REVIEW 2 major objections 5 minor 33 references
Thermodynamic formalism for coarse expanding dynamical systems
T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Weakly coarse expanding dynamical systems admit unique equilibrium states for every Hölder potential, along with the central limit theorem, law of iterated logarithm, exponential decay of correlations, and large deviations.
desk verdict Genuinely new and mostly sound paper on equilibrium states for weakly coarse expanding systems, but Theorem 1.1(5) as printed is an ill-posed large-deviation statement that needs to be corrected. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The argument runs through two constructions. First, paths in $W_0$ joining a basepoint to each of its $d$ preimages and avoiding the countable post-branch set build a geometric coding tree: the branches give a Hölder semiconjugacy $\pi$ from the full shift $\Sigma$ onto the repellor $X$, and a no-entropy-drop lemma (the number of depth-$n$ cylinders over a point grows subexponentially when no critical point is periodic) lets equilibrium states and statistical laws be pulled back from the shift. Second, for periodic critical points on the sphere, each such point and its grand orbit is blown up to a circle, giving a system without periodic critical points on a carpet-like space; a metrization lemma produces the exponentially contracting metric on the new repellor.
What would settle it
Find (or prove impossible) a weakly coarse expanding system on a compact, locally connected space that is path connected but not strongly path connected—for instance, a finite tree-like branched cover—and test whether a Hölder potential on its repellor has a unique equilibrium state. If such a system admits two equilibrium states, strong path connectivity is a necessary assumption, not an artifact of the proof.
Extended reading notes
Core claim
The paper introduces weakly coarse expanding dynamical systems—continuous finite branched coverings of locally connected, strongly path connected spaces that expand in a weak metric sense—and proves that whenever no critical point is periodic, every Hölder continuous potential $\phi$ on the repellor $X$ has exactly one equilibrium state, and every Hölder observable $\psi$ satisfies the central limit theorem, the law of the iterated logarithm, exponential decay of correlations, and the large deviation principle. If $W_0$ is an open subset of the 2-sphere, the same conclusions hold even with periodic (repelling) branch points: the authors blow up each periodic critical point and its grand orbit to circles, obtaining a carpet-like space with no periodic critical points, then run the non-critical argument.
Load-bearing premise
The entire construction rests on the phase space being strongly path connected: after deleting any countable set of points, the space must remain path connected, because the coding tree is built from paths that avoid the countable post-branch set. A merely path connected space, such as a tree, can be disconnected by removing a single point.
Editorial extensions
If this is right
- Every Hölder potential on the repellor has a unique equilibrium state, and it is the pushforward of the unique Gibbs state for a Hölder potential on the full shift.
- Every Hölder observable obeys the central limit theorem, law of iterated logarithm, exponential decay of correlations, and a large deviation principle with rate function given by topological pressure.
- The variance in the CLT is zero exactly when the centered observable is a continuous coboundary, and two potentials share an equilibrium state exactly when they differ by a coboundary plus a constant.
- All of this remains true on an open subset of the 2-sphere even when periodic branch points are present.
- The results apply to a class of examples—iterated function systems, semigroup skew products, maps on carpet-like fractals, expanding polynomial-like maps—that need not be conformal, holomorphic, or postcritically finite.
Reading between the lines
- This suggests that strong path connectivity could be relaxed to the existence of a finite covering by path connected sets whose intersections remain path connected after removing countable sets; if a coding tree could be replaced by such a covering, the theorems would extend to dendrites and trees.
- The paper's criterion for zero variance in the CLT is immediately testable numerically on a postcritically infinite rational map: compute sample variances of a Hölder observable along orbits and check whether they vanish precisely for coboundaries.
- The blow-up construction points to a two-way dictionary between periodic branch points and invariant circles, which could generate new examples by starting from a circle map on a carpet and collapsing the invariant circles.
- Since the proofs use only oscillation decay along pullback covers, the results should extend to observables with weaker moduli of continuity than Hölder, as long as the topologically Hölder condition is met.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops thermodynamic formalism for weakly coarse expanding systems: continuous finite branched covers of locally compact strongly path connected spaces satisfying topological expansion and irreducibility. Its main results assert existence and uniqueness of equilibrium states for Hölder potentials and the validity of the central limit theorem, law of the iterated logarithm, exponential decay of correlations, and a large deviation principle, both for systems without periodic critical points (Theorem 1.1) and for open subsets of S² with periodic critical points allowed (Theorem 1.2). The strategy is to encode the system by a geometric coding tree semiconjugate to the full shift, prove that entropy does not drop under the coding map, and transfer statistical laws from the shift; in the S² case, periodic critical points and their grand orbits are blown up to circles, producing a weakly coarse expanding system without periodic critical points.
Significance. The framework unifies and extends earlier work on expanding Thurston maps and coarse expanding conformal systems, allowing postcritically infinite behavior and non-conformal dynamics. The paper contains detailed constructive proofs, including two independent constructions of an exponentially contracting metric (via Frink's lemma and an explicit chain metric), and the coding-tree/no-entropy-drop argument is robust. If the statistical laws are stated correctly, the paper would be a valuable contribution to the thermodynamic formalism of non-uniformly hyperbolic systems. The present version, however, contains a defective large-deviation statement in the headline theorem, so the paper needs revision.
major comments (2)
- [Theorem 1.1(5); Section 4, proof of Theorem 1.1(1)-(5)] The displayed large-deviation assertion in Theorem 1.1(5) is not a well-formed statement. The event contains the expression tψ(x) on the right-hand side of the inequality, and the right-hand side of the limit also contains tψ, so the claimed limit is point-dependent and the equality is not meaningful as a statement about a fixed real t. Moreover, even under the minimal correction tψ→0 on the right-hand side, the formula does not give the rate inherited from the full shift. For the full shift on two symbols, φ≡0, ψ=1_{first coordinate}, and t=0.1, the exact rate for {S_nψ ≥ 0.6n} is −D(0.6‖0.5) ≈ −0.0202, whereas the displayed right-hand side evaluates to P(0.1ψ)−P(0)−0.1∫ψ ≈ +0.0012 under the first correction and to ≈ +0.1012 if tψ is replaced by t. Neither equals the Cramér rate, and the sign is wrong in the first interpretation. The transfer argument in Section 4 can at best yield the standard LDP for the full shift, so Theorem 1.1(5) should be replaced by a correct statement, for example with the rate given by the Legendre transform associated to t↦P(φ+tψ)−P(φ)−t∫ψ. This is a load-bearing defect in the main theorem, not a typographical issue in a proof.
- [Section 5.1, proof of Theorem 1.2(1)-(5)] Theorem 1.2 asserts that claim (5) of Theorem 1.1 holds also in the presence of periodic critical points, and the proof in Section 5.1 says that statistical laws follow from the shift by pushforward. Since claim (5) as printed is ill-posed and the displayed rate is not the shift rate, this proof cannot establish the stated theorem. The correction to Theorem 1.1(5) must be propagated to Theorem 1.2; otherwise the main theorems remain defective even if the rest of the thermodynamic formalism argument is sound.
minor comments (5)
- [Section 4, Lemma 4.3] In the proof of Lemma 4.3, the equality "π⋆~µ = ~µ" should read "π⋆~µ = µ"; the current text appears to be a typo.
- [Section 5, after Proposition 5.1] The symbol ~W0 is used both for the blown-up space and for the lifted cover ("Denote this cover by ~W0"), which creates ambiguity in Lemma 5.3; a different letter, such as ~U0, should be used for the cover.
- [Theorem 1.1(5)] The function sgn(t) is undefined at t=0; the corrected large-deviation statement should specify t∈R\{0} or include a separate convention for t=0.
- [Section 2.6, proof of Proposition 2.16] The phrase "a curve which is the branch of f^{-(n-1)}(γ_{i_n})" is not formally precise, since f^{-(n-1)} of a curve is not a single curve; it should say that γ_n(α) is the lift of γ_{i_n} by f^{n-1} starting at z_{n-1}(α).
- [Appendix A, after equation (14)] The parameter c is introduced as "a constant which will be determined later" but the proof only chooses it in Step 3; the text should state explicitly that c can be made small enough to satisfy both cd²e^{α₂}<1 and cd²<1.
Circularity Check
No circularity: the coding-map reduction to the full shift is self-contained; self-citations are to established, independently verified thermodynamic formalism.
full rationale
The paper's derivation is not circular. The central reduction constructs an explicit Holder semiconjugacy pi : Sigma -> X (Proposition 2.16), proves that entropy does not drop under pi (Lemma 3.3), compares pressures (Lemma 4.2), and transfers equilibrium states and statistical laws from the full shift. These steps are proven in the paper, not assumed. The shift-space CLT, LIL, exponential decay, and large deviations are cited to [24] (Przytycki-Urbanski) and [10]; although [24] is by two of the present authors, these are published, standalone classical results with independent proofs, so the citation is real evidence and does not make the argument circular. No parameter is fitted to data, and no prediction is equivalent to an input by construction. The large-deviation display in Theorem 1.1(5) is ill-posed as printed (the right-hand side depends on the point x through the term t psi(x)), but that is a correctness/typographical defect, not a circularity: the proof does not derive that displayed formula from itself.
Assumptions & free parameters
assumptions (8)
- standard math Variational principle for topological pressure on compact metric spaces.
- standard math Thermodynamic formalism for the one-sided full shift on d symbols: unique equilibrium states, Gibbs property, CLT, LIL, exponential decay of correlations, large deviations for Holder potentials.
- standard math Riesz extension theorem and Riesz representation theorem.
- standard math Frink's metrization lemma for constructing metrics from expanding sequences of covers.
- standard math Topological facts: Jordan curve theorem, Caratheodory's theorem on extensions of Riemann maps, Whyburn's theorem on open maps on 2-manifolds.
- domain assumption The space W0 is strongly path connected (Definition 2.3), i.e. removing any countable set leaves it path connected.
- domain assumption The system satisfies the weakly coarse expanding axioms: [Expansion], [Irreducibility] (leo), finite branch set, and repellor X not a single point (Definition 2.10).
- domain assumption For Theorem 1.2, W0 is an open subset of the 2-sphere S^2 with Euclidean topology.
invented entities (1)
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Blown-up space ~W0 (and repellor Y), in which each point in the grand orbit of a periodic critical point is replaced by a circle, producing a Sierpinski carpet.
Cite this review
Pith. "Pith review of Thermodynamic formalism for coarse expanding dynamical systems." pith.science (2026). https://pith.science/paper/QN3EEAAP
@misc{pith2026190808270,
author = {Pith},
title = {Pith review of: Thermodynamic formalism for coarse expanding dynamical systems},
year = {2026},
howpublished = {\url{https://pith.science/paper/QN3EEAAP}},
note = {Machine review of arXiv:1908.08270}
}
read the original abstract
We consider a class of dynamical systems, which we call weakly coarse expanding, which is a generalization to the postcritically infinite case of expanding Thurston maps as discussed by Bonk-Meyer and is closely related to coarse expanding conformal systems as defined by Haissinsky-Pilgrim. We prove existence and uniqueness of equilibrium states for a wide class of potentials, as well as statistical laws such as a central limit theorem, law of iterated logarithm, exponential decay of correlations and a large deviation principle. Further, if the system is defined on the 2-sphere, we prove all such results even in presence of periodic (repelling) branch points.
Reference graph
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Reviewed August 14, 2026 · model on record in the stance chip above.
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