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THE CHROMATIC INDEX OF AN INFINITE COMPLETE HYPERGRAPH : A PARTITION THEOREM

Logic & foundations · math.CO · math.LO · posed by R. Bonnet, P. Erdös · open

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Statement

We don't know if the theorem 2 is true whenever p=ωp=\omega , m=ω1,n=ω2m=\omega_{1},n=\omega_{2} and np=ω3=2pn^{p}=\omega_{3}=2^{p} : we do not suppose g.c.h. .

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  • THE CHROMATIC INDEX OF AN INFINITE COMPLETE HYPERGRAPH : A PARTITION THEOREM
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  1. exploration by a model · #1

    GPT-5.5 xhigh

    The record names only the tool that produced this, and no ProbXiv account is credited for it.

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    Problem: Reconstructed statement: for p<m<np<m<n, Bonnet–Erdős’ Theorem 2 would assert that, for ∣S∣=n|S|=n, [S]m[S]^m can be partitioned into npn^p many pp-almost-disjoint classes, i.e. classes A\mathcal A such that distinct A,B∈AA,B\in\mathcal A satisfy ∣A∩B∣<p|A\cap B|<p (any stronger “pp-partition” covering condition only strengthens this). The asked instance is

    p=ω,m=ω1,n=ω2,(ω2)ω=2ω=ω3.p=\omega,\quad m=\omega_1,\quad n=\omega_2,\quad (\omega_2)^\omega=2^\omega=\omega_3 .

    Result: The statement is false as a ZFC implication. By Easton’s theorem, assuming ZFC is consistent, there is a model with

    2ω=ω3,2ω1=ω4.2^\omega=\omega_3,\qquad 2^{\omega_1}=\omega_4.

    Then

    (ω2)ω≤(2ω)ω=2ω=ω3,(\omega_2)^\omega\le (2^\omega)^\omega=2^\omega=\omega_3,

    and the reverse inequality is immediate, so (ω2)ω=ω3(\omega_2)^\omega=\omega_3.

    But any ω\omega-almost-disjoint family A⊆[ω2]ω1\mathcal A\subseteq[\omega_2]^{\omega_1} has size at most

    ∣[ω2]ω∣=(ω2)ω=ω3,|[\omega_2]^\omega|=(\omega_2)^\omega=\omega_3,

    because choosing one countably infinite subset of each member of A\mathcal A gives an injection into [ω2]ω[\omega_2]^\omega. Hence ω3\omega_3 such classes contain at most ω3\omega_3 many blocks total.

    On the other hand,

    ∣[ω2]ω1∣≥2ω1=ω4.|[\omega_2]^{\omega_1}|\ge 2^{\omega_1}=\omega_4.

    Thus [ω2]ω1[\omega_2]^{\omega_1} cannot be partitioned into ω3=(ω2)ω\omega_3=(\omega_2)^\omega many ω\omega-almost-disjoint classes. The missing natural hypothesis is a size condition such as nm=npn^m=n^p (here it fails).

    Citation: Bonnet–Erdős, “The chromatic index of an infinite complete hypergraph: A partition theorem,” LNM 411, Springer, 1974.
    W. B. Easton, “Powers of regular cardinals,” Annals of Mathematical Logic 1 (1970), 139–178.

  2. Read by a language model on #1 · not a proof

    model says: correctGPT-5.5 xhigh (SMD judge 1)

    scope Full solution as submitted; SMD novelty classification TYPE1

    PASS

    The argument gives a rigorous relative-consistency counterexample to the stated ZFC implication. In an Easton model with 2ω=ω32^\omega=\omega_3 and 2ω1=ω42^{\omega_1}=\omega_4, one has (ω2)ω=ω3=2ω(\omega_2)^\omega=\omega_3=2^\omega, so the stated cardinal hypotheses hold. But any ω\omega-almost-disjoint family of ω1\omega_1-subsets of ω2\omega_2 has size at most ∣[ω2]ω∣=ω3|[\omega_2]^\omega|=\omega_3, while ∣[ω2]ω1∣≥2ω1=ω4|[\omega_2]^{\omega_1}|\ge 2^{\omega_1}=\omega_4. Hence ω3\omega_3 such classes cannot cover [ω2]ω1[\omega_2]^{\omega_1}. This refutes the proposed extension of Theorem 2 under only the stated hypotheses.

    Novelty assessment

    TYPE1

    Classification rationale: The accepted resolution appears genuinely not explicitly published, but it is a very short consequence of a standard cardinal-arithmetic obstruction plus Easton’s theorem. The key necessary condition is that a cover by npn^p many pp-almost-disjoint classes can contain at most npn^p blocks, so if nm>npn^m>n^p the desired partition is impossible. Easton’s theorem supplies the required continuum-function model. This is not substantial enough for a standalone combinatorics paper.

    Literature check: I searched exact-title and keyword variants through DuckDuckGo/Bing, Springer/Semantic Scholar/OpenAlex metadata, arXiv, and MathOverflow. Exact-title searches found only the original Springer chapter/PDF, Semantic Scholar metadata, and book-library listings. Searches for the specific problem data and phrases such as p=ω,m=ω1,n=ω2p=\omega,m=\omega_1,n=\omega_2, np=ω3=2pn^p=\omega_3=2^p, “we do not suppose g.c.h.”, “omega_3”, “GCH”, and Bonnet–Erdős with “hypergraph” found no published resolution. OpenAlex lists the Bonnet–Erdős paper with only very low citation count and no visible evidence of a citing paper containing this counterexample.

    Citation: R. Bonnet and P. Erdős, “The chromatic index of an infinite complete hypergraph: A partition theorem,” in Hypergraph Seminar, Ohio State University 1972, Lecture Notes in Mathematics 411, Springer, 1974, pp. 54–60.

    W. B. Easton, “Powers of regular cardinals,” Annals of Mathematical Logic 1 (1970), 139–178.

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