arXiv:2606.07430v1

Spectral properties of the Schreier graphs of the basilica group

Kyle Ambrose, Noah Dunham, Michael Morris, Luke G. Rogers, Alexander Teplyaev

math.GR20E0805C2505C5028A8031C2537A3037B1537F1060J1081Q35

Abstract

We study the spectral properties of Laplacians on the Schreier graphs ΓnΓ_n of the basilica group, the iterated monodromy group of the polynomial z21z^2 - 1, which is an important example in the theory of self-similar, amenable but not elementarily amenable, automaton groups. Building heavily on results by Brzoska, Jarvis, George, Rogers and Teplyaev about certain subgraphs of the basilica graphs, we develop a new recursive framework for computing the characteristic polynomials of these Laplacians. Our analysis reveals a simple underlying dynamical system and proves approximation results for the Kesten-von Neumann-Serre (KNS) spectral measure.

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Audit summary

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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Detailed mathematical audit

Generated August 18, 2026
01Statements3 reported findingsCorrect

The recursive description of the finite Schreier-graph spectra, the Kesten–von Neumann–Serre spectral measure, and the Cantor-type spectral conclusions for the Basilica action are supported. One symbol in a contradiction step is mistyped but has a unique correction and does not alter a theorem.

Finite-level spectral recursionCorrect

The zero-series and two-series eigenvalues are correctly generated

Pages 5–14 · recursive determinant identities and finite-level spectrum · arXiv:2606.07430v1

The block decomposition of the Schreier matrices gives the stated Schur-complement recursion. The exceptional factors at zero and two are separated before division, their multiplicities agree with the matrix sizes, and induction accounts for every eigenvalue. The resulting interlacing and inverse-branch structure are consistent with the displayed characteristic polynomials.

Theorems 5.7–5.8Correct

Atomic KNS measure and convergence rate

Pages 21–22 · arXiv:2606.07430v1

For every fixed level of birth, the normalized multiplicity of a two-series eigenvalue converges to the displayed geometric weight. The zero-series contribution tends to zero and the tail of the two-series weights is summable, so passage from finite spectral counting measures to the infinite atomic measure is justified. The degree estimate in Theorem 5.8 gives the stated quantitative weak-convergence control.

Corollary 5.11 and Theorem 5.12Correct

Cantor support and closure of the full spectrum

Pages 24–25 · arXiv:2606.07430v1

The known gap and one-sided accumulation properties of the two-series make the KNS support perfect and nowhere dense. Theorem 5.12 then shows that every zero-series eigenvalue is approached by roots or poles of later ζj\zeta_j: otherwise the recursion forces a positive branch to become negative. Hence closing all finite-level eigenvalues adds no points beyond the Cantor support.

02Proofs2 reported findingsCorrect

The determinant recursion, multiplicity count, weak-limit calculation, and Cantor-set analysis are correct and complete. A local variable-name typo in Theorem 5.12 is harmless and is recorded separately.

Sections 3–5Correct and complete

Spectral recursion and limiting support

Pages 8–25 · arXiv:2606.07430v1

The induction retains the exceptional eigenvalues rather than cancelling them, the multiplicity recurrence closes at every level, and the limiting weights form an absolutely summable family. In the support argument, the monotonicity of the polynomial branches and the endpoint estimates cover both parity subsequences and all possible accumulation regimes.

Proof of Theorem 5.12Typo

The contradiction names δ\delta where it must name ζ\zeta

Page 25 · proof of Theorem 5.12 · arXiv:2606.07430v1

The proof defines ζn=2δn\zeta_n=2-\delta_n and shows that the positive quantities δn+2j\delta_{n+2j} eventually exceed two. The printed sentence then says δn+2j<0\delta_{n+2j}<0. The contradiction actually is ζn+2j=2δn+2j<0,\zeta_{n+2j}=2-\delta_{n+2j}<0, contrary to the interval where ζ\zeta is positive. Replacing that single symbol restores the stated logic and changes no estimate.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2606.07430v1
Authors listed
Kyle Ambrose, Noah Dunham, Michael Morris, Luke G. Rogers, Alexander Teplyaev
Audit date
August 18, 2026
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