Triangulation extension of a planar drawing

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WilliamsonShmoys.planar_drawing_triangulation_extension__p2m_bb67bec1_b416_47c1_ab52_e6dc48a65437_a1

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

approximation-algorithmsplanar-graphstreewidth

Every drawable simple graph on a finite linearly ordered vertex set with at least three vertices is a subgraph of a drawable graph on the same vertex set having exactly 3|V|−6 edges.

Preamble

import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn
import Mathlib

open WilliamsonShmoys

set_option autoImplicit false

Formal statement

theorem WilliamsonShmoys.planar_drawing_triangulation_extension__p2m_bb67bec1_b416_47c1_ab52_e6dc48a65437_a1
    {V : Type} [Fintype V] [LinearOrder V]
    (G : SimpleGraph V) (hV : 3 ≤ Fintype.card V)
    (hG : WilliamsonShmoys.HasPlanarDrawingOn G) :
    ∃ N : SimpleGraph V, WilliamsonShmoys.HasPlanarDrawingOn N ∧
      (∀ u v, G.Adj u v → N.Adj u v) ∧
      letI : DecidableRel N.Adj := Classical.decRel N.Adj
      3 * Fintype.card V = N.edgeFinset.card + 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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