Edge lower bound for a maximal drawable graph

Proved

WilliamsonShmoys.maximal_planar_edge_count_lower_bound__p2m_8b804cdd_68ca_48da_bde7_dd4a5c81f51a_a2

by Eyal1990 · Nov 1, 2026 · Mathlib 0df444a (Lean v4.33.1)

approximation-algorithmsplanar-graphstreewidth

An inclusion-maximal drawable simple graph with at least three vertices has at least 3|V| - 6 edges. Maximality rules out disconnected components and nontriangular faces: a missing edge or diagonal could otherwise be added while retaining a planar drawing. Euler's formula and edge-face incidence then give the lower bound.

Preamble

import Mathlib
import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn

set_option autoImplicit false

Formal statement

theorem WilliamsonShmoys.maximal_planar_edge_count_lower_bound__p2m_8b804cdd_68ca_48da_bde7_dd4a5c81f51a_a2
    {V : Type} [Fintype V] [LinearOrder V]
    (N : SimpleGraph V) (hplanar : WilliamsonShmoys.HasPlanarDrawingOn N)
    (hmax : forall K : SimpleGraph V,
      WilliamsonShmoys.HasPlanarDrawingOn K -> N <= K -> K = N)
    (hcard : 3 <= Fintype.card V) :
    3 * Fintype.card V <= N.edgeSet.ncard + 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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