REVIEW 2 major objections 5 minor 2 cited by
Interpolating chromatic and homomorphism thresholds
T0 review · 2 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Odd cycles have blowup threshold 1/(2k−1), and clique thresholds interpolate smoothly via VC-dimension.
desk verdict A substantial paper that resolves the blowup threshold for all odd cycles and gives a genuinely new interpolation between chromatic and homomorphism thresholds, conditional only on one unpublished result used in Theorem 1.1. 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 arguments run on two engines. The first is the theory of VC-dimension: a Haussler packing lemma (Lemma 3.1) partitions the vertex set into classes with nearly identical neighborhoods, and an iterative refinement (Definition 4.13) sharpens this partition until each class is either complete or anti-complete to every other class, yielding the blowup structure. The second is a code-on-graphs theorem (Theorem 1.4), which bounds the total ℓ1-weight of binary vectors assigned to vertices of a dense graph under a constraint on common 1-coordinates of any K_{s−2}; this theorem supplies the clique-number bound on the homomorphic image in Theorem 1.1. For the odd-cycle results, Bollobás's set-pair inequality bounds the VC-dimension of dense maximal C_{2k−1}-free graphs (Theorem 1.5), and Andrásfai graphs provide the sharp lower-bound constructions.
What would settle it
For k = 2, search for a maximal C_5-free graph with minimum degree at least (1/5 + ε)n for some ε > 0 that is not a blowup of any bounded-size graph; Theorem 1.3 predicts none exists, so a single example would refute it. Independently, verify the value δVC_χ(K_4) = 2/3; a different value would break the upper-bound argument of Theorem 1.1.
Extended reading notes
Core claim
The paper proves two main structural claims. First, for integers t ≥ s ≥ 3, the bounded-VC homomorphism threshold for mapping K_s-free graphs to K_t-free images is δVC_hom(K_s;K_t) = ((s−3)(t−s+2)+1)/((s−2)(t−s+2)+1), with matching constructions showing optimality. As t grows this value descends smoothly from δVC_hom(K_s) = (2s−5)/(2s−3) to δVC_χ(K_s) = (s−3)/(s−2), showing that the coincidence of chromatic and homomorphism thresholds for cliques is governed by VC-dimension. Second, the paper defines the blowup threshold δ_B(H) and proves δ_B(C_{2k−1}) = 1/(2k−1) for all k ≥ 2: any maximal C_{2k−1}-free graph with minimum degree above (1/(2k−1)+ε)n is a blowup of a graph whose size is bounded in terms of k and ε, and the Andrásfai graphs show the threshold cannot be lowered. Along the way the paper proves that such graphs have bounded VC-dimension and that removing O_{ε,k}(1) vertices destroys all short odd cycles.
Load-bearing premise
The upper bound for the clique interpolation theorem assumes the still-unpublished result δVC_χ(K_s) = (s−3)/(s−2); if that result were false, the bounded-coloring step in Section 3 would collapse, though Theorems 1.2 and 1.3 do not depend on it.
Editorial extensions
If this is right
- For every k ≥ 2, every maximal C_{2k−1}-free graph with minimum degree at least (1/(2k−1)+ε)n is a blowup of a graph of size bounded by a tower function in k and 1/ε.
- The Andrásfai graphs show the threshold is sharp: at minimum degree n/(2k−1) there are maximal C_{2k−1}-free graphs that are twin-free and hence not blowups of any smaller graph.
- Since 1/(2k−1) → 0 as k → ∞, blowup thresholds have 0 as an accumulation point, in contrast to chromatic thresholds, which are all jumps.
- The interpolation formula for cliques, δVC_hom(K_s;K_t) = ((s−3)(t−s+2)+1)/((s−2)(t−s+2)+1), is exact and decreases to δVC_χ(K_s) as t → ∞.
- For mixed odd-cycle families, δVC_hom(C_{2k+1}; C_{2k−1}) = 0, extending the VC-dimension transition for homomorphism thresholds from 1/(2k+1) to 0.
Reading between the lines
- The paper does not compute δ_B for other graphs; testing Conjecture 5.1 (δ_hom = δ_B) on even cycles or on K_{s,t} would be a natural next step, and Theorem 1.4's code bound may be the right tool.
- The iterative refinement that forces blowup structure suggests a general sufficient condition: any H-free graph whose VC-dimension is bounded and whose minimum degree is a positive fraction of n should be near a blowup, potentially extending beyond odd cycles.
- Theorem 1.2's zero threshold for {C_{2k+1}, C_{2k−1}}-free graphs suggests the blowup threshold of the family {C_{2k+1}, C_{2k−1}} might also be 0; verifying or refuting this would be a concrete test of the framework.
- The paper leaves open whether δVC(C_{2k−1}) equals 1/(2k−1); resolving this would pin down when bounded VC-dimension alone, rather than maximality, forces blowup structure.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper introduces two new threshold parameters for H-free graphs: the bounded-VC homomorphism threshold δVC_hom(G1;G2), which interpolates between chromatic and homomorphism thresholds, and the blowup threshold δ_B(H). The main results are Theorem 1.1, computing δVC_hom(K_s;K_t)=((s-3)(t-s+2)+1)/((s-2)(t-s+2)+1); Theorem 1.2, giving two zero thresholds for families of odd cycles; Theorem 1.3, proving δ_B(C_{2k-1})=1/(2k-1); and Theorem 1.5, bounding the VC-dimension of dense maximal C_{2k-1}-free graphs. The proofs combine Haussler's packing lemma, an iterative partition-refinement process, a new inequality for codes on graphs (Theorem 1.4), and Andrásfai-graph lower-bound constructions.
Significance. If the results are correct, this is a substantial contribution: Theorem 1.1 gives the first smooth interpolation between chromatic and homomorphism thresholds, and Theorem 1.3 resolves the blowup threshold for all odd cycles while answering a question of Schacht. The odd-cycle arguments appear largely self-contained and introduce a useful framework for refining VC-based partitions. Theorem 1.4 is a clean statement of independent interest. The main caveat is that the upper bound of Theorem 1.1 rests on an unpublished result, and one proof in Section 4 appears to misapply a stated lemma; these issues must be resolved before the full significance can be assessed.
major comments (2)
- [Section 3, first paragraph] The proof of the upper bound of Theorem 1.1 invokes the equality δVC_χ(K_s)=(s-3)/(s-2), cited from [34] (Liu, Shangguan, Xue, 'In preparation'), to conclude that G has a bounded coloring before applying Lemma 3.1. This step is load-bearing: it is what makes the refined partition have O_{ε,d,r}(1) parts, and hence what makes the auxiliary graph H have bounded size. Because no proof or published reference is supplied, the upper bound of Theorem 1.1 is conditional on [34]. The authors should include a proof of this equality in an appendix or state Theorem 1.1 as conditional on the unpublished result.
- [Section 4.3, proof of Theorem 1.2] The first assertion of Theorem 1.2 states δVC_hom(C_{2k+1}; C_{2k-1})=0, so the input graph G is assumed only to be C_{2k+1}-free. The proof, however, applies Lemma 4.16(2), whose stated hypothesis is that G is {C_{2k+1}, C_{2k-1}}-free. Moreover, the proof of Lemma 4.16 explicitly uses C_{2k-1}-freeness of G to rule out singular C_{2k-1} cycles in H_k(G,ε/3). Thus the proof as written does not establish the stronger first assertion of Theorem 1.2. Please correct the statement of Lemma 4.16, add the missing argument, or restrict the theorem accordingly.
minor comments (5)
- [Section 1.1] The name is misspelled as 'Obsen and Schacht'; it should be 'Ebsen and Schacht'.
- [Section 4.2, heading] The heading 'Proof of Lemma 1.5' should refer to Theorem 1.5.
- [Section 3.2 and Claim 3.2] In the proof of Theorem 1.4, the symbols V(H), N_H, and H are used where the graph under discussion is G; for example, the maximum in equation (5) should be over V(G) and the neighborhoods should be in G.
- [Throughout] Several LaTeX control sequences appear literally in the text, such as '/suppress Luczak' and '/suppress' before Thomassé; these should be typeset properly.
- [Figure 1.1] Figure 1.1 is difficult to read and is not needed for the proofs; consider simplifying or removing it.
Circularity Check
Theorem 1.1's upper bound relies on an unpublished co-authored threshold result; the rest of the derivation chain is self-contained.
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self citation load bearing
[Section 3, first paragraph (upper bound of Theorem 1.1)]
"As δpGq ě p s´3 s´2 ` εqn and δVC χ pKsq “ s´3 s´2, G can be colored by a bounded number of colors, say C colors. Then, apply Lemma 3.1 to refine the color classes of th is coloring to obtain a partition V pGq “ U1 \ ¨ ¨ ¨ \UK"
The upper-bound proof of Theorem 1.1 needs the partition U1,...,UK to have K = O(1) so that the auxiliary graph H = H(U1,...,UK) is a bounded-size homomorphic image. The boundedness of K comes entirely from the bounded coloring step, which invokes the equality δVC_χ(K_s) = (s−3)/(s−2). This equality is cited to [34] (Liu, Shangguan, Xue, 'In preparation'), whose author list overlaps with the present paper (Hong Liu). No proof or published version is supplied, so the step is not independently verifiable. If the cited equality is false or unavailable, K is not O(1), and the upper bound for δVC_hom(K_s;K_t) is not established.
full rationale
The derivation chain for the odd-cycle results (Theorems 1.2, 1.3, 1.5) is self-contained: Theorem 1.5 is proved in Section 4.2 from Bollobás's set-pair inequality and a Ramsey argument; Lemma 4.16 is proved by internal claims; the lower bound for δB(C_{2k−1}) uses an explicit Andrásfai construction verified in Proposition 4.21. The lower-bound construction for Theorem 1.1 is an explicit family of K_s-free graphs whose VC-dimension is bounded and whose homomorphic images are forced to contain K_t; it does not rely on any fitted parameter or cited threshold. The only load-bearing step that reduces to a non-independent input is the bounded-coloring step in the upper bound of Theorem 1.1, which invokes δVC_χ(K_s) = (s−3)/(s−2) from reference [34], an unpublished manuscript co-authored by one of the present authors. Since the rest of the upper-bound proof (Haussler partition, Theorem 1.4, the code-on-graphs argument) is a genuine derivation from that coloring, the overall result is not equivalent to its inputs by construction; but the first main theorem's upper bound is not self-contained and rests on a self-citation whose correctness is not independently established. Citations to [33] for the Haussler partition lemma and the clique homomorphism-threshold observation are also self-citations, but those statements are either directly derivable from Haussler's lemma or not needed for the final conclusions. This warrants a score of 4 rather than 0–2.
Assumptions & free parameters
assumptions (6)
- standard math Haussler packing lemma based partition lemma (Lemma 3.1) providing a partition with small neighborhood symmetric differences for graphs with bounded VC-dimension.
- domain assumption δVC_χ(K_s) = (s-3)/(s-2) (reference [34], 'In preparation').
- standard math Bollobás set-pair inequality (Theorem 4.6).
- standard math Ramsey number upper bound R(s,t) ≤ binom(s+t-2, s-1).
- standard math Lemma 4.1 from Ebsen and Schacht [14]: a C_{2k-1}-free graph with δ ≥ (1/(2k-1)+ε)n is C_ℓ-free for every odd ℓ with k ≤ ℓ ≤ 2k-1.
- standard math Basic properties of VC-dimension (shattering, packing), treated as standard.
invented entities (2)
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Bounded-VC homomorphism threshold δVC_hom(G1; G2)
independent evidence
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Blowup threshold δ_B(H)
independent evidence
Cite this review
Pith. "Pith review of Interpolating chromatic and homomorphism thresholds." pith.science (2026). https://pith.science/paper/XT3DSGXU
@misc{pith2026250209576,
author = {Pith},
title = {Pith review of: Interpolating chromatic and homomorphism thresholds},
year = {2026},
howpublished = {\url{https://pith.science/paper/XT3DSGXU}},
note = {Machine review of arXiv:2502.09576}
}
abstract
The problem of chromatic thresholds seeks for minimum degree conditions that ensure $H$-free graphs to have a bounded chromatic number, or equivalently a bounded size homomorphic image. The strengthened homomorphism thresholds problem further requires that the homomorphic image itself is $H$-free. The purpose of this paper is two-fold. First, we define a generalized notion of threshold which encapsulates and interpolates chromatic and homomorphism thresholds via the theory of VC-dimension. Our first result shows a smooth transition between these two thresholds when varying the restrictions on homomorphic images. In particular, we proved that for $t \ge s \ge 3$ and $\epsilon>0$, if $G$ is an $n$-vertex $K_s$-free graph with VC-dimension $d$ and $\delta(G) \ge (\frac{(s-3)(t-s+2)+1}{(s-2)(t-s+2)+1} + \epsilon)n$, then $G$ is homomorphic to a $K_t$-free graph $H$ with $|H| = O(1)$. Moreover, we construct graphs showing that this minimum degree condition is optimal. This extends and unifies the results of Thomassen, {\L}uczak and Thomass\'e, and Goddard, Lyle and Nikiforov, and provides a deeper insight into the cause of existences of homomorphic images with various properties. Second, we introduce the blowup threshold $\delta_B(H)$ as the infimum $\alpha$ such that every $n$-vertex maximal $H$-free graph $G$ with $\delta(G)\ge\alpha n$ is a blowup of some $F$ with $|F|=O(1)$. This notion strengthens homomorphism threshold. While the homomorphism thresholds for odd cycles remain unknown, we prove that $\delta_B(C_{2k-1})=1/(2k-1)$ for any integer $k\ge 2$. This strengthens the result of Ebsen and Schacht and answers a question of Schacht and shows that, in sharp contrast to the chromatic thresholds, 0 is an accumulation point for blowup thresholds. Our proofs mix tools from VC-dimension theory and an iterative refining process, and draw connection to a problem concerning codes on graphs.
Figures
Forward citations
Cited by 2 Pith papers
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Regularity for hypergraphs with bounded VC$_2$ dimension
For 3-graphs of bounded VC2 dimension, an (ε,ψ)-regular partition exists with twr(twr(poly(1/ε))) vertex parts, improving the generic wowzer bound to tower type.
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On the spectrum and structure of blowup thresholds
Blowup thresholds are always positive for non-bipartite H, fail monotonicity under induced subgraphs, and equal 1/4 for certain constrained odd-cycle blowups.
Reference graph
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