Edge count of a maximal planar graph

Proved

WilliamsonShmoys.maximal_planar_exact_edge_count__p2m_e74de359_8181_429e_92f3_9fe027354fb5_a1

by Eyal1990 · Oct 30, 2026 · Mathlib 0df444a (Lean v4.33.1)

approximation-algorithmsplanar-graphstreewidth

An inclusion-maximal drawable simple graph on at least three vertices has exactly 3|V| − 6 edges.

Preamble

import Mathlib
import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn

set_option autoImplicit false

open WilliamsonShmoys

Formal statement

theorem WilliamsonShmoys.maximal_planar_exact_edge_count__p2m_e74de359_8181_429e_92f3_9fe027354fb5_a1
    {V : Type} [Fintype V] [LinearOrder V]
    (K : SimpleGraph V) (hK : HasPlanarDrawingOn K)
    (hcard : 3 <= Fintype.card V)
    (hmax : forall L : SimpleGraph V, K <= L -> HasPlanarDrawingOn L -> L = K) :
    letI : DecidableRel K.Adj := Classical.decRel _
    K.edgeFinset.card = 3 * Fintype.card V - 6 := by sorry

Source

David P. Williamson and David B. Shmoys, The Design of Approximation Algorithms, Cambridge University Press, 2011, author electronic manuscript, Section 10.2, proof of Theorem 10.11, pp. 270-272. https://doi.org/10.1017/CBO9780511921735. Bounded tree-width of the pieces: Robertson–Seymour (planar graphs of radius r have tree-width at most 3r+1) / Bodlaender (k-outerplanar graphs have tree-width at most 3k-1).

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