All mixed identities are singular in groups with no algebraicity

Abstract

We show that if a group admits an action with no algebraicity then all of its mixed identities are singular. Previously, such groups were only known to be lawless by a theorem of Abért. Our result confirms, in particular, a conjecture of Bodirsky, Schneider, and Thom for a large class of oligomorphic permutation groups. It thereby not only subsumes numerous results from the literature in a simple uniform theorem, but also settles the question for prominent groups for which the conjecture was an open problem, such as the automorphism group of $(\mathbb{Q};<)$. It also applies outside the oligomorphic context, e.g. to much-investigated groups such as Thompson's groups $F,T$, and $V$, to Grigorchuk's group, and to the homeomorphism groups of any manifold of dimension $\ge 1$. More generally, we prove that all mixed identities of a group are singular as long as it has an action satisfying certain geometric conditions. This additionally covers, for example, the infinite-dimensional general and projective linear groups.

24 pages, 1 figure