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Sanity check: f(0,1)=1f(0,1) = 1f(0,1)=1

Proved
Erdos20.f_0_1

by Lucas · Sep 25, 2026 · Mathlib 0df444a (Lean v4.33.1)

combinatoricserdos-problemssunflower

For the sunflower threshold f(n,k)f(n,k)f(n,k) (the least mmm such that every nnn-uniform family with at least mmm members contains a kkk-sunflower),

f(0,1)=1.f(0,1) = 1.f(0,1)=1.

This is a test case from the source formalization: one member always forms a 111-sunflower, while the empty family has none.

Preamble
import Definitions.Def_Erdos20_defs
import Mathlib
Formal statement
namespace Erdos20
theorem f_0_1 : f 0 1 = 1 := by sorry
end Erdos20
Source
Formal Conjectures, `FormalConjectures/ErdosProblems/20.lean` (Erdős Problem 20), https://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/20.lean ; https://www.erdosproblems.com/20 (theorem `f_0_1`)
Read-back

What the Lean code literally says, in plain math · Aristotle (Harmonic) — same agent that drafted the statements; non-blind

Non-blind read-back — not independent testimony. This read-back was written by the same agent that drafted the Lean statements of this proposal (not by a blind auditor with a fresh context), and that agent had seen the source material and knew the intended meaning while writing it. Reviewers should not treat it as independent evidence of faithfulness; compare the Lean code against the source directly.

The statement asserts the equality f(0,1)=1f(0,1) = 1f(0,1)=1 of natural numbers, with no hypotheses. Here f(n,k)f(n,k)f(n,k) is the sunflower threshold: the least m∈Nm\in\mathbb Nm∈N (with inf⁡∅:=0\inf\emptyset := 0inf∅:=0) such that for every type α\alphaα and every family F\mathcal FF of subsets of α\alphaα all of whose members have ncard⁡=n\operatorname{ncard} = nncard=n and with m≤ncard⁡(F)m \le \operatorname{ncard}(\mathcal F)m≤ncard(F), some subfamily S⊆F\mathcal S\subseteq\mathcal FS⊆F with ncard⁡(S)=k\operatorname{ncard}(\mathcal S)=kncard(S)=k has all pairwise intersections of distinct members equal to one common set (ncard⁡\operatorname{ncard}ncard counts elements of finite sets and is 000 on infinite sets). With n=0n=0n=0 and k=1k=1k=1 it therefore says: the least mmm such that every family of sets of ncard⁡\operatorname{ncard}ncard 000 (i.e. empty sets or infinite sets) having ncard⁡\operatorname{ncard}ncard at least mmm contains a one-member subfamily (a one-member family is automatically a sunflower), equals 111.

Human review
  • Endorsed by Shuze Chen · Sep 26, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Lucas · Sep 26, 2026

    Confirmed by the mission captain (proposal self-audit).

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