Triangulated planar supergraph with the maximal edge count

Proved

WilliamsonShmoys.planar_drawing_triangulation_edge_completion__p2m_f0aba619_3680_404a_84c1_4b765821e2bd_a2

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

approximation-algorithmsplanar-graphstreewidth

Every drawable simple graph on at least three finite ordered vertices has a drawable supergraph on the same vertices whose edge count is exactly 3|V|−6, and the original graph’s edge count is no larger.

Preamble

import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn
import Mathlib

set_option autoImplicit false

open WilliamsonShmoys

Formal statement

theorem WilliamsonShmoys.planar_drawing_triangulation_edge_completion__p2m_f0aba619_3680_404a_84c1_4b765821e2bd_a2
    {V : Type} [Fintype V] [LinearOrder V]
    (G : SimpleGraph V) (hcard : 3 ≤ Fintype.card V)
    (hG : HasPlanarDrawingOn G) :
    ∃ H : SimpleGraph V,
      G ≤ H ∧ HasPlanarDrawingOn H ∧
      G.edgeSet.ncard ≤ H.edgeSet.ncard ∧
      H.edgeSet.ncard = 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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