arXiv:2606.04696v1

The No-Core Principle for Stationary Actions and Ends of Stationary Random Subgroups

Yair Hartman, Nadav Kalma

math.DSmath.GR37A3020P0560J10

Abstract

We prove a No-Core Principle for stationary actions of countable groups. Namely, if a Borel set intersects almost every orbit in finitely many points and has positive measure, then it is supported, modulo null sets, on the finite-orbit part of the action. This extends to stationary actions a basic regularity phenomenon known for measure-preserving actions. We apply this principle to the geometry of Stationary Random Subgroups. For a finitely generated group, we prove that the Schreier graph of a stationary random subgroup has almost surely 0,1,2, or infinitely many ends. Finally, we contrast this probabilistic regularity with the topological notion of Boomerang subgroups: for every k3k\geq 3, including k=0k=\aleph_0, we construct a Boomerang subgroup of F3\mathbb{F}_3 whose Schreier graph has exactly kk ends.

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Audited against arXiv v1

Not a correctness certificate. A “Correct” result may include yellow typos or minor formal corrections that do not affect substantive soundness. It means this audit found no unresolved substantive error under the stated criteria; it does not replace expert scrutiny or formal verification.

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Generated August 18, 2026
01Statements3 reported findingsCorrect

The no-core principle for stationary actions, the end-rigidity theorem for stationary random subgroups, and the contrasting Boomerang-subgroup examples are supported.

No-Core PrincipleCorrect

Stationarity forbids a positive-measure recurrent core unless it is finite

Pages 2 and 8–11 · Theorem 1.1 and Section 3 · arXiv:2606.04696v1

For the irreducible Markov chain induced on an orbit, Kac's formula shows that positive recurrence forces the stabilizer to have finite index, hence the orbit to be finite. On an infinite orbit the transition probabilities into any finite set tend to zero. Intersecting the core with each orbit gives just such a finite set; dominated convergence then sends its random-walk averages to zero, while stationarity keeps their integral equal to the positive core measure. This contradiction proves the theorem without assuming measure preservation.

Theorem 1.2Correct

End-rigidity for stationary random subgroups

Pages 3 and 11–13 · Section 4 · arXiv:2606.04696v1

If a Schreier graph has a finite number k>2k>2 of ends, the subgroups whose radius-nn ball separates those ends form a core for the conjugation action. The no-core principle forces almost every subgroup in a positive-measure such core to have finite conjugacy class, while Abels' theorem rules that out for a graph with finitely many ends greater than two. This leaves exactly 0,1,20,1,2, or infinitely many ends almost surely.

Theorem 1.3Correct

Boomerang subgroups with prescribed exceptional end counts

Pages 3 and 14–15 · Section 5 · arXiv:2606.04696v1

This example is deterministic, not a stationary random subgroup. The construction rewires kk copies of a one-ended Cayley graph of an infinite exponent-pp Burnside group into a labelled Schreier graph with exactly the prescribed finite or countable number of ends. For every word gg, the powers gnpg^{np} lie in the kernel of the Burnside quotient and reroot the graph at an isomorphic distinguished vertex, so an infinite subsequence of conjugates equals the original subgroup. This verifies the Boomerang property.

02Proofs1 reported findingCorrect

The recurrence, dominated-convergence, finite-core, and construction arguments are correct and complete.

Sections 3–5Correct and complete

Recurrence and end-structure proofs

Pages 8–15 · arXiv:2606.04696v1

The return-time identity is used only for the irreducible orbit chain, and the pointwise transition probabilities are uniformly bounded, so dominated convergence applies. In the SRS argument, removing two disjoint separating balls would create at least 2k2>k2k-2>k ends, which proves the core property. In the Boomerang construction, the rewiring remains a regular labelled Schreier graph, the one-ended pieces give the asserted end count, and the exponent-pp quotient supplies an infinite subsequence of conjugates equal to the original subgroup.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2606.04696v1
Authors listed
Yair Hartman, Nadav Kalma
Audit date
August 18, 2026
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