REVIEW 2 major objections 5 minor 1 cited by
Induced rational exponents and bipartite subgraphs in $K_{s, s}$-free graphs
T0 review · 2 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Every rational exponent in (1,2) is realized by an induced Turán family of size at most $2^a$ in $K_{s,s}$-free host graphs.
desk verdict Strong transfer results in induced Turán theory; one repairable counting error in Section 4.2. 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 central objects are the $(p;S)$-lifts $F_p(T;R)$ of a rooted tree $(T;R)$: graphs obtained by gluing $p$ vertex-disjoint copies of $T$ along the root set $R$ and an additional set $S$, where $S$ ranges over subsets of the non-root vertices. The paper proves a Ramsey-type lemma that, from many induced copies of $T$ sharing a fixed root set, extracts $p$ copies whose intersections are regular and whose union is induced, forming a lift in $F_p(T;R)$. The extraction is powered by two ingredients: a greedy embedding that uses $K_{s,s}$-freeness to control bad common neighborhoods, and an almost-regular reduction (Lemma 2.1) that keeps degrees and codegrees bounded. For the cycle-based theorems, the machinery combines Sidorenko's property for even cycles, the Kővári–Sós–Turán theorem, and Janzer's regularization lemma to show that most closed $2\ell$-walks in a dense $K_{s,s}$-free graph are induced $2\ell$-cycles, from which induced $\theta$ and prism subgraphs are assembled.
What would settle it
Take the degenerate closed walk formed by two internally vertex-disjoint $\ell$-cycles meeting at exactly one vertex, and count the ordered pairs $(u,v)$ of vertices at distance $\ell$ along the walk: the walk lies in several distinct sets $A_{u,v}$, demonstrating that the inequality $\sum_{(u,v)\in\Gamma}|A_{u,v}|\le \#\{\text{degenerate $2\ell$-cycles}\}$ fails by a factor of at least $\ell$; checking whether the corrected factor still fits inside the slack constants of the proof settles whether Theorem 4.2 stands as written.
Extended reading notes
Core claim
The central discovery is that the induced Turán number $\mathrm{ex}^*(n,\mathcal{H},s)$ — the maximum number of edges in an $n$-vertex $K_{s,s}$-free graph with no induced copy of any graph in $\mathcal{H}$ — satisfies the same rational exponents as the ordinary Turán number for a carefully chosen subfamily of the Bukh–Conlon lifting family. For every rational $q=\frac{a}{b}\in(1,2)$, the paper finds a family $\mathcal{H}$ of at most $2^a$ bipartite graphs with $\mathrm{ex}^*(n,\mathcal{H},s)=\Theta_s(n^q)$ (Theorem 1.2). It also proves that for $\theta$ graphs $\Theta^t_{\ell}$, $\mathrm{ex}^*(n,\Theta^t_{\ell},s)=\Theta_{\ell,t}(n^{1+1/\ell})$ up to $s$-dependent constants for all sufficiently large $t$ (Theorem 1.5), and that for even prism graphs $C^{\square}_{2\ell}$ with $\ell\ge 10$, $\mathrm{ex}^*(n,C^{\square}_{2\ell},s)=\Theta_{\ell}(n^{3/2})$ (Theorem 1.6). In both cases the $n$-exponent matches the ordinary Turán exponent, thereby confirming the transfer phenomenon for these families.
Load-bearing premise
The proof of Theorem 4.2 assumes that every degenerate homomorphic $2\ell$-cycle is counted in at most one of the sets $A_{u,v}$; a closed walk consisting of two cycles sharing a single vertex is counted in several such sets under different rotations, so the asserted uniqueness is false and the argument needs an extra factor of $\ell$.
Editorial extensions
If this is right
- Every rational number in $(1,2)$ is the induced Turán exponent of a family of at most $2^a$ bipartite graphs, with the family size independent of $s$ and of $b$.
- For theta graphs, the induced Turán number has the same $n$-exponent as the ordinary Turán number, $n^{1+1/\ell}$, for all sufficiently large $t$; for even prisms with $\ell\ge 10$, the induced Turán number is $\Theta_{\ell}(n^{3/2})$.
- The supersaturation theorem shows that any $K$-almost-regular $K_{s,s}$-free graph with average degree $d$ contains at least $n(d/2K)^{t-1}$ labeled induced copies of every $t$-vertex tree, which is optimal up to constants.
- The transfer phenomenon implies that ordinary bipartite Turán results can be lifted to induced statements in locally sparse hosts, making the Hunter–Milojević–Sudakov–Tomon conjecture plausible for a wider class of bipartite graphs.
- The clique-blowup construction (Proposition 1.4) converts any ordinary lower bound $\mathrm{ex}(n,H)=\Omega(n^{1+\alpha})$ into an induced lower bound $\Omega_h(s^{1-\alpha}n^{1+\alpha})$, so for connected bipartite $H$ the induced Turán number is never much smaller than the ordinary one.
Reading between the lines
- The double-counting gap in Section 4.2, where a degenerate closed walk formed by two cycles sharing a single vertex is counted in several sets $A_{u,v}$, is most likely absorbed by the very loose constants in the proof; a corrected factor of $\ell$ should not change the theorem's truth.
- The transfer phenomenon probably extends to any bipartite $H$ whose ordinary Turán exponent is realized by lifted-tree constructions, as long as an induced-supersaturation statement holds for the building blocks; the paper's tree and even-cycle supersaturation results are natural templates.
- The exponential $s$-dependence in the upper bounds for theta and prism graphs, contrasted with polynomial lower bounds, is an artifact of the method; closing this gap is a test of whether the transfer is tight in the parameters beyond $n$.
- A further reduction from the $2^a$-element family to a single forbidden graph would settle Conjecture 1.1 but requires a fundamentally new idea, since the Ramsey cleaning step inherently needs many copies to find a regular intersection.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies induced Turán numbers under a K_{s,s}-freeness assumption on the host graph, proposing a transfer phenomenon from non-induced to induced extremal problems. The main results are: (i) every rational exponent q in (1,2) is realized as the induced Turán exponent of a subfamily of the Bukh–Conlon family of size at most 2^a (Theorem 1.2); (ii) supersaturation theorems for induced trees and induced even cycles in K_{s,s}-free almost-regular graphs (Theorems 1.3, 4.2, 5.2); and (iii) optimal-order upper bounds for induced theta graphs and induced even prisms (Theorems 1.5 and 1.6), matching the known non-induced exponents. The lower bounds are obtained by clique blowups of known constructions, while the upper bounds are proved by passing to almost-regular subgraphs and using auxiliary Ramsey and dependent-random-choice arguments.
Significance. If the proofs are correct, the paper gives substantial new evidence for the Hunter–Milojević–Sudakov–Tomon conjecture and for the proposed transfer phenomenon. The induced-rational-exponent result improves the Bukh–Conlon family size from an uncontrolled number to at most 2^a, and the induced theta and prism bounds match the best known non-induced exponents. The paper is largely self-contained: it supplies new supersaturation machinery, explicit constants, and a clean lower-bound construction via clique blowups. These are genuine contributions. The two technical gaps I identify below are localized and appear repairable within the paper's existing framework, so the high-level claims are plausible despite the need for revision.
major comments (2)
- [Section 4.2, Eq. (2)] The assertion before (2) that every degenerate homomorphic 2ℓ-cycle is counted in at most one set A_{u,v} is false. For ℓ=3, the closed walk a-b-c-a-d-c-a is degenerate, and it is counted in A_{b,d} when read starting at b with midpoint d and in A_{d,b} when read starting at d; the two ℓ-paths share the interior vertex c in both representations. More generally, a figure-eight walk can be counted under several rotations, so the left side of (2) should be bounded by O(ℓ) times the number of degenerate homomorphic cycles, not by that number itself. This error is load-bearing: with the uncorrected coefficient, the displayed inequality before the averaging step is no longer valid once the extra factor from (2) is inserted, and the derivation of the pair (u*,v*) is unsupported. The argument is repairable: replacing the coefficient of Σ|A| by its 1/(2ℓ) multiple (equivalently, enlarging the parameter δ in the application of Lemma 4.4 by a factor 2ℓ) is harmless because C>(ℓt)^{100ℓ} leaves enormous slack. As written, however, the proof of (2) and the subsequent averaging step are incorrect.
- [Section 3.2, proof of Theorem 1.3] The counting of embeddings does not exclude previously embedded vertices from the candidate set V_k. If w_i is an earlier leaf of the prefix tree whose only earlier neighbor is w_{k'}, then v_i lies in N(v_{k'}), is not in N(v_j) for any j≠k', and need not belong to any X(v_j); hence v_i is counted as an available choice for v_k even though reusing v_i would make the map non-injective. The lower bound |V_k|≥d/(2K) therefore overcounts valid embeddings. This gap is also repairable: subtracting at most t previously embedded vertices from each candidate set is absorbed by the slack in the assumption d≥(4Kt)^{6s}s^3, and the final constant in Theorem 1.3 would still have the required form. As stated, however, the proof does not justify the claimed number of induced copies.
minor comments (5)
- [Throughout] Displayed results are referred to by inconsistent labels: Lemma 2.1 is called Theorem 2.1 in its proof; Lemma 3.3 is cited as Theorem 3.3; Claim 3.4 as Theorem 3.4; Lemmas 4.3, 4.4, 5.3, 5.4, 5.5, 5.9 and Claim 5.10 are cited as Theorems 4.3, 4.4, 5.3, 5.4, 5.5, 5.9, 5.10; and Proposition 1.4 is called Theorem 1.4 in Sections 4 and 5. These labels should be reconciled.
- [Section 4.2, after Lemma 4.4] The text states that Lemma 4.4 yields 'at least δr²/2 = εr²/(64ℓ) pairs' among the selected paths, but the quantity used in Lemma 4.4 is the blue-edge parameter λ=ε/(32ℓ), not δ; the displayed equality is correct only if 'δ' is replaced by 'λ'.
- [Section 3.1, proof of Theorem 3.1] In the displayed lower bound for the number of induced copies of (T;R) under a fixed root orientation, the final term '≥ C/(4K)' appears to be missing the exponent and the cancellation of the powers of n; as written the algebra is unclear. The intended estimate should be that the ratio is at least (C/(4K))^{t-1}, since the exponent of n is 0 after substituting α=1-x/y and y=t-1.
- [Section 1.1 and Section 3.2] The notation for rational exponents is inconsistent: Theorem 1.2 writes q=a/b with q∈(1,2), while the remark in Section 3.2 discusses 'a/b∈(0,1)' as the deficit from 2 and uses rooted trees of density b/a. The change of variables should be stated explicitly to avoid confusion.
- [Section 5.3, Claim 5.6] The claim that at most 4 of the 2ℓ rotations of a homomorphic 2ℓ-cycle are special is asserted without proof; a one-sentence explanation of why the forbidden positions {1,ℓ+1} account for at most four rotations per edge would improve readability.
Circularity Check
No significant circularity; upper bounds derived internally, lower bounds from external constructions; only a minor non-load-bearing self-citation to the authors' prism paper.
full rationale
This paper derives its main upper bounds (Theorems 1.2, 1.5, 1.6) from first-principles supersaturation results (Theorems 1.3, 4.2, 5.2) proved inside the paper using KST-type lemmas and external tools (Sidorenko, Janzer, Hunter et al. lemmas). Lower bounds are imported from external constructions (Bukh-Conlon rational exponent families, Conlon's theta graphs, known C4 lower bounds) via Proposition 1.4. The only self-citation is [23] (Gao-Janzer-Liu-Xu), which includes an author; it is used merely as a proof-strategy precedent in Section 5.1 ('We use a similar strategy as in the proof of [23, Theorem 1.1]'), and the induced-prism proof is fully self-contained, so the citation is not load-bearing. I find no step where a claimed prediction reduces by definition to a fitted parameter or to a self-citation chain. The skeptic's concern about the assertion in Section 4.2 that each degenerate 2-ell cycle is counted in at most one A_{u,v} is a potential correctness issue (the uniqueness claim appears unproved and may be false under multiple rotations), but an overcount factor is not a circular reduction of the conclusion to the hypothesis; it is a bounded-error artifact repairable by a constant factor. No circularity is present.
Assumptions & free parameters
assumptions (7)
- standard math Kővári-Sós-Turán theorem: ex(n,K_{s,s}) is O(n^{2-1/s})
- standard math Turán's theorem for graphs with bounded independence number
- standard math Sidorenko property for even cycles: hom(C_{2ℓ},G) is at least d^{2ℓ}
- domain assumption Bukh-Conlon random algebraic construction supplies ex(n,T)=Ω(n^q) for their rooted-tree families
- domain assumption Conlon's theorem ex(n,Θ^t_ℓ)=Θ(n^{1+1/ℓ}) for t sufficiently large
- standard math External lemmas from Hunter et al. [28] and Janzer [30] used for degenerate walks, dependent random choice, and regularization are correct
- standard math ex(n,C4)=Ω(n^{3/2})
Cite this review
Pith. "Pith review of Induced rational exponents and bipartite subgraphs in $K_{s, s}$-free graphs." pith.science (2026). https://pith.science/paper/LDLLEY6U
@misc{pith2026250609020,
author = {Pith},
title = {Pith review of: Induced rational exponents and bipartite subgraphs in $K_s, s$-free graphs},
year = {2026},
howpublished = {\url{https://pith.science/paper/LDLLEY6U}},
note = {Machine review of arXiv:2506.09020}
}
abstract
In this paper, we study a general phenomenon that many extremal results for bipartite graphs can be transferred to the induced setting when the host graph is $K_{s, s}$-free. As manifestations of this phenomenon, we prove that every rational $\frac{a}{b} \in (1, 2), \, a, b \in \mathbb{N}_+$, can be achieved as Tur\'{a}n exponent of a family of at most $2^a$ induced forbidden bipartite graphs, extending a result of Bukh and Conlon [JEMS 2018]. Our forbidden family is a subfamily of theirs which is substantially smaller. A key ingredient, which is yet another instance of this phenomenon, is supersaturation results for induced trees and cycles in $K_{s, s}$-free graphs. We also provide new evidence to a recent conjecture of Hunter, Milojevi\'{c}, Sudakov, and Tomon [JCTB 2025] by proving optimal bounds for the maximum size of $K_{s, s}$-free graphs without an induced copy of theta graphs or prism graphs, whose Tur\'{a}n exponents were determined by Conlon [BLMS 2019] and by Gao, Janzer, Liu, and Xu [IJM 2025+].
Figures
Forward citations
Cited by 1 Pith paper
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Supersaturation of induced even cycles in locally sparse graphs
For every integer ℓ≥2, any (1−ε,t)-sparse n-vertex graph with at least C t^{1−1/ℓ} n^{1+1/ℓ} edges contains at least C' d^{2ℓ} induced copies of the even cycle C_{2ℓ}, where d is its average degree.
Reference graph
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