Finite maximal drawable supergraph

Proved

WilliamsonShmoys.planar_extension_maximal_supergraph__p2m_06bdf4fe_cd1d_4093_b45f_6118505c9d7e_a1

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

approximation-algorithmsplanar-graphstreewidth

Every connected drawable graph in the finite simple-graph universe has a drawable supergraph on the same vertices that is maximal under inclusion. It remains connected, and its edge count is at least the original graph’s.

Preamble

import Mathlib
import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn

set_option autoImplicit false

Formal statement

theorem WilliamsonShmoys.planar_extension_maximal_supergraph__p2m_06bdf4fe_cd1d_4093_b45f_6118505c9d7e_a1
    {V : Type} [Fintype V] [LinearOrder V]
    (K : SimpleGraph V)
    (hcard : 3 ≤ Fintype.card V)
    (hKconn : K.Connected)
    (hKdraw : WilliamsonShmoys.HasPlanarDrawingOn K) :
    ∃ H : SimpleGraph V, K ≤ H ∧
      (letI : DecidableRel K.Adj := Classical.decRel K.Adj
       letI : DecidableRel H.Adj := Classical.decRel H.Adj
       K.edgeFinset.card ≤ H.edgeFinset.card) ∧
      H.Connected ∧ WilliamsonShmoys.HasPlanarDrawingOn H ∧
      (∀ J : SimpleGraph V, H ≤ J → WilliamsonShmoys.HasPlanarDrawingOn J → J ≤ H) := 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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