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A computer-assisted proof of the four-uniform Erdős matching conjecture

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Abstract

We prove the Erdős matching conjecture for four-uniform hypergraphs for every matching parameter and every admissible number of vertices. The proof combines the finite-board method of Hou, Hu and Liu with ordered local configurations, exact integer counts and certified inequalities. Small matching parameters are treated on full labelled configurations. Two fixed counting inequalities and two endpoint anchors propagate polynomially across the remaining range; 51 endpoint records and the two fixed inequalities suffice for the intermediate cardinality inputs. Dense triple supports admit direct slab and majority proofs. A classical shadow bound and a deletion argument complete the passage from these local bounds to the global theorem. The strict margins also yield clique stability with explicit constants for shifted families whose matching number is preserved on deleting the first vertex, and classifications of extremizers in specified ranges. The finite parts are supported by exact certificates and exhaustive computations with stated mathematical specifications.

The complete four-uniform bound

All vertex and matching parameters are integers. A matching is a collection of pairwise disjoint sets. Write ν(F)\nu(\mathcal{F}) for the largest size of a matching in F⊆([n]4)\mathcal{F} \subseteq\binom{[n]}{4}, and define

Br(n,t)=(nr)−(n−tr),A4(s)=(4s+34).B_r(n,t)=\binom{n}{r}-\binom{n-t}{r}, \qquad A_4(s)=\binom{4s+3}{4}.

For a tt-set T⊆[n]T \subseteq[n], the complete cover

CT(n)={E∈([n]4):E∩T≠∅}\mathcal{C}_T(n)=\left\{E\in\binom{[n]}{4}: E\cap T\ne\varnothing\right\}

has B4(n,t)B_4(n,t) members and matching number at most tt. Write Ct=C[t](n)\mathcal{C}_t=\mathcal{C}_{[t]}(n) when the ground set is clear. The other candidate is the complete four-graph on 4s+34s+3 vertices, with all remaining vertices isolated.

Theorem 1.1 (Four-uniform matching bound). Let s≥1s\ge1. For every n≥4s+4n\ge4s+4 and F⊆([n]4)\mathcal{F}\subseteq\binom{[n]}{4} with ν(F)≤s\nu(\mathcal{F})\le s,

∣F∣≤M(n,s):=max⁡{A4(s),B4(n,s)}.|\mathcal{F}|\le M(n,s):=\max\{A_4(s),B_4(n,s)\}.

For s≤4s\le4, the classical shadow input in § 2 and the small full-carrier certificates suffice.

The clique on 4s+34s+3 vertices and the complete ss-cover attain the two terms. For n<4s+4n<4s+4 the complete family has no forbidden matching and gives the trivial extremal value (n4)\binom{n}{4}. The theorem concerns cardinality. § 5 identifies the exact extremizers in explicit cover and clique domains and gives quantitative clique stability for shifted ONE families. It does not classify every crossing case or prove arbitrary-family stability.

Contribution and antecedents. Erdős formulated the matching problem in [1 equation (9)]. Frankl [13] settled uniformity three; the short proof of Frankl–Rödl–Ruciński [4] covers matching number at least 33. Hou–Hu–Liu [12 Theorem 1.1] prove the four-uniform bound for s≥6961s \ge6961, using a finite-board reduction. Theorem 1.1 extends that range to every positive ss. Its ingredients retain local order in the trace constraints, couple the resulting inequalities, and use integer orbit counts where a continuous relaxation loses necessary information. The board method, selected-residual construction and consumed HHL programs remain attributed to that work. The reduction to Property ONE and the deletion inequality have antecedents in [13 Proposition 5.1 and Corollary 5.1].

Notation and proof route. A family is shifted if every four-set coordinatewise below one of its members also belongs to it. It has Property ONE if a maximum matching avoids its first vertex. Put

as=n4(s)=min⁡{n≥4s+4:B4(n,s)≥A4(s)}.a_s = n_4(s) = \min\{n \ge4s + 4 : B_4(n,s) \ge A_4(s)\}.

For s≥2s \ge2, the direct inputs are the two crossings as−1,asa_s - 1, a_s. § 2 proves that these inputs suffice, before the finite configurations are analysed. § 3 gives the trace reduction, an ordered board and the certificates for the small and intermediate parameters. § 4 proves the necessary layer inequalities and covers the infinite tail. The division of the critical inputs is

s=1s = 1partition and classical link bound
s=2,3,4s = 2, 3, 4partition and four full-carrier certificates
s≥5s \ge551 endpoint records, two fixed inequalities and propagation
s≥73s \ge73stronger ONE-clique bound directly from the layers.

Theorem 1.1 is a cardinality statement. Table 1 distinguishes it from the structural consequences.

ConclusionFamiliesParameter domain
Exact cardinalityAll four-uniform families, ν≤s\nu\le ss≥1s \ge1, n≥4s+4n \ge4s + 4
Clique stability 71/143Shifted ONE, ν=s\nu= ss≥122s \ge122, 4s+4≤n≤as4s + 4 \le n \le a_s
Clique extremizersArbitrary, ν≤s\nu\le s, maximum sizes≥25s \ge25, 4s+4≤n<as4s + 4 \le n < a_s
Cover extremizersArbitrary, ν≤s\nu\le s, maximum sizes=5s = 5, n≥33n \ge33; s=6s = 6, n≥34n \ge34; 7≤s≤1217 \le s \le121, n≥asn \ge a_s; s≥122s \ge122, n≥as+1n \ge a_s + 1

Table 1. Domains of the global and structural results. The constants 71 and 143 control respectively the number of edges outside the canonical clique and the symmetric difference. The precise statements are in § 5.

Dependence on mathematical and computational inputs. The global cardinality proof imports Frankl’s shadow inequality [3 Theorem 1.2]; it does not import the complete three-uniform matching theorem. The necessary finite HHL statements are reconstructed with explicit searches and certificate checks. The cover-uniqueness consequence also uses [3 Theorem 1.1] at s=4s = 4, n≥32n \ge32. The complementary shifted cover-stability and graph results in Appendix § 9 have their additional imports listed there. Exact equality can be transferred through shifts by the two inverse lemmas in § 5; quantitative stability for arbitrary families is not established. § 7 compares the precise statements with earlier work, including the clique-side results of Guo–Lu. § 8 separates the global computations from the additional checks required by the appendices.

From the critical bounds to the global theorem

Write ∂H\partial\mathcal{H} for the immediate shadow and Kj(A)K_j(\mathcal{A}) for the number of jj-sets all of whose appropriate uniform subsets belong to A\mathcal{A}. Integer binomial coefficients are zero when the upper argument is nonnegative and smaller than the lower one. For polynomial identities we use the product extension of binomials.

We use shifting, which preserves cardinality and does not increase the matching number, and the following classical quantitative input.

Shadow. Frankl [3 Theorem 1.2] gives ∣H∣≤u∣∂H∣|\mathcal{H}| \leq u|\partial\mathcal{H}| when ν(H)=u\nu(\mathcal{H}) = u. Consequently ν(H)≤c\nu(\mathcal{H}) \leq c implies ∣H∣≤c∣∂H∣|\mathcal{H}| \leq c|\partial\mathcal{H}|, with the empty case immediate.

Lemma 2.1 (Shifted completion). Let n≥4tn \geq4t and let F⊆([n]4)\mathcal{F} \subseteq\binom{[n]}{4} be shifted with ν(F)≤t\nu(\mathcal{F}) \leq t. There is a shifted G⊇F\mathcal{G} \supseteq\mathcal{F} with ν(G)=t\nu(\mathcal{G}) = t. Consequently, for K≥0K \geq0, a bound

∣G∖Ct∣≤K(B4(n,t)−∣G∣)|\mathcal{G} \setminus\mathcal{C}_t| \leq K\bigl(B_4(n,t)-|\mathcal{G}|\bigr)

for every shifted G\mathcal{G} of exact matching number tt implies the same bound for every shifted F\mathcal{F} of matching number at most tt.

Proof. While the matching number is below tt, add a coordinatewise minimal missing four-set. Every strict predecessor is present, so the enlarged family is still shifted. Adding one edge increases the matching number by at most one. The process is finite and reaches tt, because the complete family has matching number ⌊n/4⌋≥t\lfloor n/4\rfloor\geq t. Inclusion then gives

∣F∖Ct∣≤∣G∖Ct∣≤K(B4(n,t)−∣G∣)≤K(B4(n,t)−∣F∣).|\mathcal{F} \setminus\mathcal{C}_t| \leq|\mathcal{G} \setminus\mathcal{C}_t| \leq K\bigl(B_4(n,t)-|\mathcal{G}|\bigr) \leq K\bigl(B_4(n,t)-|\mathcal{F}|\bigr).

A section bound already sufficient for the links

Lemma 2.2 (A consequence of Frankl’s section method). For r≥2r \geq2, t≥1t \geq1, put

Dr(t)=(2r−1)t+r−(r−1)⌈t+1r⌉.D_r(t)=(2r-1)t+r-(r-1)\left\lceil\frac{t+1}{r}\right\rceil.

Every rr-uniform family with matching number at most tt on n≥Dr(t)n \geq D_r(t) vertices has size at most Br(n,t)B_r(n,t). In particular D4(3)=22D_4(3)=22, and D3(t)≤n4(t)D_3(t) \leq n_4(t) for all t≥1t \geq1.

Proof. Shift and complete to matching number exactly tt, by the argument of Lemma 2.1 in uniformity rr. An edge of a maximum matching avoids [t−1][t-1], so shiftedness forces the complete rr-graph on [t+r−1][t+r-1]. Put X=[t+1]X=[t+1], Y=[n]∖XY=[n]\setminus X, q0=⌊(t+1)/r⌋q_0=\lfloor(t+1)/r\rfloor, and c=t−q0c=t-q_0. The section H\mathcal{H} disjoint from XX has matching number at most cc, using q0q_0 disjoint edges inside XX. For the nested singleton sections A1,…,At+1\mathcal{A}_1,\ldots,\mathcal{A}_{t+1} on YY, the shadow input gives ∣H∣≤c∣At+1∣|\mathcal{H}| \leq c|\mathcal{A}_{t+1}|. They are cross-dependent.

For a matching of v=2t−q0+1=t+c+1v=2t-q_0+1=t+c+1 sets of size r−1r-1 in YY, write did_i for its section counts. The nested-neighbourhood argument gives an x∈[0,t]x\in[0,t] such that the first t−xt-x counts are at most vv and the last x+1x+1 are at most xx. Hence

∑i=1tdi+(c+1)dt+1≤(t−x)v+x2+(c+1)x=tv+x(x+c+1−v)≤tv.\sum_{i=1}^{t}d_i+(c+1)d_{t+1}\leq(t-x)v+x^2+(c+1)x=tv+x(x+c+1-v)\leq tv.

Such a matching exists because n≥Dr(t)n\geq D_r(t) is precisely ∣Y∣≥(r−1)v|Y|\geq(r-1)v. Averaging gives

∣H∣+∑i=1t+1∣Ai∣≤t(∣Y∣r−1).|\mathcal{H}|+\sum_{i=1}^{t+1}|\mathcal{A}_i|\leq t\binom{|Y|}{r-1}.

Every section using at least two vertices of XX already meets [t][t]; bounding it by the complete section proves the cover inequality. The argument uses the shadow theorem and the weighted nested-section method of [3 Sections 3–5].

References

  1. [1]P. Erdős, A problem on independent r-tuples, Ann. Univ. Sci. Budapest. Eötvös Sect. Math. 8 (1965), 93–95. Original paper, equation (9).
  2. [2]M. Guo and H. Lu, A stability result for almost perfect matchings, J. Graph Theory 112 (2026), 5–14. doi:10.1002/jgt.70007. Theorem locators follow arXiv:2404.09720v1.DOI
  3. [3]P. Frankl, Improved bounds for Erdős’ Matching Conjecture, J. Combin. Theory Ser. A 120 (2013), 1068–1072. doi:10.1016/j.jcta.2013.01.008. Theorems 1.1–1.2 and Sections 3–5.DOI
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