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Disjoint pairs in set systems and combinatorics of low rank matrices
T0 review · 1 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read This paper proves the optimal disjoint-pairs bound for large set families, resolving the 40-year-old problem, and extends the same covering and discrepancy machinery to dense disjointness graphs, nonzero intersections, and low-rank…
desk verdict This is a strong paper that settles the Daykin–Erdős problem with optimal dependence and proves the Singer–Sudan conjecture; the main proofs hold up, but Section 3.1 contains a false inequality and Theorem 1.10 has a fixable small-family gap. 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
Three mechanisms carry the proof. The first is the bad-set covering argument: a set $U$ is bad for a family $\mathcal F$ if it covers fewer than $2^{-2n/k}|\mathcal F|$ members; the union of $k$ independent random sets is bad with probability at most $2^{-n}$, while the expected common neighbourhood of $k$ random vertices in a $\delta$-dense disjointness graph is at least $\delta^k|\mathcal B|$, so a typical union yields large cross-disjoint subfamilies. The second is the entropy covering lemma: for every $\mathcal F$ of size $2^{\alpha n}$, independent uniform draws $F_1,\dots,F_k$ satisfy $\mathbb E|\bigcup_i F_i|\ge \alpha n - Cn/2^k$, proved by writing the expected union as $\sum_i(1-(1-p_i)^k)$ and comparing each term with the binary entropy $H(p_i)$; this makes random unions provably large and creates the contradiction with a small common neighbourhood. The third is a discrepancy bound for low-rank matrices: representing $M$ through singular vectors scaled by singular values and applying Grothendieck's inequality gives a half-size submatrix $M'$ with $\|M\|_C \ge c\sqrt{mn}\,\|M'\|_F^2/(\sqrt r\|M\|_F)$, where $\|\cdot\|_C$ is the cut norm; iterating this discrepancy while reducing the average entry produces large all-zero or constant submatrices.
What would settle it
Exhibit a family $F\subseteq 2^{[n]}$ of size $2^{(1/2+\delta)n}$ whose disjoint-pair density is $2^{-o(\delta/\log(1/\delta))}$, which would beat the theorem's upper bound for every constant $c$; alternatively, find a family for which the expected union of $k$ uniform random members is smaller than $\alpha n - Cn/2^k$, which would refute the lemma that drives the proof.
Extended reading notes
Core claim
The central claim is that the density of disjoint pairs in a family is controlled by the exponent $\delta$ through the ratio $\delta/\log(1/\delta)$. For $F\subseteq 2^{[n]}$ with $|F|=m\ge 2^{(1/2+\delta)n}$, the number of disjoint unordered pairs is at most $m^2 2^{-c\delta/\log(1/\delta)}$, and the construction from [2] with a split ground set and low-intersection layers has size $2^{(1/2+\delta)n}$ and contains $m^2 2^{-O(\delta/\log(1/\delta))}$ disjoint pairs, so the upper bound is optimal up to the constant $c$. In the bipartite form, two families of size at least $c_d\,2^{n/2} n^d$ have at most $(1+o(1))2^{-2d}|A||B|$ disjoint pairs. The same covering mechanism proves that families with a constant density of pairs having intersection exactly $\lambda\neq 0$ still have size at most $2^{(1/2+o(1))n}$, and that disjointness density $\delta$ forces cross-disjoint subfamilies of product size $|A||B|2^{-O(\sqrt{n\log(1/\delta)})}$. For matrices, the paper shows that an $m\times n$ rank-$r$ matrix with entries $0$ or at least $1$ and average $\varepsilon\le 1/2$ contains an all-zero submatrix of size $n2^{-O(\log r+\sqrt{\varepsilon r})}$, optimal when $\varepsilon\gg(\log r)^2/r$, and that integer matrices with entries $0,\dots,t$ contain constant submatrices of size $2^{-O(t\sqrt r)}m\times 2^{-O(t\sqrt r)}n$.
Load-bearing premise
The load-bearing premise is the entropy covering lemma: for every family $\mathcal F$ of size $2^{\alpha n}$, the union of $k$ independent uniform random sets from $\mathcal F$ has expected size at least $\alpha n - Cn/2^k$ with an absolute constant $C$; if that universal union-growth bound failed, the dependent-random-choice contradiction behind Theorems 1.2 and 1.3 would collapse.
Editorial extensions
If this is right
- The 1985 construction is optimal up to a constant factor, so the maximum disjoint-pair density for families of size $2^{(1/2+\delta)n}$ is $2^{-\Theta(\delta/\log(1/\delta))}$.
- The biclique conjecture of [26] holds in strong quantitative form: disjointness density $\delta$ always yields cross-disjoint subfamilies of size at least $|A||B|2^{-O(\sqrt{n\log(1/\delta)})}$.
- For every distribution $\mu$ on $2^{[n]}$, the probability that $A_0\subseteq A_1\cup\cdots\cup A_r$ for independent draws is at least $2^{-n/r-2}$, which is tight up to the constant and sharpens the known supersaturation bound for $r$-cover-free families.
- The sparse low-rank matrix conjecture from [22] is confirmed for separated matrices: if the average entry of a rank-$r$ matrix is $\varepsilon\le1/2$, an all-zero submatrix of size $n2^{-O(\log r+\sqrt{\varepsilon r})}$ exists, and this is optimal in the regime $\varepsilon\gg(\log r)^2/r$.
- Log-rank-type rectangle bounds extend beyond binary matrices: integer matrices with entries $0,\dots,t$ and rank $r$ contain a constant submatrix of size $2^{-O(t\sqrt r)}m\times 2^{-O(t\sqrt r)}n$.
Reading between the lines
- The entropy covering lemma suggests a transferable principle: for any large family, $k$ uniform random draws cover essentially as many coordinates as $k$ draws from a subcube of dimension $\log_2|\mathcal F|$. If this principle extends to other forbidden intersection patterns, the $\lambda\neq0$ variant of the disjoint-pairs problem could be sharpened from constant-density to subconstant density.
- Theorem 1.12 shows the additive $\log r$ loss in the all-zero rectangle bound is only needed when the average entry is very small; reaching the conjectured $2^{-O(\sqrt{\varepsilon r})}$ for all $\varepsilon$ would require a new argument near the boundary $\varepsilon\sim(\log r)^2/r$, which the construction in the paper shows is the delicate regime.
- The bad-set covering proof of Lemma 1.11 is distribution-free and yields a quantitative covering probability; the same union-bound-over-bad-sets trick should give supersaturation bounds for other union-closed properties, such as $k$-wise disjointness or prescribed intersection sizes, with the same $2^{-O(n/r)}$ shape.
- The bipartite reduction in the proof of Theorem 1.3 indicates that sharp constants for other intersection constraints might follow from the same discrepancy and entropy template; Problem 6.2 of the paper, whether a large family of pairs with intersection $\lambda$ contains a large subfamily with all intersections exactly $\lambda$, is a natural first test.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies extremal problems about disjoint pairs in set systems and zero/constant submatrices in low-rank matrices. Its main results are: an optimal bound for the Daykin-Erdős problem with the conjectured dependence on δ (Theorem 1.2); a bipartite strengthening with asymptotically sharp constants (Theorem 1.3); a Singer–Sudan type biclique theorem for disjointness graphs (Theorem 1.10); a tight lower bound for r-cover-free families (Lemma 1.11); a nonzero-intersection analogue (Theorem 1.5); an even/odd intersection bound (Theorem 1.6); new low-rank matrix results for sparse separated matrices (Theorem 1.12) and for integer matrices with bounded average (Theorem 1.13). The proofs combine dependent random choice, entropy, Fourier analysis, discrepancy, and additive combinatorics, and the paper also provides constructions showing optimality of several bounds.
Significance. If the proofs are correct, these are substantial contributions. Theorem 1.2 resolves the Daykin-Erdős problem with the optimal dependence on δ, matching Construction 1 up to the constant c. Theorem 1.10 proves the Singer–Sudan conjecture in a strong quantitative form, and Lemma 1.11 settles the Alon–Gilboa–Gueron problem up to a constant in the exponent. The matrix results generalize the best known log-rank bounds and are optimal in a natural parameter range. The entropy proof of Theorem 1.2 via Theorem 3.3 and Lemma 3.5 is sound; I checked the dependent-random-choice chain and the constants. The combinatorial proof of Theorem 3.2 is also essentially correct once the notation '104' is read as 10^4, matching the hypothesis of Theorem 3.2; with that reading the numerical inequalities are valid. The main weakness is in the appendix proof of Theorem 1.13, which has a missing verification of a key hypothesis.
major comments (1)
- [Appendix, proof of Theorem 1.13] In the key statement of the proof, Proposition A.5 is applied to the matrix M − ℓJ, but the hypothesis of Proposition A.5 that all entries lie in (−r^3, r^3) is not verified. Lemma 4.7(1) only shows that a submatrix has entries bounded by 400r^2 p(M), which can exceed r^3 when ℓ grows, and M − ℓJ can have negative entries of similarly large magnitude. This matters because the proof of Proposition A.5 uses the entry bound to assert q0 ≤ r^6 and hence f(p0) + g(q0) = O(√r + log r); without that bound the potential could be much larger and the claimed O(√r) iteration count does not follow. The theorem may still be true, and the gap may be repairable by a capping argument or by splitting off the regime where t is large, but as written the proof of Theorem 1.13 is incomplete.
minor comments (4)
- [Section 3.1, proof of Theorem 3.2] The apparent numerical contradiction in the displayed inequality disappears when '104' is read as 10^4, as in the theorem statement; with that reading 32/100 + 192/10^4 < 1/2 and the analogous final bound 35/100 + 210/10^4 < 1 also hold. The text should use a clear superscript to avoid confusion.
- [Section 4, Claim 4.1] The proof of the first bullet of Claim 4.1 is omitted with a pointer to the authors' own paper [28]. Since this claim is used in the proof of Theorem 1.12, the manuscript should either include the short proof or state the precise claim from [28] being invoked.
- [Statement of Theorem 1.12] The theorem is stated for an m × n matrix but the conclusion says 'an all-zero submatrix of size at least n2^{-O(log r+√εr)}'. If m is much smaller than n this is false as stated; the proof actually gives a submatrix of size 2^{-O(...)}m × 2^{-O(...)}n. The statement should be corrected to use min(m,n) or to state both dimensions.
- [Section 2, proof of Theorem 1.10] The small-family case is written only for |B| ≤ 2^{2√{n log δ^{-1}}}; if instead |A| is small while |B| is large, the symmetric argument picking one element of A and its neighbourhood in B should be stated explicitly.
Circularity Check
No significant circularity; the main theorems are proved from first principles against external benchmarks and constructions.
full rationale
The central results do not reduce to their inputs by construction. Theorem 1.2 is proved twice: the combinatorial proof in Section 3.1 is self-contained given Lemma 3.1, which is proved in the text; the entropy proof in Section 3.2 rests on Lemmas 3.4 and 3.5, both proved in-text using only elementary inequalities and the standard subadditivity of entropy (cited to Alon–Spencer). Theorem 1.3 follows from Theorem 3.3, whose proof is again self-contained; the Alon–Frankl theorem is used only in the reduction to the bipartite setting, not as an input to the main bound. Theorem 1.5 uses the external results of Gowers–Green–Manners–Tao, Ben-Sasson–Lovett–Ron-Zewi, Bhowmick–Dvir–Lovett, and Sgall; none of these assume the paper's conclusions. Theorems 1.12 and 1.13 are proved via discrepancy lemmas developed in Section 4 and the Appendix; Lemma 4.2 is credited to Kwan and Sauermann, and the iteration arguments are proved in-text. The only deferred item is Claim 4.1, whose proof is said to be 'almost identical' to Claim 2.2 of the authors' own [28]; this is a parameter-free technical averaging fact about discrepancy and does not assume any of the target theorems, so the citation is real evidence rather than a load-bearing self-citation. The lower bounds in Constructions 1, 2, and 3 serve as external benchmarks, and the upper bounds are matched against them rather than derived from them. No fitted parameter is renamed as a prediction, no uniqueness theorem from the authors' prior work is invoked to force a choice, and no known result is merely renamed. Hence the derivation chain is self-contained apart from minor, non-circular deferrals.
Assumptions & free parameters
assumptions (6)
- standard math Grothendieck's inequality: the cut norm and the γ_2^* norm of a real matrix are equivalent up to an absolute constant.
- standard math Approximate Duality Theorem (Theorem 3.7 of the paper, from Ben-Sasson-Lovett-Ron-Zewi [5] and Bhowmick-Dvir-Lovett [7]): sets A,B in F_p^n with bias ≥ 2/(3p^{3/2}) contain large subfamilies with constant inner product.
- standard math Polynomial Freiman-Ruzsa theorem over F_2^n, proven by Gowers-Green-Manners-Tao [14], used to remove the conditionality from Theorem 3.7.
- standard math Sgall's bound: if A,B ⊆ 2^[r] and |A∩B| is constant over A×B, then |A||B| ≤ 2^r (Corollary 3.5 of [25]).
- standard math Szemerédi's regularity lemma and Mantel's theorem, used in the non-bipartite reduction from Theorem 1.3 to the original Daykin-Erdős problem.
- standard math Alon-Frankl bound [2] for disjoint pairs in families on smaller ground sets, used in the same non-bipartite reduction.
Cite this review
Pith. "Pith review of Disjoint pairs in set systems and combinatorics of low rank matrices." pith.science (2026). https://pith.science/paper/FDTBICA4
@misc{pith2026241113510,
author = {Pith},
title = {Pith review of: Disjoint pairs in set systems and combinatorics of low rank matrices},
year = {2026},
howpublished = {\url{https://pith.science/paper/FDTBICA4}},
note = {Machine review of arXiv:2411.13510}
}
abstract
We study and solve several problems in two closely related settings: set families in $2^{[n]}$ with many disjoint pairs of sets and low rank matrices with many zero entries. - More than 40 years ago, Daykin and Erd\H{o}s asked for the maximum number of disjoint pairs of sets in a family $F\subseteq 2^{[n]}$ of size $2^{(1/2+\delta)n}$ and conjectured it contains at most $o(|F|^2)$ such pairs. This was proven by Alon and Frankl in 1985. In this paper we completely resolve this problem, proving an optimal dependence of the number of disjoint pairs on the size of family $F$. We also prove the natural variant of the Daykin-Erd\H{o}s conjecture in which disjoint pairs are replaced by pairs with intersection $\lambda\neq 0$. - Motivated by a conjecture of Lovett related to the famous log-rank conjecture, Singer and Sudan asked to show that for two families $A, B \subseteq 2^{[n]}$ with a positive constant fraction of set pairs $(a,b)\in A\times B$ being disjoint, there are $R\subset A$ and $S\subset B$ such that all set pairs $(r, s)\in R\times S$ are disjoint, and $|R|\geq 2^{-O(\sqrt{n})}|A|$ and $|S|\geq 2^{-O(\sqrt{n})}|B|$. We prove this conjecture in a strong quantitative form. - We prove the following generalizations of the best known bounds for the log-rank conjecture. If $M$ is an $n\times n$ non-negative integer matrix of rank $r$ in which the average of the entries is $\varepsilon\leq 1/2$, then $M$ contains an all-zero submatrix of size at least $2^{-O(\sqrt{\varepsilon r})}n$. Unlike the known bounds for the log-rank conjecture, this result is optimal. Moreover, using similar methods, we also prove that any $n\times n$ matrix of rank $r$ with entries from $\{0,\dots,t\}$ contains a constant submatrix of size at least $2^{-O(t\sqrt{r})}n$. Our proofs use probabilistic, entropy and discrepancy methods and explore connections to additive combinatorics and coding theory.
Forward citations
Cited by 1 Pith paper
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Factorization norms and an inverse theorem for MaxCut
Boolean matrices with bounded γ2-norm or normalized trace norm contain linear-sized homogeneous submatrices, and graphs with near-minimal MaxCut contain a large clique.
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