Maximal planar graphs have the triangulation edge count
ProvedWilliamsonShmoys.maximal_planar_edge_count__p2m_9bfbc735_ec6d_4d27_8e0c_d58af3810de3_a1approximation-algorithmsplanar-graphstreewidth
An inclusion-maximal drawable simple graph on at least three vertices has exactly 3|V|−6 edges.
Preamble
import Definitions.Def_WilliamsonShmoys_PlanarDrawingOn set_option autoImplicit false open scoped BigOperators
Formal statement
theorem WilliamsonShmoys.maximal_planar_edge_count__p2m_9bfbc735_ec6d_4d27_8e0c_d58af3810de3_a1
{V : Type} [Fintype V] [LinearOrder V]
(K : SimpleGraph V) (hcard : 3 ≤ Fintype.card V)
(hK : WilliamsonShmoys.HasPlanarDrawingOn K)
(hmax : ∀ L : SimpleGraph V, K ≤ L →
WilliamsonShmoys.HasPlanarDrawingOn L → L ≤ K) :
3 * Fintype.card V - 6 = K.edgeSet.ncard := by sorrySource
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).