Planar simple graph edge upper bound

Proved

WilliamsonShmoys.maximal_planar_edge_count_upper__p2m_d4d2ff2f_8400_446a_8d4e_186156e78298_a2

by Eyal1990 · Oct 31, 2026 · Mathlib 0df444a (Lean v4.33.1)

approximation-algorithmsplanar-graphstreewidth

Every finite simple graph with a crossing-free drawing and at least three vertices has at most 3|V|−6 edges. This removes the general planar edge-bound argument from the parent. A proof route uses Euler’s formula and face-boundary counting, with the disconnected case handled as well.

Preamble

import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn
import Mathlib

open WilliamsonShmoys

Formal statement

theorem WilliamsonShmoys.maximal_planar_edge_count_upper__p2m_d4d2ff2f_8400_446a_8d4e_186156e78298_a2
    {V : Type} [Fintype V] [LinearOrder V]
    (K : SimpleGraph V)
    (hK : HasPlanarDrawingOn K)
    (hcard : 3 ≤ Fintype.card V) :
    (letI : DecidableRel K.Adj := Classical.decRel K.Adj;
      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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