Conditional Non-Soficity of p-adic Deligne Extensions: on a Theorem of Gohla and Thom

Abstract

A long standing problem asks whether every group is sofic, i.e., can be separated by almost-homomorphisms to the symmetric group $Sym(n)$. Similar problems have been asked with respect to almost-homomorphisms to the unitary group $U(n)$, equipped with various norms. One of these problems has been solved for the first time in [De Chiffre, Gelbsky, Lubotzky, Thom, 2020]: some central extensions $\tilde{\Gamma }$ of arithmetic lattices $\Gamma$ of $Sp(2g,{\mathbb{Q}}_p)$ were shown to be non-Frobenius approximated by almost homomorphisms to $U(n)$. Right after, it was shown that similar results hold with respect to the $p$-Schatten norms in [Lubotzky, Oppenheim, 2020]. It is natural, and has already been suggested in [Chapman, Lubotzky, 2024] and [Gohla, Thom, 2024], to check whether the $\tilde{\Gamma }$ are also non-sofic. In order to show that they are (also) non-sofic, it suffices: (a) To prove that the permutation Cheeger constant of the simplicial complex underlying $\Gamma$ is positive, generalizing [Evra, Kaufman, 2016]. This would imply that $\Gamma$ is stable. (b) To prove that the (flexible) stability of $\Gamma$ implies the non-soficity of $\tilde{\Gamma }$. Clause (b) was proved by Gohla and Thom. Here we offer a more algebraic/combinatorial treatment to their theorem.

13 pages, added example for a non-stable central extension of a stable group