The idempotent fim measure–stabilizer correspondence for automorphism groups

Let μMmˉinv(C,M)\mu\in\mathfrak{M}^{\operatorname{inv}}_{\bar m}(\mathfrak{C},M) be fim over MM. Suppose that Stab(μ)=Gπ,C\operatorname{Stab}(\mu)=G_{\pi,\mathfrak{C}} for a partial type π(xˉ;yˉ)xˉyˉ\pi(\bar x;\bar y)\vdash\bar x\equiv_{\emptyset}\bar y, where

Gπ,C={σAut(C):π(σ(mˉ);mˉ)}.G_{\pi,\mathfrak{C}}=\{\sigma\in\operatorname{Aut}(\mathfrak{C}):\models\pi(\sigma(\bar m);\bar m)\}.

The automorphism-group idempotent fim measure conjecture. The following conditions are equivalent: (i) μ\mu is idempotent with respect to the new convolution product; (ii) μ\mu is the unique left Gπ,CG_{\pi,\mathfrak{C}}-invariant measure in Mπ(xˉ;mˉ)inv(C,M)\mathfrak{M}^{\operatorname{inv}}_{\pi(\bar x;\bar m)}(\mathfrak{C},M). In particular, idempotent fim measures in Mmˉinv(C,M)\mathfrak{M}^{\operatorname{inv}}_{\bar m}(\mathfrak{C},M) correspond to relatively mˉ\bar m-type-definable-over-MM fim subgroups of Aut(C)\operatorname{Aut}(\mathfrak{C}). This generalizes the preceding definable-group problem from groups in the monster model to relatively type-definable subgroups of its automorphism group; the supplied text does not state a resolution.

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Primary source

Kyle Gannon, Daniel Max Hoffmann and Krzysztof Krupiński, “Convolution semigroups for automorphism dynamics”, arXiv:2507.23503 (2025).

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