Recognition: no theorem link
Continuity properties of partial entropy
Pith reviewed 2026-05-14 18:31 UTC · model grok-4.3
The pith
For C^{1+α} diffeomorphisms, partial entropy in every direction is upper semi-continuous whenever the sums of Lyapunov exponents are continuous.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We establish a general criterion on the upper semi-continuity of partial entropy in all directions for C^{1+α} diffeomorphisms: it holds when the respective sums of Lyapunov exponents are continuous. This addresses, in arbitrary dimensions, the converse aspect of the entropic continuity of the Lyapunov exponents established by Buzzi, Crovisier, and Sarig. Consequently, the entropy (and all the partial entropies) is always upper semi-continuous at generic ergodic measures of every C^{1+α} diffeomorphism.
What carries the argument
The criterion that upper semi-continuity of partial entropy holds in all directions precisely when the corresponding sums of Lyapunov exponents are continuous.
If this is right
- Entropy itself is upper semi-continuous at every generic ergodic measure.
- The result applies to measures with dominated splittings and to SRB measures.
- The same upper semi-continuity holds for average expanding diffeomorphisms and for singular flows.
- The criterion covers standard maps and symbolic codings of diffeomorphisms.
Where Pith is reading between the lines
- If future work shows that Lyapunov sums are continuous for a larger class of maps, the new criterion would immediately give upper semi-continuity of entropy there as well.
- The argument may adapt to time-one maps of flows once the corresponding Lyapunov sums are known to be continuous.
- Generic ergodic measures now inherit upper semi-continuity without needing infinite differentiability, which could simplify proofs that rely on entropy continuity.
Load-bearing premise
The map must be C^{1+α} and the sums of Lyapunov exponents must be continuous; the conclusion can fail if either requirement is removed.
What would settle it
A C^{1+α} diffeomorphism in which the sums of Lyapunov exponents are continuous at some ergodic measure yet at least one partial entropy fails to be upper semi-continuous at that measure.
Figures
read the original abstract
We establish a general criterion on the upper semi-continuity of partial entropy in all directions for $C^{1+\alpha}$ diffeomorphisms: it holds when the respective sums of Lyapunov exponents are continuous. This addresses, in arbitrary dimensions, the converse aspect of the entropic continuity of the Lyapunov exponents established by Buzzi, Crovisier, and Sarig. Consequently, the entropy (and all the partial entropies) is always upper semi-continuous at generic ergodic measures of every $C^{1+\alpha}$ diffeomorphism, which extends the $C^{\infty}$ result of Newhouse. Numerous applications and examples are provided, including topics related to measures with dominated splittings, SRB measures, average expanding diffeomorphisms, singular flows, standard maps, and symbolic codings for diffeomorphisms.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript establishes a general criterion for C^{1+α} diffeomorphisms: the upper semi-continuity of partial entropy in all directions holds precisely when the corresponding sums of Lyapunov exponents are continuous. This supplies the converse direction to the Buzzi–Crovisier–Sarig theorem on entropic continuity of Lyapunov exponents. As a consequence, entropy and all partial entropies are upper semi-continuous at generic ergodic measures for every such diffeomorphism, extending Newhouse’s C^∞ result. The paper supplies applications and examples involving dominated splittings, SRB measures, average expanding diffeomorphisms, singular flows, standard maps, and symbolic codings.
Significance. If the stated criterion holds, the work is significant: it furnishes a dimension-independent, if-and-only-if link between partial-entropy upper semi-continuity and continuity of Lyapunov sums, thereby converting a continuity question about entropy into one about more tractable exponent sums. The automatic upper semi-continuity at generic ergodic measures strengthens the global picture of entropy regularity in smooth dynamics and extends Newhouse’s theorem. The breadth of applications (dominated splittings, SRB measures, singular flows, standard maps) indicates the criterion is a practical tool rather than an isolated statement.
minor comments (3)
- Introduction: the list of applications should be accompanied by explicit section or theorem references so that readers can locate each example without searching the entire text.
- Notation section: the symbol for partial entropy (presumably h_μ(·,ξ) or similar) should be defined before its appearance in the statement of the main criterion.
- References: confirm that the Buzzi–Crovisier–Sarig citation includes the precise journal or arXiv details and that Newhouse’s C^∞ result is cited with the correct reference.
Simulated Author's Rebuttal
We thank the referee for the careful reading and positive assessment of the manuscript. The summary accurately reflects the main result: an if-and-only-if criterion linking upper semi-continuity of partial entropies to continuity of the corresponding Lyapunov exponent sums for C^{1+α} diffeomorphisms, together with the automatic upper semi-continuity at generic ergodic measures. We are pleased that the breadth of applications is viewed as a practical contribution.
Circularity Check
No circularity: criterion conditioned on external continuity assumption
full rationale
The derivation establishes an if-then criterion for upper semi-continuity of partial entropy in C^{1+α} diffeomorphisms precisely when the corresponding Lyapunov exponent sums are continuous. This is framed as the converse to the independent Buzzi–Crovisier–Sarig result on entropic continuity of Lyapunov exponents and extends Newhouse's C^∞ case. No step reduces by definition to its own inputs, no parameter is fitted and relabeled as a prediction, and no load-bearing premise rests on a self-citation chain. The argument remains self-contained against the stated external continuity hypothesis.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Standard properties of Lyapunov exponents, entropy, and C^{1+α} diffeomorphisms from prior literature
Reference graph
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