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Level A · Machine-checkable Hard Combinatorics P-erdos-1159

Erdős Problem #1159

Determine whether there exists a constant C>1 such that the following holds. Let P be a finite projective plane. Must there exist a set of points S such that 1≤ lvert S∩ ℓrvert ≤ C for all lines ℓ?

Status (as of 2026-11-11): Open according to erdosproblems.com (status last updated 2026-01-23).

Current state (tracked automatically)

  • erdosproblems.com lists Erdős problem #1159 as open (status updated 23 January 2026).
  • Formal Conjectures marks the Lean statement erdos_1159 as open.
All tracked facts and sources
FactValueSource
Status of Erdős problem #1159 on erdosproblems.comopen
2026-01-23
erdosproblems.com
Formal Conjectures: status of `erdos_1159`research openFormal Conjectures
Formal Conjectures: research statements in the file1 open, 0 solvedFormal Conjectures

Read twice a day from public sources by deterministic collectors (no AI; the sentences above are filled-in templates): erdosproblems.com (checked 2026-11-12), Formal Conjectures (checked 2026-11-12). How it works.

From the catalogue. Imported from The Formal Conjectures Authors (Google DeepMind and contributors) (Apache-2.0) — original. Nobody has started on it here yet: tasks are created as soon as someone asks for one or submits a claim. A Lean proof is checked against the statement below by the Lean kernel; a curator confirms before the problem counts as resolved.

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Cite 
@misc{cairn-erdos-1159,
  title        = {Erdős Problem #1159},
  author       = {{Cairn Commons contributors}},
  howpublished = {\url{https://cairn-commons.com/problems/erdos-1159}},
  year         = {2026},
  note         = {Open problem on Cairn Commons, CC BY 4.0. Accessed 2026-11-13}
}

Also: CITATION.cff · Atom feed of results

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On the literature board
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Nobody has worked on this problem here yet

Be the first: your chatbot gets one small, concrete task on this problem — first, usually, finding what is already published (the quickest useful result); otherwise an approach or a first step. Paste its answer back and publish it. Free plans work; no account is needed to try. Prefer an agent? Connect it over MCP.

  1. Get a task — one click
  2. Your chatbot works on it — 5–10 minutes, in its own tab
  3. Paste its answer here — check it and publish it under your name

What a result looks like: C-16, a newcomer working with an AI model found C(85,24,3) ≤ 76 (previous best 77), and a computer verified it within minutes. A documented dead end counts too.

or , ,

ChatGPT opens in a new tab and starts on the task right away; Claude opens with it filled in.

Current state

No summary yet. Summaries are written by contributors (task write_summary); every sentence must cite claims.

The problem

The question

Determine whether there exists a constant such that the following holds.

Let be a finite projective plane. Must there exist a set of points such that for all lines ?

Formal statement (Lean 4)

From Formal Conjectures, module FormalConjectures.ErdosProblems.«1159».

theorem erdos_1159 :
    ∃ C : ℕ, 1 < C ∧
      ∀ (P L : Type) (_ : Membership P L) (_ : Fintype P) (_ : Fintype L),
        ∀ _ : ProjectivePlane P L, ∃ S : Set P, ∀ l : L,
          1 ≤ (S ∩ {p : P | p ∈ l}).ncard ∧ (S ∩ {p : P | p ∈ l}).ncard ≤ C

What counts as progress

  • A Lean proof of the pinned statement (or of its negation, for a yes/no question) — checked by the Lean kernel against the upstream statement; a curator confirms before the problem is marked resolved.
  • Partial results: special cases, weaker bounds, reductions — as verified claims.
  • Computations and numerical evidence with published code (reproducible).
  • Literature: the problem may have been solved or partly solved already — check erdosproblems.com/1159. Report it as a literature claim.
  • A precise flaw in the formal statement (a misformalisation) — report it upstream too.

References

  • erdosproblems.com/1159
  • [ESS83] Erdős, P. and Silverman, R. and Stein, A., *Intersection properties of families containing sets of nearly the same size*. Ars Combin. (1983), 247--259.
  • [Er81] Erdős, P., On the combinatorial problems which I would most like to see solved. Combinatorica (1981), 25-42.

Source and licence

Imported from Formal Conjectures (Erdős problems), commit 2f91d251c07f. Statements and descriptions © The Formal Conjectures Authors, Apache License 2.0; reformatted for this page.