The Aldous--Lyons Conjecture I: Subgroup Tests

TL;DR

This paper introduces subgroup tests as a framework to study invariant random subgroups of free groups, ultimately disproving the Aldous–Lyons Conjecture by showing the existence of non co-sofic invariant random subgroups and linking the problem to undecidable computational problems.

Contribution

It develops subgroup tests for invariant random subgroups, establishes their connection to non-local games, and proves the existence of non co-sofic invariant random subgroups of free groups.

Findings

Subgroup tests provide a new framework for studying invariant random subgroups.

Approximating the sofic value of a subgroup test is undecidable.

Existence of non co-sofic invariant random subgroups of free groups is proven.

Abstract

This paper, and its companion [BCV24], are devoted to a negative resolution of the Aldous--Lyons Conjecture [AL07, Ald07]. This conjecture, originated in probability theory, is well known (cf. [Gel18]) to be equivalent to the statement that every invariant random subgroup of the free group is co-sofic. We disprove this last statement. In this part we introduce subgroup tests. These tests are finite distributions over continuous functions from the space of subgroups of the free group to $\{0,1\}$. Subgroup tests provide a general framework in which one can study invariant random subgroups of the free group. Classical notions such as group soficity and group stability arise naturally in this framework. By the correspondence between subgroups of the free group and Schreier graphs, one can view subgroup tests as a property testing model for certain edge-labeled graphs. This correspondence also provides the connection to random networks. Subgroup tests have values, which are their asymptotic optimal expectations when integrated against co-sofic invariant random subgroups. Our first main result is that, if every invariant random subgroup of the free group is co-sofic, then one can approximate the value of a subgroup test up to any positive additive constant. Our second main result is an essentially value preserving correspondence between certain non-local games and subgroup tests. By composing this correspondence with a stronger variant of the reduction in MIP*=RE [JNV+21], proved in the companion paper [BCV24], we deduce that approximating the sofic value of a subgroup test is as hard as the Halting Problem, and in particular, undecidable. The combination of our two main results proves the existence of non co-sofic invariant random subgroups of the free group.