REVIEW 3 major objections 4 minor 48 references
Size-Ramsey numbers of tight paths
T0 review · 3 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read This paper proves that the s-colour size-Ramsey number of the r-uniform tight path on n vertices is linear in n for every fixed r and s, resolving a 2017 question.
desk verdict Major result—linear size-Ramsey for all tight paths—but there is a real gap in the proof of Lemma 4.24 that needs patching before the paper is complete. 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 is carried by a hierarchy of rooted, ordered forests attached to the vertices of a bounded-degree $\varepsilon$-expander graph $G$. The host hypergraph is $G^{p'}$, the $r$-uniform hypergraph whose edges are $r$-tuples of vertices pairwise within distance $p'$ in $G$. A colouring with no long monochromatic tight path is shown to be $k$-disconnected: no short monochromatic tight walk joins two independent $(r-1)$-sets of leaves of the same 'type' in nearby trees. Ramsey properties of powers of expanders (Lemma 3.9) then force either a long monochromatic path in an auxiliary colouring or many disjoint monochromatic cliques; each clique is used to 'augment' the forest into taller trees that remain disconnected, while Lemma 4.25 bounds the possible height of any such disconnected family. The contradiction proves the existence of a long monochromatic tight walk in the original colouring.
What would settle it
A direct refutation would be a sequence of $r$-uniform hypergraphs with $O(n)$ edges and bounded maximum degree whose edges can be $s$-coloured so that every monochromatic tight path has $o(n)$ vertices. Concretely, finding such a construction for $r=3$, $s=2$ for infinitely many $n$ would contradict Theorem 1.1, since the theorem promises that every $O(n)$-edge bounded-degree host forces a monochromatic tight path of linear length.
Extended reading notes
Core claim
On the paper's own terms, the central claim is Theorem 1.1: for fixed integers $r,s \ge 1$, the $s$-colour size-Ramsey number of the $r$-uniform tight path $P_n^{(r)}$ is $O(n)$. Equivalently, for any fixed uniformity and any fixed number of colours, an $n$-vertex hypergraph with $O(n)$ edges can be built that is Ramsey for that path in every colouring. The stronger technical engine is Theorem 5.1, which produces, for every $r \ge 3$ and $s \ge 2$, a bounded-degree $n$-vertex $r$-uniform hypergraph in which every $s$-colouring contains a monochromatic tight walk of length $\Omega(n)$ that uses each vertex only a bounded number of times; a short blow-up argument converts such a walk into an actual tight path. This settles in the affirmative the 2017 question and supersedes the previous $O((n \log n)^{r/2})$ upper bound for $r \ge 4$.
Load-bearing premise
The load-bearing premise is that arbitrarily large connected expander graphs of bounded degree exist for every small expansion parameter; the paper cites this to an unpublished companion manuscript and offers only a brief probabilistic sketch, yet the host hypergraph cannot be built without such graphs.
Editorial extensions
If this is right
- For every fixed $r$ and $s$, $\hat{r}_s(P_n^{(r)}) = O(n)$, so the size-Ramsey number matches the order of the number of vertices of the target path.
- The result holds for an arbitrary fixed number of colours, not just two, and the host hypergraph can be chosen with maximum degree bounded by a constant depending only on $r$ and $s$.
- The earlier upper bound $O((n \log n)^{r/2})$ for $r \ge 4$ is improved to linear, and the $r=3$ case is recovered as a special case.
- A monochromatic tight walk with bounded vertex repetition can be converted into a genuine monochromatic tight path, so the proof's walk-based formulation directly implies the path statement.
- According to the paper, the same proof approach also gives linear size-Ramsey numbers for powers of tight paths, tight hypergraph trees, and long subdivisions of bounded-degree hypergraphs.
Reading between the lines
- Beyond the paper: the constants hidden in the $O(n)$ bound grow rapidly with $r$ and $s$, and known lower bounds suggest the linear coefficient genuinely depends on both parameters; determining the correct order in $r$ and $s$ is a natural next problem.
- Beyond the paper: the expander-power construction is a plausible template for proving linear size-Ramsey bounds for other sparse hypergraph families, such as bounded-degree tight trees or grid-like hypergraphs, by adapting the tree-hierarchy and type machinery.
- Beyond the paper: the contradiction is formulated in terms of monochromatic tight walks with bounded vertex repetition, a stronger notion than simple paths, so the method may transfer to 'blow-up Ramsey' problems or to structured walks in other sparse hypergraphs.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves that the s-colour size-Ramsey number of the r-uniform tight path P_n^{(r)} is O(n) for every fixed r and s, answering a question of Dudek, La Fleur, Mubayi and Rödl (2017). The proof constructs a bounded-degree expanding graph G and shows that an appropriate power G^p, viewed as an r-uniform hypergraph, is Ramsey for tight paths. The main technical engine is Theorem 5.1, which asserts the existence of bounded-degree r-uniform hypergraphs on n vertices in which every s-colouring contains a long monochromatic tight walk with bounded vertex multiplicity. The proof introduces ordered forests, types of leaf subsets, clean colourings, disconnected tree assignments, and versatile sets, and culminates in a structural contradiction (Lemma 4.25). The final section derives the size-Ramsey bound from Theorem 5.1 via a blow-up argument.
Significance. The result resolves an open problem from Dudek, La Fleur, Mubayi and Rödl (2017), extending the known case r = 3 and improving the best previous bound for r ≥ 4. The paper introduces a substantial new toolkit for hypergraph size-Ramsey problems, and the overall strategy is coherent and ambitious. If the technical gaps identified below are repaired, the paper would be a significant contribution to extremal combinatorics. The final blow-up argument is clean, and the dependence on expanders is standard; the paper also gives a concise reduction of the main theorem to a single structural statement (Theorem 5.1).
major comments (3)
- [Section 4.3, Lemma 4.24] The proof of Lemma 4.24 applies Lemma 4.17 to the tree T, but Lemma 4.17 requires an r-uniform hypergraph H of maximum degree at most Δ and a V(H)-forest F. Here H is the complete r-uniform hypergraph on L(T), whose maximum degree is unbounded (it is binomial in |L(T)|), and T is not a V(H)-forest with V(H) = L(T). Since Lemma 4.25 and hence Theorem 5.1 rely on Lemma 4.24, this is a load-bearing gap. The desired statement is plausible and can be proved by iterating Lemma 4.16 over the finite set Types(h, r), reducing the arity by a controlled amount at each step; please provide this argument explicitly.
- [Section 4.3, Observation 4.19 and Lemma 4.25] Observation 4.19 is false as stated: the claimed equivalence between e being an edge of G^t ⊗_r F and ||e||_G ≤ t fails for r-sets e with fewer than r distinct π0-values, because {v} need not be contained in any r-edge of G^t. For example, if G is a path, t = 1, and r = 3, then G^1 has no edges and no r-subset of L(F(v)) with π0(e) = {v} is an edge, yet ||e||_G = 0. The proof of Lemma 4.25 uses this false converse to conclude that the sequence P constructed from Lemma 4.24 is a tight walk in G^t ⊗ F. In the intended application (t = c_h ≥ 2 and G an expander with Δ ≫ r) the needed direction does hold, because every vertex has at least r vertices in its closed neighbourhood and all r-subsets of L(F(v)) are then contained in edges of G^{c_h}; but the paper must state and prove this, and the statement of Lemma 4.25 should be restricted to the range where the complete hypergraph on L(F(v)) embeds into G^t ⊗ F.
- [Section 4.3, Lemma 4.25] Lemma 4.25 is stated for every non-negative integer t, but it is false for small t: if G is a path and t = 1 with r = 3, then G^1 ⊗ F has no edges, so every colouring is vacuously 1-disconnected regardless of the height of F. The proof implicitly assumes that every r-subset of leaves with constant π0 is an edge, which requires t large enough relative to the local structure of G. Since Theorem 5.1 only needs t = c_h for the large constants produced by the hierarchy, the lemma should be reformulated for that range, with the missing embedding argument supplied.
minor comments (4)
- [Lemma 3.9] In the proof of Lemma 3.9, 'we either get a colour 1 copy of K_d' should read 'colour d+1' (or 'colour s+1'), since the K_d in Lemma 3.8 is in the last colour, not colour 1.
- [Theorem 5.1] In the final paragraph of the proof of Theorem 5.1, 'a balanced d_h-ary V(G)-forest' and 'G^{d_h} ⊗ F_h' should be 'c_h-ary' and 'G^{c_h} ⊗ F_h', consistent with properties (F2) and (F5).
- [Lemma 4.25] The statement assumes r ≤ s, but the proof does not use this inequality. If the result is intended for all s, remove the restriction; otherwise, Theorem 5.1's application for r > s needs a short reduction (e.g., viewing an s-colouring as an r-colouring with unused colours).
- [Sections 2–3] The notation G^k is defined as the r-uniform hypergraph power, but in Section 3 (Lemmas 3.7–3.9) it is used for the ordinary graph power; please disambiguate, for instance by writing G^k_{(r)} for the hypergraph in the Ramsey lemmas or by explicitly saying that Section 3 uses graph powers.
Circularity Check
No circular derivation: the size-Ramsey bound is obtained from a self-contained Ramsey engine, with one standard self-cited expander fact and one non-circular proof gap.
full rationale
Theorem 1.1 is derived from Theorem 5.1 by a direct blow-up argument: Theorem 5.1 provides a bounded-degree host H with a long monochromatic tight walk, and the proof converts that walk to a tight path in H[d] using the cleaning lemma (Lemma 4.17) and the repetition bound from Theorem 5.1; the target statement is not assumed at any point. Theorem 5.1 is proved by building forests F_i and invoking Lemma 4.25 for the final contradiction, with no fitted parameter or renamed known result. Two caveats deserve explicit flags, but neither is circular. (1) Lemma 3.2, the existence of arbitrarily large bounded-degree epsilon-expanders, is cited to the authors' own unpublished manuscript [38]: 'It is a standard result that there exist arbitrarily large epsilon-expanders of bounded degree (e.g. G(n, C/n) satisfies this after deleting high degree vertices. See [38], Proposition 3.2).' This is a self-citation, but it is a parameter-free existence fact whose stated assumptions do not include the size-Ramsey conclusion, and the paper itself gives the G(n,C/n) probabilistic sketch, so it is independent support rather than circular dependence. (2) In Lemma 4.24, the line 'By Lemma 4.17 applied to T, there is a balanced binary subtree T′ ≤ T of height h, such that r-sets of leaves of the same type have the same colour' applies Lemma 4.17 outside its stated hypotheses: Lemma 4.17 requires an r-uniform hypergraph H of maximum degree at most Δ and a V(H)-forest, while the natural H here is the complete r-uniform hypergraph on L(T), whose maximum degree is unbounded, and T is not a V(H)-forest on L(T). This is a load-bearing proof gap (the claim is plausible via iteration of Lemma 4.16), but it is a missing-support or correctness issue, not a reduction of the theorem to its own input. No equation or construction in the paper identifies the predicted quantity with a fitted parameter or with the theorem's hypothesis, so the circularity score is 0.
Assumptions & free parameters
assumptions (5)
- domain assumption Existence of arbitrarily large connected epsilon-expanders with maximum degree epsilon^{-2} (Lemma 3.2).
- standard math Path-partition lemma of [6]: every graph can be partitioned into a path P and two equal-size sets A,B with no edges between A and B (used in Lemma 3.3).
- standard math Ramsey's theorem (used in Lemmas 3.5, 4.16, 4.24 and in the cleaning lemma 4.17).
- standard math Chernoff bounds (used in Lemma 4.14 to find disjoint subtrees).
- standard math The line graph of an r-uniform hypergraph with maximum degree Delta has chromatic number at most r*Delta + 1 (used in Lemma 4.17 to partition edges into matchings).
Cite this review
Pith. "Pith review of Size-Ramsey numbers of tight paths." pith.science (2026). https://pith.science/paper/ZR6MMDUH
@misc{pith2026250701498,
author = {Pith},
title = {Pith review of: Size-Ramsey numbers of tight paths},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZR6MMDUH}},
note = {Machine review of arXiv:2507.01498}
}
abstract
The $s$-colour size-Ramsey number of a hypergraph $H$ is the minimum number of edges in a hypergraph $G$ whose every $s$-edge-colouring contains a monochromatic copy of $H$. We show that the $s$-colour size-Ramsey number of the $r$-uniform tight path on $n$ vertices is linear in $n$, for every fixed $r$ and $s$, thereby answering a question of Dudek, La Fleur, Mubayi and R\"odl (2017).
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Reference graph
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