REVIEW 3 major objections 4 minor 24 references
Generalized Tur\'an problem for directed cycles
T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read For every k and l, the paper determines how many directed k-cycles an oriented graph can have without a directed l-cycle: order n^k when k does not divide l, order n^{k-1} when it does.
desk verdict Strong, original paper on the directed generalized Turán problem; the stress-test's objection to Claim 16 rests on a misreading of the path notation, and the real soft spots are the unpublished [17] and insufficiently documented computer-assisted steps. 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 runs through three devices. First, a clearing step uses the directed regularity lemma and the blow-up lemma to delete $o(n^2)$ arcs so that all homomorphic images of $\vec C_\ell$ (closed directed walks of length $\ell$) disappear, losing only $o(n^k)$ copies of $\vec C_k$. Second, a weighting lemma counts copies of $\vec C_k$ by averaging probabilities of 'good sequences' around each cycle; together with the AM-GM inequality it bounds the count by $\frac nk(\frac nd)^{k-1}$ whenever every vertex has at most $2k/d$ neighbours on any $\vec C_k$, and this neighbourhood bound is forced by forbidding cycles with one arc reversed through a coin-representation theorem. Third, in the bipartite exception, a high-minimum-degree plus no-short-odd-cycle argument forces the extremal graph to be bipartite, and an eigenvalue majorization inequality bounds the cycle count by $\frac2k(\frac n4)^k$.
What would settle it
Check the unpublished inducibility bound by computing, for small $n$, the maximum number of induced copies of each orientation of $C_4$ in an oriented graph and comparing the extremal construction with the iterated blow-up of $\vec C_4$; a higher density or a different extremal construction would invalidate $\mathrm{ex}(n,\vec C_4,\vec C_3)=\frac{n^4}{4^4-1}+o(n^4)$. Equivalently, a computer search for directed-triangle-free oriented graphs whose directed 4-cycle count beats the iterated blow-up on the same vertex set would decide the matter.
Extended reading notes
Core claim
Writing $\mathrm{ex}(n,\vec C_k,\vec C_\ell)$ for the maximum number of directed $k$-cycles in an $n$-vertex oriented graph with no directed $\ell$-cycle, the paper establishes the dichotomy $\mathrm{ex}(n,\vec C_k,\vec C_\ell)=\Theta(n^k)$ if $k\nmid\ell$ and $=\Theta(n^{k-1})$ if $k\mid\ell$ (Theorem 4). In the dense case, for $\ell\ge 2(k-1)^2$ and $k\nmid\ell$, let $d$ be the smallest divisor of $k$ greater than $2$ that does not divide $\ell$; then, when $k$ is odd, $\ell$ is even, or $d\le 4$, the value is $\frac{n}{k}(\frac{n}{d})^{k-1}+o(n^k)$, attained by a balanced blow-up of $\vec C_d$. When $k$ is even, $4\nmid k$, $\ell$ is odd, and $3\nmid k$ or $3\mid\ell$, with $\ell>33k^2$, the value is $\frac{2}{k}(\frac n4)^k+o(n^k)$, attained by a random orientation of a complete balanced bipartite graph. For small $\ell$ the paper computes several exact leading constants: $\frac{n^4}{4^4-1}+o(n^4)$ for $(4,3)$ via an iterated blow-up of $\vec C_4$, $(\frac n4)^4+o(n^4)$ for $(4,6)$, $\frac{n^5}{512}+o(n^5)$ for $(5,3)$, $\frac{27}{16}(\frac n5)^5+o(n^5)$ for $(5,7)$, and $(\frac n5)^5+o(n^5)$ for $(5,8)$ and $(5,13)$. It also solves the sparse example $(3,6)$ with value $\frac14 n^2+o(n^2)$. The directed-graph analogue for large $\ell$ not divisible by $k$ is $\frac nk(\frac nd)^{k-1}+o(n^k)$ with $d$ the smallest positive divisor of $k$ not dividing $\ell$.
Load-bearing premise
The exact constant for the $(k,\ell)=(4,3)$ case rests on an unpublished manuscript's claim about the largest possible density of induced 4-cycles in an oriented graph; if that claim is wrong or does not transfer to counting directed 4-cycles, the stated constant is unsupported.
Editorial extensions
If this is right
- The order of magnitude is now known for every pair $(k,\ell)$, closing the directed-cycle version of the generalized Turán problem at the $\Theta$ level.
- For large $\ell$, the extremal graph is almost always a balanced blow-up of a shorter cycle: the shortest divisor $d>2$ of $k$ that does not divide $\ell$ determines both the construction and the constant.
- In the one parity exception—$k$ even, $\ell$ odd, $d\ge 5$—the extremal graph is a random orientation of a complete balanced bipartite graph, giving $\frac2k(\frac n4)^k+o(n^k)$ copies.
- For small $\ell$ no single construction dominates: iterated blow-ups, transitive tournaments inside blobs, and unbalanced blow-ups with bipartite patches each win for different pairs, showing many genuinely different extremal regimes.
- The directed-graph version of the problem has the same order of magnitude and the same type of blow-up extremal value for large $\ell$, with $d$ now allowed to be $2$.
Reading between the lines
- The same divisibility dichotomy should survive when the forbidden object is any fixed orientation of a cycle of length $\ell$: the paper's final remarks show such a cycle is homomorphic to $\vec C_\ell$, so its upper bounds apply, but the extremal constructions may need to be re-optimized for each orientation.
- The threshold-orientation construction used for the open case $(5,4)$ suggests that between pure blow-ups and random orientations there is a continuous family of extremal graphs parameterized by monotone functions; testing this family computationally for other unsolved pairs could reveal a general interpolation principle.
- The conjectured formula $\mathrm{ex}(n,\vec C_3,\vec C_{3t})\sim \frac{t-1}{4}n^2$ indicates a family of extremal graphs with one small blob and two large parts; verifying $t=3$ would give the first sparse-case constant beyond the $(3,6)$ example and might expose a general sparse construction.
- On the open question of $\mathrm{ex}(n,\vec C_5,\vec C_4)$, the paper's lower bound $0.0517\binom n5$ comes from a monotone orientation of a blow-up of a 7-cycle plus chords; a natural testable extension is to optimize the monotone threshold function beyond the form $\min(x+c,1)$ to see whether the constant can be pushed closer to the computer-assisted upper bound.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the generalized Turán problem for oriented graphs, determining the maximum number of directed k-cycles in an n-vertex oriented graph that contains no directed l-cycle. The main results are an order-of-magnitude classification for all k,l (Theorem 4), asymptotic values in the dense setting for large l (Theorems 8 and 11), several small-cycle cases (Theorems 20, 23, 24, 26, 27, 28, 31, and 33), and an analogous theorem for directed graphs (Theorem 35). Theorems 8 and 11 together are intended to settle the leading constant whenever l is large and k does not divide l, with extremal constructions given by balanced blow-ups of a directed cycle or by a random orientation of a complete bipartite graph.
Significance. If the results were established, they would constitute a substantial contribution to generalized Turán theory for directed graphs: the paper gives a complete order of magnitude for every pair (k,l) and, for large forbidden cycle lengths, identifies the leading constant and the extremal construction. The paper also contains some sound and elegant components, notably the induction proof of Theorem 4, the weight argument of Lemma 9, and the eigenvalue bound of Lemma 13, and it provides a reproducible Flagmatic script for one of the small cases. However, two load-bearing statements in the proof machinery are false as written, and a third main theorem depends on an unpublished external manuscript. These issues leave the paper's headline results unproven in the submitted form.
major comments (3)
- [3.2.1] Claim 16 is false. It asserts p_{2j} ≤ n(n/4)^{2j-1} for every positive integer j. In the proof, the displayed base equality reads Σ_v d^+(v)p_1^2(v) = e(G), but p_1(v) is defined as the number of directed P_1 ending at v, so p_1(v) = d^-(v); the left-hand side is therefore Σ_v d^+(v)d^-(v)^2, not e(G). This is not a harmless typo: in a balanced random orientation of K_{n/2,n/2}, the expected number of directed P_2 subgraphs is n^3/16 + o(n^3), while the claimed bound for j=1 is n(n/4) = n^2/4. The inequality fails already at j=1. Since Claim 17 uses p_{k-4} ≤ n(n/4)^{k-5} to control the number of ways to complete a k-cycle through a low-degree vertex, the proof of the minimum-degree bound δ(ẽG) > 2n/23 collapses. Consequently Lemma 12, which is the entire proof of Theorem 11, is unsupported, and the large-ℓ random-bipartite case is not proved.
- [1.1] Observation 1 is false as stated. Let F = C_6 and let H be the oriented graph consisting of two directed triangles sharing one vertex. Under the paper's definition, H is a homomorphic image of C_6: map the six vertices of C_6 to the sequence a,b,c,a,d,e, where a,b,c and a,d,e are the two triangles. A balanced blow-up of H contains no C_6, because the underlying undirected graph has no cycle of length 6. However, deleting o(n^2) arcs cannot destroy all copies of H: to eliminate H one must delete all arcs of at least one of the two directed triangles, which requires Θ(n^2) deletions. Thus the reduction used at the start of Section 3 — 'remove o(n^2) arcs using Observation 1 to delete from G all homomorphic images of C_ℓ' — is not valid. This clearing step is used in the proofs of Theorems 8, 11, 24, 26, 28, 31, and 35, so those upper-bound proofs lack a valid foundation and would need a substantially different reduction.
- [3.3.2] The upper bound for ex(n,C_4,C_3) is imported entirely from the unpublished manuscript [17] (Hu–Lidický–Volec, 'Inducibility of orientations of C_4'). No proof of the relevant inducibility result is given in this paper, and no public version of [17] is referenced. The stated constant n^4/(4^4-1) + o(n^4) is therefore conditional on an external result that the present manuscript does not make available to the reader. For a main theorem, this dependency should be removed or the external result should be published and cited in a verifiable form.
minor comments (4)
- [3.3.3, Theorem 27] The flag-algebra proof is presented only as a Flagmatic script; the generated certificate is not included, and the target bound 15/64 is not reconciled with the claimed constant 1/512 n^5. Without the certificate or an explanation of the normalization, the proof is not independently verifiable from the manuscript.
- [2, Lemma 9] The same symbol w(D) is used for both the forward and reverse sampling weights in the proof of Lemma 9; renaming the second weight would improve readability.
- [1] The text contains numerous formatting artifacts and typos, such as missing spaces in the abstract, 'forex(n,C_4,C_3)' in Section 3.3.2, and inconsistent arrow/bold notation for cycles. A careful copyedit is needed.
- [3.3.3, Theorem 28] The final optimization step in the proof of Theorem 28 is only sketched ('Optimizing the sizes of the six blobs ... gives in both cases the same blob sizes ...'); the computation should be shown to confirm the claimed constant 27/16.
Circularity Check
No significant circularity: the derivations are self-contained or rely on external benchmarks, and the self-citations are not load-bearing.
full rationale
The paper contains no fitted parameters and no prediction that is defined in terms of the target quantity. The order-of-magnitude bounds in Theorem 4 are proved directly: the lower bounds come from explicit blow-up constructions, and the upper bounds follow from a thick-arc argument together with the Frobenius-type Corollary 3. In the dense setting, Theorem 8 derives the upper bound from Lemma 9, whose proof is a self-contained adaptation of the Král'-Norin-Volec weighting method, and the required neighborhood bound is established inside the proof using Corollary 3 and the definition of d; the extremal construction is a balanced blow-up of C_d, but its size is not inserted as an input to the upper-bound calculation. Theorem 11 similarly derives the random-bipartite bound from the Andrásfai-Erdős-Sós theorem and an eigenvalue estimate (Lemma 13) that is proved in full, including the inequality whose proof is credited to [15] but also included inline. The self-citations to [13], [14], and [15] cite published, externally checkable results (e.g., the maximum number of pentagons in triangle-free graphs) that do not contain the present theorem as an assumption. Theorem 23's use of the unpublished manuscript [17] for the inducibility of orientations of C4 is an external dependency rather than a circular reduction, and the cited result is about induced copies of C4, not about ex(n,C4,C3) itself. A suspected error in Claim 16 or the reliance on [17] would be a correctness risk, not a circularity, because neither reduces the claimed result to its own inputs by definition or by a fitted parameter. Overall, the derivation chain is not circular.
Assumptions & free parameters
assumptions (10)
- standard math Directed Szemeredi regularity lemma and blow-up lemma: a subgraph-free oriented graph can be made free of all homomorphic images by deleting o(n^2) arcs (Observation 1).
- standard math Brauer's theorem and Corollary 3 (Frobenius coin problem): sufficiently large multiples of the gcd are nonnegative integer combinations of given integers.
- standard math Mantel's theorem: a triangle-free graph on n vertices has at most n^2/4 edges.
- standard math The Andrasfai-Erdos-Sos theorem, in the form used by the authors to conclude bipartiteness from the minimum degree and short odd cycle conditions.
- standard math Perron-Frobenius theorem for nonnegative matrices and Ky Fan's eigenvalue inequality (Proposition 18).
- standard math The complex-number inequality Re(z^k) <= k|z|^(k-1) Re(z) for Re(z) >= 0 and k = 2 mod 4.
- standard math Erdos' theorem: a K4-free undirected graph on n vertices has at most (n/3)^3 + O(n^2) triangles, attained by a balanced blow-up of K3.
- standard math The maximum number of C5 in a triangle-free undirected graph is (n/5)^5 + o(n^5) (Grzesik [13], Hatami et al. [16]).
- domain assumption The inducibility of orientations of C4 is maximized by the iterated blow-up of C4, claimed in the unpublished manuscript [17] (see also [6]).
- domain assumption The flag algebra method with Flagmatic is a valid and complete proof system, and the script in Theorem 27 generates a verifiable certificate.
Cite this review
Pith. "Pith review of Generalized Tur\'an problem for directed cycles." pith.science (2026). https://pith.science/paper/73J7KMHY
@misc{pith2026250522189,
author = {Pith},
title = {Pith review of: Generalized Tur\'an problem for directed cycles},
year = {2026},
howpublished = {\url{https://pith.science/paper/73J7KMHY}},
note = {Machine review of arXiv:2505.22189}
}
abstract
For integers $k, \ell \geq 3$, let $\mathrm{ex}(n, \overrightarrow{C_k}, \overrightarrow{C_\ell})$ denote the maximum number of directed cycles of length $k$ in any oriented graph on $n$ vertices which does not contain a directed cycle of length $\ell$. We establish the order of magnitude of $\mathrm{ex}(n, \overrightarrow{C_k}, \overrightarrow{C_\ell})$ for every $k$ and $\ell$ and determine its value up to a lower error term when $k \nmid \ell$ and $\ell$ is large enough. Additionally, we calculate the value of $\mathrm{ex}(n, \overrightarrow{C_k}, \overrightarrow{C_\ell})$ for some other specific pairs $(k, \ell)$ showing that a diverse class of extremal constructions can appear for small values of $\ell$.
Figures
Figures from the paper (8 more)
Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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