Structure of a least-order planar counterexample
ProvedOPG169.minimal_counterexample_structureLet be a least-order counterexample to acyclic two-colorability among all finite planar orientations. Then is nonempty and strongly connected, and every vertex of its underlying graph has at least three neighbors:
The theorem is conditional: it does not assert that a counterexample exists, and minimality is by vertex count over the full planar class.
import Definitions.Def_opg169_planar_dichromatic
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 OPG169Read-back
What the Lean code literally says, in plain math · gpt-5.6-luna
Let be a type in a fixed universe with a finite-type structure, let be a simple graph on , and let be a binary relation on . Assume that there is an injective map 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 has no loops, every arc of follows an edge of , and every edge of is oriented in exactly one direction by . Assume further that there is no coloring for which, for each of the two colors , no list of length at least 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 . Finally, assume that for every finite type in the same universe, every simple graph on , and every relation on , if has an embedding satisfying the same planarity conditions, orients every edge of in exactly one direction without loops, and , then such a coloring exists for . Under these assumptions, is nonempty, every ordered pair of vertices is connected by a directed walk from to of zero or more arcs, and every vertex of has at least three neighbors.
Confirmed by the mission captain (proposal self-audit).