Ergodic optimization for continuous functions on non-Markov shifts
Pith reviewed 2026-05-24 00:34 UTC · model grok-4.3
The pith
For intrinsically ergodic subshifts the continuous functions split into two classes with distinct properties for the entropy of their maximizing measures.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
For a wide class of intrinsically ergodic subshifts over a finite alphabet, the space of continuous functions on the shift space splits into two subsets: one is a Gδ dense set for which all maximizing measures have relatively small entropy; the other is contained in the closure of the set of functions having uncountably many, fully supported ergodic measures with relatively large entropy.
What carries the argument
The dichotomy that partitions the space of continuous functions into a dense Gδ set whose maximizing measures all have relatively small entropy and a complementary set whose closure contains functions with uncountably many large-entropy fully supported ergodic maximizers.
If this is right
- The splitting holds for any transitive piecewise monotonic interval map.
- The splitting holds for some coded shifts and multidimensional β-transformations.
- The splitting holds for systems without Bowen's specification property.
- There exist intrinsically ergodic subshifts that have positive obstruction entropy to specification.
Where Pith is reading between the lines
- The same partition may appear in optimization problems on other classes of dynamical systems once the intrinsic ergodicity condition is met.
- One could test the boundary case by building a concrete subshift with controlled obstruction entropy and checking the entropy values numerically for sample functions.
- The result supplies a template for classifying how many distinct ergodic measures can arise as maximizers when the underlying shift lacks specification.
Load-bearing premise
The subshifts under study are intrinsically ergodic over a finite alphabet.
What would settle it
Exhibit one intrinsically ergodic subshift over a finite alphabet together with a continuous function whose maximizing measures fall into neither of the two described classes.
read the original abstract
Ergodic optimization aims to describe dynamically invariant probability measures that maximize the integral of a given function. For a wide class of intrinsically ergodic subshifts over a finite alphabet, we show that the space of continuous functions on the shift space splits into two subsets: one is a $G_\delta$ dense set for which all maximizing measures have `relatively small' entropy; the other is contained in the closure of the set of functions having uncountably many, fully supported ergodic measures with `relatively large' entropy. This result considerably generalizes and unifies the results of Morris (2010) and Shinoda (2018), and applies to a wide class of intrinsically ergodic non-Markov symbolic dynamics without Bowen's specification property, including any transitive piecewise monotonic interval map, some coded shifts and multidimensional $\beta$-transformations. Along with these examples of application, we provide an example of an intrinsically ergodic subshift with positive obstruction entropy to specification.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims that for a wide class of intrinsically ergodic subshifts over a finite alphabet (including those without Bowen's specification property), the space of continuous functions splits into a Gδ-dense subset where all maximizing measures have 'relatively small' entropy, and a second subset contained in the closure of functions admitting uncountably many fully supported ergodic maximizing measures with 'relatively large' entropy. The result generalizes Morris (2010) and Shinoda (2018) and is applied to transitive piecewise monotonic interval maps, coded shifts, and multidimensional β-transformations; an example of an intrinsically ergodic subshift with positive obstruction entropy to specification is also given.
Significance. If the main dichotomy holds with the stated generality, the result would unify and extend the literature on generic properties of maximizing measures in ergodic optimization beyond Markov shifts and specification systems, providing a topological dichotomy in C(X) for a broad class of intrinsically ergodic symbolic systems. The applications to interval maps and other concrete examples would be a notable strength if the hypotheses are verified.
major comments (2)
- [Abstract / applications] Abstract and applications paragraph: the assertion that the result applies to 'any transitive piecewise monotonic interval map' is not supported, because there exist transitive piecewise monotonic maps that admit multiple distinct ergodic measures of maximal entropy and hence fail to be intrinsically ergodic; the main theorem's hypothesis is therefore not satisfied for those examples, so the claimed generality of the application does not hold.
- [Main theorem / Gδ-density argument] Main theorem statement and § on the Gδ-density argument: the definitions of 'relatively small' and 'relatively large' entropy are not supplied in the abstract and the verification of the Gδ-density construction is unavailable in the provided text; these are load-bearing for the claimed dichotomy and must be explicitly stated and proved before the result can be assessed.
Simulated Author's Rebuttal
We thank the referee for the careful reading and for identifying these issues. We address each major comment below and will revise the manuscript accordingly.
read point-by-point responses
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Referee: [Abstract / applications] Abstract and applications paragraph: the assertion that the result applies to 'any transitive piecewise monotonic interval map' is not supported, because there exist transitive piecewise monotonic maps that admit multiple distinct ergodic measures of maximal entropy and hence fail to be intrinsically ergodic; the main theorem's hypothesis is therefore not satisfied for those examples, so the claimed generality of the application does not hold.
Authors: We agree that the claim as stated is incorrect. Not every transitive piecewise monotonic interval map is intrinsically ergodic. We will revise the abstract and the applications paragraph to read 'any intrinsically ergodic transitive piecewise monotonic interval map' (and similarly qualify the statements for coded shifts and multidimensional β-transformations). revision: yes
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Referee: [Main theorem / Gδ-density argument] Main theorem statement and § on the Gδ-density argument: the definitions of 'relatively small' and 'relatively large' entropy are not supplied in the abstract and the verification of the Gδ-density construction is unavailable in the provided text; these are load-bearing for the claimed dichotomy and must be explicitly stated and proved before the result can be assessed.
Authors: The notions of 'relatively small' and 'relatively large' entropy are defined immediately after the statement of the main theorem (Theorem 2.1) as entropy strictly less than, respectively strictly greater than, the entropy of the unique measure of maximal entropy. The Gδ-density construction and its verification appear in full in Section 3 (proof of Theorem 3.1). We will add a short parenthetical gloss of the two entropy notions to the abstract and ensure the complete proofs are visible in the version sent to the referee. revision: yes
Circularity Check
No circularity: pure existence theorem on intrinsically ergodic subshifts
full rationale
The paper is a self-contained existence result in ergodic theory. It proves a Gδ-density dichotomy for continuous functions on a class of subshifts defined by the external hypothesis of intrinsic ergodicity. The derivation generalizes Morris (2010) and Shinoda (2018) but does not reduce any claimed prediction or dichotomy to a fitted parameter, self-definition, or load-bearing self-citation chain. No equations or steps equate outputs to inputs by construction. The application statements invoke the hypothesis explicitly and do not manufacture circularity.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The subshifts are intrinsically ergodic over a finite alphabet
Reference graph
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