REVIEW 2 major objections 4 minor 2 cited by
Ramsey numbers of trees
T0 review · 2 major / 4 minor · reviewed 2026-08-04 · deepseek-v4-flash
Pith's one-line read Low-degree trees have exact Ramsey numbers
desk verdict Exact Ramsey numbers for small-linear-degree trees: real advance, coherent proof, but load-bearing Theorem 5.1 is asserted from HLT rather than proved; conditional as written. 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 proof's load-bearing mechanism is a four-stage stability analysis on the reduced graph of a regularity partition. It starts from a scaled-down copy of the structure produced by the 2002 asymptotic proof (an 'A-situation': a vertex joined to one side of a nearly complete bipartite matching, with cluster sizes in ratio t1:t2), then passes through B-, C- and D-situations, at each stage either embedding T or concluding the reduced graph is extremal. The embedding workhorse is Lemma 4.2, which cuts T into tiny components through a homomorphism into a fixed auxiliary graph S, randomly assigns components to regular pairs, and uses concentration to keep cluster loads below capacity; specialised
What would settle it
Apply the 2002 proof to an explicit red/blue colouring of K_{t1+2t2-1} with t1≈2t2 and check whether the asserted starting structure appears: clusters of sizes m and (t1/t2)m, three equal parts each covering about (1−2ε)t2 vertices, a perfect matching, and a vertex joined to one side. Any colouring whose reduced graph cannot be partitioned this way while leaving the leftover clusters regular would invalidate the stability start. Alternatively, one counterexample tree with Δ(T)≤cn and R(T) > max{2t1, t1+2t2}−1 would directly falsify Theorem 1.1.
Extended reading notes
Core claim
The central claim is Theorem 1.1: there is an absolute c>0 such that every n-vertex tree T with Δ(T)≤cn and bipartition classes of sizes t1≥t2 has Ramsey number R(T)=max{2t1, t1+2t2}−1. The two extremal colourings from 1974 show R(T) is at least this; the paper proves the matching upper bound by showing every red/blue colouring of K_{max{2t1,t1+2t2}-1} contains a monochromatic copy of T. The proof works by a stability dichotomy: if the colouring is not close to either of those extremal colourings, a regularity-based argument embeds T; if it is close, separate extremal theorems still embed T using structure of the tree and sparse random choices. Since the theorem holds for every tree meeting
Load-bearing premise
The stability part starts from a version of the 2002 asymptotic proof that is asserted, not derived here, to produce the scaled-down structure while remembering the unused regularity clusters; if that extraction fails, the whole four-stage argument has no foundation.
Editorial extensions
If this is right
- For every tree with maximum degree at most cn, the Ramsey number is read off from two integers t1,t2 rather than from the tree's shape.
- The 1974 formula is exact for all small-linear-degree trees, so any counterexample to the conjecture must have maximum degree exceeding the fixed constant c.
- The stability theorem gives a usable dichotomy: non-extremal colourings on exactly the conjectured number of vertices force a monochromatic copy of the tree.
- The extremal lemmas show that colourings approximating the two extremal constructions still contain the tree, despite a known obstruction that makes naive embedding fail.
Reading between the lines
- The constant c is produced by hierarchy arguments and is almost certainly not optimal; a natural next step is to determine the largest c for which the formula survives, with known double-star examples bounding any possible c by 7/11+o(1).
- The unstated starting lemma of the stability part, if true, is a reusable 'remembered clusters' version of the 2002 proof; formalising it could simplify future exact Ramsey results that work with a one-vertex deficit.
- The sparse-cut and random leaf-embedding techniques in the extremal part may apply to other spanning tree problems in graphs that are almost complete or almost complete bipartite under a few forbidden edges.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves an exact Ramsey-number formula for every n-vertex tree whose maximum degree is at most a small constant times n: R(T) = max{2t_1, t_1+2t_2}-1, where t_1≥t_2 are the bipartition class sizes. This sharpens the asymptotic result of Haxell, Luczak, and Tingley and resolves, in the small-linear-degree regime, a question of Stein and a positive part of Burr's 1974 conjecture. The proof is divided into a stability part, which shows that a red/blue colouring either contains T or is close to one of Burr's two extremal constructions, and an extremal part, which handles both near-extremal cases by a substantial set of new embedding lemmas.
Significance. If the main theorem is correct, it is a significant exact result in Ramsey theory for trees, converting a previously asymptotic statement into an exact one under a natural bounded-degree hypothesis. The paper is also structurally ambitious: it develops a large toolbox of regularity-based embedding methods (EM1a-c, EM2a-d, HLT), and the extremal part contains delicate new arguments needed because small linear maximum degree still permits trees of small diameter. The proof is not circular: the upper bound is compared against Burr's external lower-bound constructions, and the paper explicitly notes the known 7/11+o(1) ceiling from Norin–Sun–Zhao, so the small constant c is not being optimized. However, the entire stability argument rests on an unproved extraction from earlier work, which is a load-bearing gap in the current version.
major comments (2)
- [Section 5, Theorem 5.1] Theorem 5.1 is the starting point of the stability proof, but it is not proved in this manuscript and is not a stated theorem of [21]. The preceding paragraph says it 'easily follows from the proof of [21, Theorem 3]' applied with alpha=t1/t2 and n=(1-epsilon)(t1+2t2), while 'remembering' regularity clusters outside the HLT structure. This is exactly the kind of modification that needs a proof: one must show that the clusters outside the HLT structure can be retained with the required two sizes m and t1m/t2, that the partition into I_A,I_B,I_C with |I_A|=|I_B|=|I_C|=k and km≥(1-2ε)t2 can be produced, and that the perfect matching and star adjacency survive. No derivation or precise external reference is supplied. Since Lemma 5.9 consumes this structure and the chain Lemmas 5.9 -> 5.8 -> 5.5 -> 5.4 depends on it, the central claim is conditional on an unverified external proof obligation.
- [Section 5, Theorem 5.1, bullets] As stated, Theorem 5.1 is internally inconsistent in the colour of the structure: it says 'In R*, 0 is adjacent to every a∈I_A' but then 'R_red[I_A,I_B] contains a perfect matching'. If * is blue, these two properties are in different colours and do not form the monochromatic HLT- structure required by Lemma 5.9, whose assumptions Q3 and Q4 are both red. The statement should presumably have R*[I_A,I_B] in the matching bullet. This is not a mere presentation issue, because the colour consistency of the structure is essential for every subsequent stage.
minor comments (4)
- [Section 2.1 vs Section 5.7] The reduction to t1≤2t2+1 in Section 2.1 uses t'_2=floor(t1/2), while the analogous reduction in the proof of Theorem 2.2 (Section 5.7) uses t'_2=ceil(t1/2). The two claims are not the same and the notation should be harmonized, with the relevant inequalities checked for both parity cases.
- [Throughout] There are several typographical slips: 'Let Let I_A,3' at the start of the proof of Lemma 5.9; 'with with a partition' in Lemmas 4.6, 4.8, 4.9, 4.12, 4.13; 'F or' in Section 3; 'simiply' in Section 6.1. These do not affect the mathematics but should be corrected.
- [Section 3, Stage 2] The summary of the cascading argument is brief. The formal Lemma 5.10 is clear, but Stage 2 also uses a refinement into clusters of two different sizes (gamma m and gamma t1m/t2) and then requires an application of Lemma 5.5 after 'removing clusters with low degrees'. It would help the reader to state explicitly which hierarchy of constants makes this cluster-size conversion legitimate, since the B-situation in Lemma 5.8 has equal-sized clusters and the C-situation in Lemma 5.5 also assumes equal-sized clusters after refinement.
- [Section 5.1, Definition 5.2 and Lemma 5.3] In the Type II case of Lemma 5.3, the proof is omitted as 'similar'. Since Type II extremality is used in the final theorem, a few sentences indicating the counting and vertex-removal argument would make the dependency explicit.
Circularity Check
No significant circularity: the proof derives new embedding lemmas and combines them with an external HLT theorem; there is no reduction of the claimed result to its own inputs.
full rationale
The paper's derivation chain is: use Burr's lower bound (external, 1974) for R(T) >= max{t1+2t2,2t1}-1; prove stability Theorem 2.2 and extremal Theorems 2.3-2.4; then deduce Theorem 1.1. The upper-bound machinery is developed in Section 4 through self-contained regularity embedding lemmas (Lemmas 4.2, 4.5-4.13), none of which assume the target Ramsey number. The only imported structural input is Theorem 5.1, stated as 'easily follows from the proof of [21, Theorem 3]' and attributed to Haxell, Luczak, and Tingley. This is an independent external source, not a prior work of the present authors. No parameter is fitted to the target formula, no prediction is defined in terms of the quantity being predicted, and no known result is merely renamed. The one genuine concern about Theorem 5.1 is that its precise 'remembering' version is not proved in the manuscript and the justification is a one-sentence reference to the HLT proof; however, that is a proof gap or derivation dependency, not circularity: the cited HLT theorem and proof are independent of the present paper's conclusions. Similarly, the dependence of Stages 1-4 on Theorem 5.1 is a conditional chain, but conditionality is not equivalence. The lower bound and all upper-bound cases are checked against an external benchmark (Burr's construction), so the central claim has independent content. Score 0.
Assumptions & free parameters
free parameters (1)
- c (linear maximum-degree constant) =
not specified (existential)
assumptions (5)
- domain assumption Coloured Szemeredi regularity lemma (Theorem 2.22, cited as [24, Theorem 1.18])
- domain assumption Theorem 5.1, a modified A-situation structure asserted to follow from the proof of [21, Theorem 3]
- standard math Standard probabilistic and matching tools: Chernoff, Azuma, McDiarmid, Hall's theorem, Dirac's theorem (Lemmas 2.5-2.9)
- standard math Tree decomposition lemmas (Lemma 2.16 cited as [2, Proposition 4.1]; Lemma 2.11 cited as [25, Lemma 2.1]; Lemma 2.13 cited as [28, Proposition 3.19])
- standard math Burr's 1974 lower bound constructions (Figure 1) showing R(T) >= max{t1 + 2t2, 2t1} - 1
Cite this review
Pith. "Pith review of Ramsey numbers of trees." pith.science (2026). https://pith.science/paper/6SVQSR7F
@misc{pith2026250907934,
author = {Pith},
title = {Pith review of: Ramsey numbers of trees},
year = {2026},
howpublished = {\url{https://pith.science/paper/6SVQSR7F}},
note = {Machine review of arXiv:2509.07934}
}
abstract
We show that there exists a constant $c>0$ such that every $n$-vertex tree $T$ with $\Delta(T)\le cn$ has Ramsey number $R(T)=\max\{t_1+2t_2,2t_1\}-1$, where $t_1\ge t_2$ are the sizes of the bipartition classes of $T$. This improves an asymptotic result of Haxell, {\L}uczak, and Tingley from 2002, and shows that, though Burr's 1974 conjecture on the Ramsey numbers of trees has long been known to be false for certain `double stars', it is true for trees with up to small linear maximum degree.
Figures
Figures from the paper (20 more)
Forward citations
Cited by 2 Pith papers
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On the Ramsey number of a graph obtained by attaching a pendant to a path with length 1 modulo 4
For n ≡ 1 (mod 4), the two-colour Ramsey number of the path-with-middle-pendant tree T_n is exactly (3n+1)/2.
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Recent progress in graph theory using expansion
Sublinear expansion—weak neighbourhood growth in sparse graphs—has resolved many long-standing extremal graph theory conjectures, and this survey organizes that progress.
Reference graph
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