REVIEW 2 major objections 3 minor 1 cited by
Perfect tilings of 3-graphs with the generalised triangle
T0 review · 2 major / 3 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read For large n divisible by 5, every 3-uniform hypergraph with minimum codegree at least 2n/5 has a perfect tiling by the generalised triangle, and the bound is optimal.
desk verdict First exact codegree threshold for perfect tilings with a non-tripartite 3-graph on more than four vertices; the main theorem is sound, though Lemma 4.2 has a fixable averaging slip. 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 objects are the generalised triangle $T$ (the 3-graph with five vertices and edges $abc$, $abd$, $cde$), a fractional $T$-tiling (a weighting of copies of $T$ so every vertex has total weight 1), and the vector $a\in\mathbb{R}^n$ produced by Farkas' lemma when no perfect fractional $T$-tiling exists. Ordering vertices by $a_1\le\cdots\le a_n$ defines three families of five-vertex sets, and the central technical step is to find three copies $T_1,T_2,T_3$ of $T$, each $B$-avoiding for a small graph $B$ of forbidden pairs, each dominated by the corresponding family. Domination means the $i$-th smallest vertex of the copy is no larger than the $i$-th smallest vertex of the set, so $a\cdot\mathbf{1}_{T_i}\le a\cdot\mathbf{1}_V$ for every $V$ in that family. Summing the inequalities over the three families contradicts $a\cdot\mathbf{1}<0$, forcing a perfect fractional $T$-tiling whose pair weights are bounded by $1/(\varepsilon n)$; a matching theorem for multi-hypergraphs then turns this into an almost-perfect tiling, and absorption upgrades it to a perfect one.
What would settle it
An infinite family of 3-graphs $H_n$ with $5\mid n$, $\delta(H_n)\ge 2n/5$, and no perfect $T$-tiling would refute the main theorem; since every $\gamma$-extremal such graph is shown tileable, such a family would have to be non-extremal. A concrete way to search is to perturb the extremal example so that every copy of $T$ still needs two vertices from a blocker set of size $2n/5-1$ while every $3n/5$-vertex set induces density above $\gamma$.
Extended reading notes
Core claim
The central claim is that $\delta(H)\ge 2n/5$ is the exact barrier for perfect $T$-tilings in 3-graphs. The proof is split into an extremal case, where $H$ contains a set of $3n/5$ vertices inducing low density and a tiling is completed by finding a perfect matching in a dense auxiliary 5-partite 5-graph, and a non-extremal case, handled by absorption. In the non-extremal case the paper constructs a perfect fractional $T$-tiling with bounded pair weights via Farkas' lemma, then converts it to an almost-perfect tiling and uses a small absorber to finish. The matching threshold cannot be lowered: the hypergraph $H_{\text{ext}}$ with parts of sizes $2n/5-1$ and $3n/5+1$, containing all triples that meet the smaller part, has minimum codegree $2n/5-1$ and no perfect $T$-tiling.
Load-bearing premise
The non-extremal proof depends on a case split asserting that, for every ordering of the vertices by the separation vector, three prescribed five-vertex families can each be dominated by an actual copy of the generalised triangle that avoids a small set of forbidden pairs; if any one of the three constructions cannot be realized, the contradiction forcing a perfect fractional tiling collapses.
Editorial extensions
If this is right
- For all sufficiently large $n$ divisible by 5, any 3-uniform hypergraph with minimum codegree at least $2n/5$ has a perfect tiling by the generalised triangle.
- The threshold $2n/5$ is sharp: the constructed extremal hypergraph has codegree $2n/5-1$ and no perfect $T$-tiling.
- The rainbow version holds with the same constant asymptotically: for every $\varepsilon>0$, a family of $3n/5$ hypergraphs with common vertex set and each codegree at least $(2/5+\varepsilon)n$ admits a perfect rainbow $T$-tiling.
- This is the first exact perfect-tiling threshold for a non-tripartite 3-uniform hypergraph with more than four vertices, placing the generalised triangle alongside $K_4^3$ and $K_4^3-e$ as cases where the codegree threshold is fully determined.
Reading between the lines
- The authors conjecture that the 'sufficiently large' condition is unnecessary; if that is right, the exact $2n/5$ formula holds for every $n$ divisible by 5, and the extremal example is the only obstruction at every scale.
- Because the colour-covering threshold for $T$ is zero, the rainbow threshold equals the ordinary tiling threshold; comparing this with hypergraphs whose colour-covering threshold is positive would quantify when rainbow tilings become strictly harder.
- The structure-dependent Farkas step suggests a general template for non-tripartite $F$: identify the extremal construction, use the separation vector to define order-based vertex classes, and construct one dominating $F$-copy per class with cases chosen from neighbourhood intersections.
- Exact-cover searches on small multiples of 5 could test the no-large-$n$ conjecture and would expose the first possible non-extremal obstruction if the threshold formula fails before $n_0$.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves an exact minimum codegree threshold for perfect tilings by the generalized triangle T: for all sufficiently large n divisible by 5, every 3-uniform hypergraph H on n vertices with δ(H) ≥ 2n/5 contains a perfect T-tiling, and the constant 2/5 is best possible, as shown by H_ext with |A|=2n/5−1 and all edges meeting A. The proof combines an extremal case (Section 2), an absorbing lemma (Section 3), and an almost-perfect tiling lemma (Section 4) obtained from a fractional T-tiling via Farkas' lemma and a Pippenger–Spencer matching result. Section 5 derives an asymptotically optimal rainbow version from a theorem of Lang.
Significance. The result is a genuine step forward: K_4^3 and K_4^3−e were previously the only non-tripartite 3-graphs for which an optimal codegree threshold for perfect tilings was known, and T is the first such graph on more than four vertices. The lower-bound example is simple and correct, and the proof is detailed, with the absorbing and extremal components carefully structured. The Farkas-lemma case analysis in Section 4 is a novel technique with potential for further applications. If the Section 4 statements are corrected as described below, the main theorem is sound; the rainbow corollary is conditional on the unpublished preprint [33].
major comments (2)
- [Section 4, Lemma 4.2, first paragraph] The reduction to the case 5|n is invalid. In the 5-blow-up H', a pair u_i v_j with u,v∈V and uv∉E(H) is contained only in the triples u_i u_{j'} v_j with j'≠i, so its codegree is 4, whereas 5δ(H) is about 2n; hence δ(H') ≥ 5δ(H) is false and the case 5|n cannot be applied to H'. Even if the codegree were preserved, the projection is mis-normalized: averaging w' over all 5^5 liftings of a copy T* assigns load 1/5^4 to each vertex of H, not 1; the correct projection is to sum the weights of all liftings and divide by 5. The remainder of the proof after 'so assume that 5 divides n' is self-contained, so the cleanest repair is to state Lemmas 4.2, 4.3, and 1.5 only for 5|n, which is the only case used in the proof of Theorem 1.1.
- [Section 4, Lemma 4.3] The application of Lemma 4.2 in the proof of Lemma 4.3 is not justified as written: from W ≥ 1/(εn) one gets ∆(B) ≤ 4/W ≤ 4εn, while Lemma 4.2 requires ∆(B) ≤ εn. The argument can be repaired by applying Lemma 4.2 with parameter 4ε (or a rescaled constant), so this is a local but necessary correction.
minor comments (3)
- [Section 4, proof of Lemma 4.2, T1 construction] In the construction of T1 the displayed inequality '3n/5+αn+4εn≤βn' is false; it should read '≤3n/5+βn' for the subsequent domination by V1 to hold.
- [Section 5, proof of Theorem 1.6] The proof of δ_c=0 says 'Let H1,H2 be graphs'; these should be 3-graphs. It would also be clearer to state explicitly that for n large the condition δ(H_i)≥(δ+µ)n with δ=0 yields δ(H_i)≥3, so the construction applies.
- [Section 1.3, proof of Theorem 1.1] The notation '5||S|' is nonstandard; consider writing '5 divides |S|' for clarity.
Circularity Check
No significant circularity; Theorem 1.1 rests on external prior results and the rainbow corollary applies Lang's theorem independently.
full rationale
The main derivation is self-contained against external benchmarks. Lemma 1.3 uses the Daykin–Häggkvist matching theorem; Lemma 1.4 is obtained from Lo–Markström's absorbing theorem together with Han–Treglown and Han–Zang–Zhao; Lemma 1.5 is derived from Farkas' lemma and Pippenger–Spencer. None of these inputs states Theorem 1.1 or is fitted to its conclusion, and the extremal lower bound H_ext is constructed independently and verified directly. The rainbow corollary computes δ_c=0 by an explicit construction and δ_t=2/5 from Theorem 1.1 plus H_ext, then applies Lang's external relation δ_r=max(δ_c,δ_t); this is a legitimate deduction, not a renaming or a self-citation. The paper's citations to the authors' own previous work (e.g., Mycroft [39], Keevash–Mycroft [25]) are contextual and not load-bearing for Theorem 1.1. The only salient defect, an averaging slip in the blow-up reduction at the opening of Lemma 4.2, is a non-circular technical gap in the case where 5 does not divide n; the divisible case actually used in the proof of Theorem 1.1 is proved directly, so the circularity verdict is unaffected.
Assumptions & free parameters
assumptions (7)
- standard math Farkas' lemma (Lemma 4.1)
- domain assumption Daykin-Haggkvist theorem (Theorem 2.1)
- domain assumption Lo-Markström absorption lemma (Theorem 3.1)
- domain assumption Han-Treglown partition theorem (Theorem 3.3)
- domain assumption Han-Zang-Zhao lattice theorem (Theorem 3.5)
- domain assumption Pippenger-Spencer matching theorem (Theorem 4.4) and Corollary 4.5
- domain assumption Lang's theorem δ_r = max(δ_c, δ_t) (Theorem 5.1)
Cite this review
Pith. "Pith review of Perfect tilings of 3-graphs with the generalised triangle." pith.science (2026). https://pith.science/paper/UPOCRUEU
@misc{pith2026250505606,
author = {Pith},
title = {Pith review of: Perfect tilings of 3-graphs with the generalised triangle},
year = {2026},
howpublished = {\url{https://pith.science/paper/UPOCRUEU}},
note = {Machine review of arXiv:2505.05606}
}
abstract
We establish a best-possible minimum codegree condition for the existence of a perfect tiling of a $3$-uniform hypergraph $H$ with copies of the generalised triangle $T$, which is the 3-uniform hypergraph with five vertices $a, b, c, d, e$ and three edges $abc$, $abd$, $cde$. We also give an asymptotically-optimal minimum codegree condition for the rainbow version of the problem.
Figures
Forward citations
Cited by 1 Pith paper
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Positive codegree thresholds for perfect matchings in hypergraphs
For every k ≥ 3, a large k-uniform hypergraph with minimum positive codegree at least (k−1)/k n − (k−2) and no isolated vertices must contain a perfect matching, and this bound is best possible.
Reference graph
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