{Z})$ and its subgroups

Kamil Bulinski·Alina Ostafe·Igor E. Shparlinski
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Abstract

We show that if one selects uniformly independently and identically distributed matrices $A_1, \ldots, A_s \in \mathrm{SL}_2(\mathbb{Z})$ from a ball of large radius $X$ then with probability at least $1 - X^{-1 + o(1)}$ the matrices $A_1, \ldots, A_s$ are free generators for a free subgroup of $\mathrm{SL}_2(\mathbb{Z})$. Furthermore, to show the flexibility of our method we do similar counting for matrices from the congruence subgroup $\Gamma_0(Q)$ uniformly with respect to the positive integer $Q\le X$. This improves and generalises a result of E. Fuchs and I. Rivin (2017) which claims that the probability is $1 + o(1)$. We also disprove one of the statements in their work that has been used to deduce their claim.

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Kamil Bulinski, Alina Ostafe, Igor E. Shparlinski. 2023-04-21. Counting embeddings of free groups into $\mathrm{SL}_2(\mathbb{Z})$ and its subgroups. https://arxiv.org/abs/2304.10980

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