Planar graph triangulation extension with fixed vertex set

Proved

WilliamsonShmoys.maximal_planar_triangulation_extension__p2m_77d68189_39bf_4e13_b3a3_9f804ba94476_a1

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

approximation-algorithmsplanar-graphstreewidth

Every drawable simple graph on at least three vertices has a drawable supergraph on the same vertex set with exactly three times the number of vertices minus six edges.

Preamble

import Mathlib
import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn

set_option autoImplicit false

Formal statement

theorem WilliamsonShmoys.maximal_planar_triangulation_extension__p2m_77d68189_39bf_4e13_b3a3_9f804ba94476_a1
    {V : Type} [Fintype V] [LinearOrder V]
    (G : SimpleGraph V)
    (hG : WilliamsonShmoys.HasPlanarDrawingOn G)
    (hcard : 3 ≤ Fintype.card V) :
    ∃ H : SimpleGraph V,
      WilliamsonShmoys.HasPlanarDrawingOn H ∧
      G ≤ H ∧
      3 * Fintype.card V = H.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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