Upside down and backwards

Kyle Gannon; Tomasz Rzepecki

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arXiv · 2512.03576

Upside down and backwards

Kyle Gannon·Tomasz Rzepecki

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Abstract

We investigate the semigroup of invariant types through the lens of Ellis theory; primarily focusing on definably amenable NIP groups. In this context, we observe that the collection of strong right $f$-generic types forms the unique minimal left ideal and thus, the Ellis subgroups are isomorphic to $G/G^{00}$ via the canonical quotient map. As consequence of the Newelski-Pillay conjecture, the Ellis subgroups of the semigroup of invariant types are abstractly isomorphic to the Ellis subgroups of the semigroup of finitely satisfiable types in the definable amenable NIP setting. We are interested in the existence of natural isomorphisms from invariant Ellis subgroups to finitely satisfiable Ellis subgroups and we determine when these isomorphisms can be witnessed by variants of the canonical NIP retraction map. Several limiting examples are provided. Outside of the NIP context, we provide an abelian group (and thus definably amenable) with an $\emptyset$-definable (dfg) type in which the invariant Ellis subgroups and finitely satisfiable Ellis subgroups not isomorphic.

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Kyle Gannon, Tomasz Rzepecki. 2025-12-03. Upside down and backwards. https://arxiv.org/abs/2512.03576

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