REVIEW 3 major objections 4 minor 2 cited by
$C_4$-free subgraphs of high degree with geometric applications
T0 review · 3 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Every graph of average degree d hides either an induced C4-free subgraph of degree at least k or a d-vertex patch with c d^2 edges, and this dichotomy yields optimal Zarankiewicz bounds across geometry.
desk verdict Theorem 1.9 is a real result; referees need to verify the imported Lemma 3.3 before this is final. 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 load-bearing object is the (c,t)-sparsity condition and the local control it gives over common neighbourhoods. The paper proves Theorem 1.12 by a four-step extraction: choose a maximal-average-degree induced subgraph with a balanced cut; use the imported Lemma 3.3 to upgrade sparsity so that vertices sharing many neighbours form a set of size O(t); randomly sample to keep edges while destroying all copies of K_{2,v+1} with two vertices on one side; then apply a known theorem converting K_{v+1,v+1}-freeness with large average degree into an induced C4-free subgraph. The 1-subdivision of $K_v^{{(h)}}$, a bipartite graph whose one side encodes all h-subsets of the other, is the obstruction gadget that makes the random sampling work.
What would settle it
Search for a sequence of graphs that are (c,t)-sparse and induced-H-free for some fixed bipartite H yet contain two sets of size at least βt with at least (1−ε)|A||B| edges; such a construction would refute Lemma 3.3 and break the proof of Theorem 1.12.
Extended reading notes
Core claim
Theorem 1.9 states that for every k there is c=c(k)>0 such that every graph G with average degree d contains either an induced C4-free subgraph of average degree at least k or a subgraph on d vertices with at least c $d^{2}$ edges. The proof runs through a stronger statement, Theorem 1.12: every (c,t)-sparse graph of sufficiently large average degree contains a bipartite induced C4-free subgraph of average degree at least k. From the dichotomy, Corollary 1.10 draws the main structural conclusion: a hereditary family that is weakly degree-bounded (bounded average degree on C4-free members) and has the density-Erdős-Hajnal property (every dense induced subgraph contains a linear-size biclique) has K_{s,s}-free members of average degree O(s). The paper claims this unified mechanism recovers and improves a long list of geometric bounds, including O(s log s) for string graphs without separator theorems, O_k(sn) for k-intersecting curve families, O(sn) for convex sets and for two families of disjoint curves, O(sn) for y-monotone pseudodisks, and O_{dx,h}(t s n (log n/log log n)^{dy-1}) for semilinear graphs.
Load-bearing premise
The proof imports the unproved lemma that a graph which avoids induced copies of a fixed bipartite graph and has no dense pair of large sets (c,t-sparse) also has no nearly complete pair of sets of size βt; if that lemma fails, the local sparsity argument collapses.
Editorial extensions
If this is right
- Any hereditary family with bounded average degree on C4-free members and large bicliques in dense induced subgraphs automatically has K_{s,s}-free members of average degree O(s).
- The string graph bound O(s log s) follows without separator theorems, purely from the dichotomy plus a dense-string-graph biclique lemma.
- For k-intersecting curve families, convex sets, and two families of disjoint curves, the dichotomy plus a new biclique-in-dense-bipartite-curve-intersection-graph theorem gives linear-in-s bounds.
- Semilinear incidence graphs of dimension (dx,dy) and complexity (h,t) have K_{s,s}-free edge bound O_{dx,h}(t s n (log n/log log n)^{dy-1}), matching the point-box bound up to dimension-dependent constants.
- A (c,t)-sparse graph of average degree Ω_{H,c}(t) contains an induced subdivision of every fixed graph H, confirming the sparse-graph conjecture.
Reading between the lines
- The dichotomy offers a reusable recipe: to prove degree-boundedness of a hereditary geometric family, check only C4-free members and dense-induced bicliques; this may shorten future proofs that currently rely on separators or epsilon-nets.
- The semilinear bound suggests the log factor is governed by the ambient dimension of the point side, so any family representable by polytopes with O(1) facet directions should satisfy the same bound; testing point-box incidences in higher dimensions would confirm the range of the method.
- If Lemma 3.3 turns out to hold only for particular bipartite H, Theorem 1.12 may still be salvageable for those H; a targeted search for counterexamples to the lemma would clarify which sparse graph classes the method covers.
- The same dichotomy may extend to longer even cycles: replace C4-free by C_{2ℓ}-free in the hypothesis and ask whether the conclusion holds with a corresponding dense patch, which would give an analogue for higher even-cycle obstructions.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces and proves a structural dichotomy for general graphs (Theorem 1.9): for every k there is c(k)>0 such that every graph of average degree at least d either contains an induced C4-free subgraph of average degree at least k, or a subgraph on d vertices with at least c d^2 edges. The dichotomy is derived from a stronger statement about (c,t)-sparse graphs (Theorem 1.12), proved through a four-step sampling argument in Section 3. The authors then use this tool to give uniform and, in several cases, optimal bounds for Zarankiewicz-type problems in geometric settings: string graphs, intersection graphs of k-intersecting curve families, intersection graphs of convex sets, incidence graphs of y-monotone pseudodisks, semilinear graphs, and polygon visibility graphs. The paper also derives a conjecture of Fox, Nenadov, and Pham on induced subdivisions in (c,t)-sparse graphs.
Significance. If the main dichotomy is accepted, the paper is a significant contribution: it unifies a broad range of geometric Zarankiewicz results and obtains tight dependence on s (often O(s n)) that previously required ad-hoc geometric arguments or separator theorems. The overall architecture is coherent and the applications are substantial. The proof of Theorem 1.9 is carefully structured, and the use of the published theorem of Du, Hunter, Girão, McCarty, and Scott (Theorem 3.8) as a black box is appropriate. The main barrier to acceptance is not the internal derivation but the reliance, at a single load-bearing point, on an unproved lemma imported from a preprint, together with a few proof gaps and typos that need to be repaired.
major comments (3)
- [Section 3, Lemma 3.3] Lemma 3.3 is imported from Ding–Gao–Liu–Luan–Sun (arXiv:2411.12659) without a proof or even a full statement of the result in the present paper. This lemma is the only mechanism in Lemma 3.4 that upgrades (c,t)-sparsity to (1−ε,βt)-sparsity; that upgraded sparsity is then used twice: to bound |S_a| < t0 and to delete vertices from the sets T_e in Claim 3.5. If Lemma 3.3 fails, the proof of Lemma 3.4 collapses, and with it Theorem 1.12 and Theorem 1.9, on which all geometric applications depend. The paper should either prove the needed special case, give a precise statement with the dependence of β on H and ε, or cite a version that has passed peer review. As written, this is a load-bearing external-preprint dependency and must be resolved before the paper is considered final.
- [Section 5, Lemma 5.5, Step 4] In the proof of Lemma 5.5, Step 4 says that a (d−1)-dimensional vertical face F is covered by at most f(d, 2(d−1)) parallelotopes, but the induction is on the dimension d, so the bound should refer to f(d−1, 2(d−1)). As written, the recurrence uses the same dimension d and is circular; this affects the proof of Lemma 5.2 and hence Proposition 5.1 and Theorem 1.7. The error appears to be a typo, but it must be corrected and the induction checked explicitly.
- [Section 2, Claim 2.7] Claim 2.7 is stated as a proof sketch, yet it is used to assert that the family of point-halfspace incidence graphs in R^4 is not degree-bounded, which the introduction presents as resolving the open case of dimension 4. Several steps are only sketched: the conversion from rectangles to inscribed ellipses, the deletion argument for close pairs, and the final lifting map. Since this is a claimed theorem rather than an aside, the proof should be completed or the claim should be explicitly downgraded to a conjecture or conditional construction.
minor comments (4)
- [Section 3, Lemma 3.4] There is a typo in 'we havεd' in the paragraph after the definition of t0; also the constant K is required to satisfy d/4 ≥ βt/ε, but the displayed inequality says K ≥ 4 C(v,h) β/ε, which should be checked for consistency with the subsequent lower bound on |N(a)|.
- [Section 4.2, Lemma 4.5] In the case e(G1) ≥ εn^2/3, the text says a point p in P' is contained in at least εn/2 pseudodisks, but the earlier deletion step only guarantees εn/6. The subsequent conclusion should be adjusted accordingly (the desired bound still follows).
- [Section 6] Theorem 6.9 and Theorem 6.11 refer to 'Theorem 1.10' in their final sentences, but the correct reference is Corollary 1.10.
- [General] The manuscript contains many typographical errors (e.g., 'strucutral', 'recieved', 'sugbgraph', 'biparite', 'indicedence', 'parrallelotope', 'the set A'' has size at least 2/3 |A''|' in the proof of Theorem 1.2). A careful proofreading pass is needed.
Circularity Check
No circular derivation: Theorem 1.9 follows from a direct averaging argument plus independent cited lemmas; the unproved external Lemma 3.3 is a support gap, not a circular step.
full rationale
The derivation of Theorem 1.9 is not circular. It is obtained from Theorem 1.12 by a direct averaging argument: setting c=1/2 and t=floor(d/C), if G is not (1/2,t)-sparse then the violating pair A,B yields a d-vertex subgraph with at least (1/4)t^2 = Omega(d^2) edges; otherwise Theorem 1.12 supplies the required induced C4-free subgraph of average degree at least k. Theorem 1.12 is then proved through Lemmas 3.2, 3.4, 3.7 and the cited Theorem 3.8. Theorem 3.8 (Du, Girao, Hunter, McCarty, Scott) does share a coauthor with the present paper, but it is a published, parameter-free theorem whose hypothesis (K_{s,s}-free with large average degree) is not the paper's target dichotomy, and it is used only as the final step after an independent sparsification argument. No fitted parameter is relabeled as a prediction, and no object is defined in terms of the target conclusion. The one notable caveat is Lemma 3.3, imported without proof from Ding, Gao, Liu, Luan, Sun (arXiv:2411.12659) and used to upgrade (c,t)-sparsity to (1-epsilon, beta t)-sparsity in Lemma 3.4. This lemma is genuinely load-bearing: if it failed, the local sparsity estimate |S_a| < t0 and the rich-set deletion in Claim 3.5 would collapse, breaking Lemma 3.4 and hence Theorem 1.12. That is a completeness and correctness risk, not circularity, because the lemma is external to the paper and not derived from the paper's own conclusions. Accordingly, the circularity score is low: there is no significant circularity, though the proof is not fully self-contained at one external-support step.
Assumptions & free parameters
assumptions (8)
- standard math Kühn-Osthus theorem: K_{s,s}-free graphs with no induced subdivisions of a fixed H have O(n) edges
- standard math Theorem 3.8 (Du, Hunter, Girão, McCarty, Scott): every K_{s,s}-free graph with average degree at least k C0 s^3 has a C4-free induced subgraph of average degree k
- domain assumption Lemma 3.3 from Ding, Gao, Liu, Luan, Sun: (c,t)-sparse graphs avoiding induced bipartite H are (1-ε,βt)-sparse
- standard math Density-EH property for semialgebraic graphs (Fox-Gromov-Lafforgue-Naor-Pach; Fox-Pach-Sheffer-Suk-Zahl)
- domain assumption Fox-Pach-Tóth Theorem 15: dense intersection graphs of convex sets contain large bicliques
- standard math Chan-Har-Peled point-box Zarankiewicz bound
- standard math Lovász-Szegedy ultra-strong regularity lemma
- domain assumption Ackerman-Keszegh double-cherry avoidance in curve visibility graphs
Cite this review
Pith. "Pith review of $C_4$-free subgraphs of high degree with geometric applications." pith.science (2026). https://pith.science/paper/NQIPSHXA
@misc{pith2026250623942,
author = {Pith},
title = {Pith review of: $C_4$-free subgraphs of high degree with geometric applications},
year = {2026},
howpublished = {\url{https://pith.science/paper/NQIPSHXA}},
note = {Machine review of arXiv:2506.23942}
}
abstract
The Zarankiewicz problem, a cornerstone problem in extremal graph theory, asks for the maximum number of edges in an $n$-vertex graph that does not contain the complete bipartite graph $K_{s,s}$. While the problem remains widely open in the case of general graphs, the past two decades have seen significant progress on this problem for various restricted graph classes -- particularly those arising from geometric settings -- leading to a deeper understanding of their structure. In this paper, we develop a new structural tool for addressing Zarankiewicz-type problems. More specifically, we show that for any positive integer $k$, every graph with average degree $d$ either contains an induced $C_4$-free subgraph with average degree at least $k$, or it contains a $d$-vertex subgraph with $\Omega_k(d^2)$ edges. As an application of this dichotomy, we propose a unified approach to a large number of Zarankiewicz-type problems in geometry, obtaining optimal bounds in each case.
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Forward citations
Cited by 2 Pith papers
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Nearly tight bounds for induced subdivisions
Every K_{s,t}-free graph of average degree Ω(h^{2(s-1)} log^{7(s-1)} h) and every C_{2k}-free graph of average degree Ω(h log^5 h) contains an induced subdivision of K_h, both nearly optimal.
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Block structure in boolean matrices of bounded factorization norm
A boolean matrix with gamma-2 norm at most lambda contains a blocky submatrix that covers at least a 1/2^{2^{O(lambda)}} fraction of its 1-entries.
Reference graph
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