Edge bound for a connected finite planar drawing

Proved

WilliamsonShmoys.planarDrawing_edge_bound_connected__p2m_af29040b_5f5f_41c7_ae26_98a3e427e1bc_a1

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

approximation-algorithmsplanar-graphstreewidth

A connected finite simple graph with at least three vertices and a crossing-free drawing has at most 3n−6 edges. Prove this from the plane embedding’s Euler formula and the lower bound of three on every face boundary walk, accounting for the connected simple cases and bridges.

Preamble

import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn
import Mathlib

set_option autoImplicit false

open WilliamsonShmoys

Formal statement

theorem WilliamsonShmoys.planarDrawing_edge_bound_connected__p2m_af29040b_5f5f_41c7_ae26_98a3e427e1bc_a1
    {V : Type} [Fintype V] [LinearOrder V]
    (G : SimpleGraph V) (dG : DecidableRel G.Adj)
    (hG : HasPlanarDrawingOn G) (hConn : G.Connected)
    (hcard : 3 ≤ Fintype.card V) :
    (letI : DecidableRel G.Adj := dG; G.edgeFinset.card) + 6 ≤ 3 * Fintype.card V := 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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