The idempotent measure conjecture for automorphism-group actions

Let mˉ\bar m enumerate a tuple from a small model MCM\preceq\mathfrak{C}, and let μMmˉdef(C,M)\mu\in\mathfrak{M}^{\operatorname{def}}_{\bar m}(\mathfrak{C},M) be fim over MM. By the cited result, there are a partial type π(xˉ;yˉ)xˉyˉ\pi(\bar x;\bar y)\vdash\bar x\equiv_{\emptyset}\bar y and a subgroup Gπ,CG_{\pi,\mathfrak{C}} such that Stabl(μ)=Gπ,C\operatorname{Stab}_l(\mu)=G_{\pi,\mathfrak{C}}. Idempotent measure conjecture. The following conditions are conjectured to be equivalent:

  1. μ\mu is idempotent.
  2. 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, this predicts a correspondence between idempotent fim measures in Mmˉinv(C,M)\mathfrak{M}^{\operatorname{inv}}_{\bar m}(\mathfrak{C},M) and relatively mˉ\bar m-type-definable over MM fim subgroups of Aut(C)\operatorname{Aut}(\mathfrak{C}). The conjecture is presented as the automorphism-group analogue and generalization of the definable-group idempotent measure conjecture; the supplied text gives no resolution, so its general status remains open.

Sources & referencesView supporting material

Primary source

Daniel Max Hoffmann and Tomasz Rzepecki, “On idempotent measure conjecture and decomposition of invariant measures”, arXiv:2511.22945 (2025).

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