English

Drawings of complete graphs in the projective plane

Combinatorics 2021-03-22 v3

Abstract

Hill's Conjecture states that the crossing number cr(Kn)\text{cr}(K_n) of the complete graph KnK_n in the plane (equivalently, the sphere) is 14n2n12n22n32=n4/64+O(n3)\frac{1}{4}\lfloor\frac{n}{2}\rfloor\lfloor\frac{n-1}{2}\rfloor\lfloor\frac{n-2}{2}\rfloor\lfloor\frac{n-3}{2}\rfloor=n^4/64 + O(n^3). Moon proved that the expected number of crossings in a spherical drawing in which the points are randomly distributed and joined by geodesics is precisely n4/64+O(n3)n^4/64+O(n^3), thus matching asymptotically the conjectured value of cr(Kn)\text{cr}(K_n). Let crP(G)\text{cr}_P(G) denote the crossing number of a graph GG in the projective plane. Recently, Elkies proved that the expected number of crossings in a naturally defined random projective plane drawing of KnK_n is (n4/8π2)+O(n3)(n^4/8\pi^2)+O(n^3). In analogy with the relation of Moon's result to Hill's conjecture, Elkies asked if limncrP(Kn)/n4=1/8π2\lim_{n\to\infty} \text{cr}_P(K_n)/n^4=1/8\pi^2. We construct drawings of KnK_n in the projective plane that disprove this.

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Cite

@article{arxiv.2002.02287,
  title  = {Drawings of complete graphs in the projective plane},
  author = {Alan Arroyo and Dan McQuillan and R. Bruce Richter and Gelasio Salazar and Matthew Sullivan},
  journal= {arXiv preprint arXiv:2002.02287},
  year   = {2021}
}