Planar triangulation extension on a fixed vertex set

Proved

WilliamsonShmoys.planar_triangulation_extension__p2m_fc022252_a12f_4a07_aef3_b2bbba00d482_a1

by Eyal1990 · Oct 30, 2026 · Mathlib 0df444a (Lean v4.33.1)

approximation-algorithmsplanar-graphstreewidth

Any planar simple graph on at least three vertices is a subgraph of a planar triangulation on the same vertex set, whose edge count is 3|V|−6.

Preamble

import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn
import Mathlib

open WilliamsonShmoys

Formal statement

theorem WilliamsonShmoys.planar_triangulation_extension__p2m_fc022252_a12f_4a07_aef3_b2bbba00d482_a1
    {V : Type} [Fintype V] [LinearOrder V]
    (K : SimpleGraph V)
    (hK : HasPlanarDrawingOn K)
    (hcard : 3 <= Fintype.card V) :
    exists L : SimpleGraph V,
      K <= L /\ HasPlanarDrawingOn L /\
        (letI : DecidableRel L.Adj := Classical.decRel L.Adj;
         L.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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