The idempotent measure conjecture for automorphism-group actions

Let mˉ\bar m enumerate a tuple from a small model M⪯CM\preceq\mathfrak{C}, and let μ∈Mmˉdef⁡(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)⊢xˉ≡∅yˉ\pi(\bar x;\bar y)\vdash\bar x\equiv_{\emptyset}\bar y and a subgroup Gπ,CG_{\pi,\mathfrak{C}} such that Stab⁡l(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.

References

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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