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Structure of a least-order planar counterexample

Proved
OPG169.minimal_counterexample_structure

by hao jia · Sep 8, 2026 · Mathlib 0df444a (Lean v4.33.1)

combinatoricsdirected-graphsgraph-theoryopen-problemplanar-graphs

Let DDD be a least-order counterexample to acyclic two-colorability among all finite planar orientations. Then DDD is nonempty and strongly connected, and every vertex of its underlying graph has at least three neighbors:

D least-order counterexample⟹D strongly connected and δ(U(D))≥3.D\text{ least-order counterexample} \quad\Longrightarrow\quad D\text{ strongly connected and }\delta(U(D))\ge3.D least-order counterexample⟹D strongly connected and δ(U(D))≥3.

The theorem is conditional: it does not assert that a counterexample exists, and minimality is by vertex count over the full planar class.

Preamble
import Definitions.Def_opg169_planar_dichromatic
Formal statement
namespace OPG169

universe u

/-- Every least-order counterexample is nonempty, strongly connected, and has
underlying minimum degree at least three. -/
theorem minimal_counterexample_structure
    {V : Type u} [Fintype V] (G : SimpleGraph V) (D : Digraph V)
    (hmin : IsLeastOrderCounterexample G D) :
    IsStronglyConnected D ∧ HasMinimumDegreeThree G := by sorry

end OPG169
Source
VibeMathing candidate_only proof draft at commit 16b9fbcf379719f4fad59a364151ff36f1bbb772, research/artifacts/candidates/opg169-a01-target-closure-20260907.md; SCC background compared with Mohar, Section 2
Read-back

What the Lean code literally says, in plain math · gpt-5.6-luna

Let VVV be a type in a fixed universe with a finite-type structure, let GGG be a simple graph on VVV, and let DDD be a binary relation on VVV. Assume that there is an injective map p:V→R2p:V\to\mathbb R^2p:V→R2 such that vertices not incident with an edge do not lie on that edge’s closed straight segment, and such that the closed segments representing any two edges with distinct endpoints are disjoint; assume also that DDD has no loops, every arc of DDD follows an edge of GGG, and every edge of GGG is oriented in exactly one direction by DDD. Assume further that there is no coloring c:V→Fin 2c:V\to\mathrm{Fin}\,2c:V→Fin2 for which, for each of the two colors kkk, no list of length at least 333 with distinct vertices, directed arcs between consecutive entries, and a directed arc from its last entry back to its first consists entirely of vertices colored kkk. Finally, assume that for every finite type WWW in the same universe, every simple graph HHH on WWW, and every relation EEE on WWW, if HHH has an embedding satisfying the same planarity conditions, EEE orients every edge of HHH in exactly one direction without loops, and ∣W∣<∣V∣|W|<|V|∣W∣<∣V∣, then such a coloring exists for EEE. Under these assumptions, VVV is nonempty, every ordered pair of vertices u,v∈Vu,v\in Vu,v∈V is connected by a directed walk from uuu to vvv of zero or more arcs, and every vertex of GGG has at least three neighbors.

Human review
  • Endorsed by Shuze Chen · Sep 8, 2026

  • Endorsed by hao jia · Sep 8, 2026

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

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