Infinitesimal embedding conjecture for a random profinite action of a free group

Let (tn)n0(t_n)_{n\geq 0} be a rapidly growing sequence of natural numbers, let FdF_d be the free group, and construct a nested sequence of finite-index subgroups (Λn)(\Lambda_n) by taking Λ0\Lambda_0 uniformly at random among subgroups of index t0t_0 and, for n1n\geq 1, taking Λn\Lambda_n uniformly at random among subgroups of index tnt_n in Λn1\Lambda_{n-1}. Let (X,μ)(X,\mu) be the inverse limit of the actions on Fd/ΛnF_d/\Lambda_n with normalized counting measures. Infinitesimal embedding conjecture. The action Fd(X,μ)F_d\curvearrowright (X,\mu) is infinitesimally embedded in FdF_d. This is presented as a candidate example of a compact, profinite action that may be infinitesimally contained in the acting group; no proof or counterexample is given.

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Primary source

Mikołaj Frączyk, “Infinitesimal containment and sparse factors of iid”, arXiv:2512.09301 (2025).

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