The Crossing Number of the Complete Graph

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Recomm. for undergrads: no
Posted by: Robert Samal
on: May 11th, 2007

The crossing number $ cr(G) $ of $ G $ is the minimum number of crossings in all drawings of $ G $ in the plane.

Conjecture   $ \displaystyle cr(K_n) =   \frac 14 \floor{\frac n2} \floor{\frac{n-1}2} \floor{\frac{n-2}2} \floor{\frac{n-3}2} $

(This discussion appears as [M].)

A drawing of a graph $ G $ in the plane has the vertices represented by distinct points and the edges represented by polygonal lines joining their endpoints such that:

  • no edge contains a vertex other than its endpoints,
  • no two adjacent edges share a point other than their common endpoint,
  • two nonadjacent edges share at most one point at which they cross transversally, and
  • no three edges cross at the same point.

The conjectured value for the crossing number of $ K_n $ is known to be an upper bound. This is shown by exhibiting a drawing with that number of crossings. If $ n = 2m $, place $ m $ vertices regularly spaced along two circles of radii 1 and 2, respectively. Two vertices on the inner circle are connected by a straight line; two vertices on the outer circle are connected by a polygonal line outside the circle. A vertex on the inner circle is connected to one on the outer circle with a polygonal line segment of minimum possible positive winding angle around the cylinder. A simple count shows that the number of crossings in such a drawing achieves the conjectured minimum. For $ n = 2m-1 $ we delete one vertex from the drawing described and achieve the conjectured minimum.

The conjecture is known to be true for $ n $ at most 10 [G]. If the conjecture is true for $ n = 2m $, then it is also true for $ n-1 $. This follows from an argument counting the number of crossings in drawings of all $ K_{n-1} $'s contained in an optimal drawing of $ K_n $.

It would also be interesting to prove that the conjectured upper bound is asymptotically correct, that is, that $ \lim \frac{cr(K_n)}{\binom{n}4} = \frac38 $.

The best known lower bound is due to Kleitman [K], who showed that this limit is at least $ 3/10 $.


[G] R. Guy, The decline and fall of Zarankiewicz's theorem, in Proof Techniques in Graph Theory (F. Harary Ed.), Academic Press, New York (1969) 63-69.

[K] D. Kleitman, The crossing number of $ K_{5,n} $, J. Combin. Theory 9 (1970) 315-323.

[M] B. Mohar, Problem of the Month

* indicates original appearance(s) of problem.

On lower bound.

It has been shown that $ \lim_{n\rightarrow\infty}\frac{cr(K_n)}{Z(n)}\geq 0.83 $, where $ Z(n) $ is the conjectured value. For the proof, see de Klerk, E.; Maharry, J.; Pasechnik, D. V.; Richter, R. B.; Salazar, G. Improved bounds for the crossing numbers of $ K\sb {m,n} $ and $ K\sb n $. (2007).

This is not the best known bound.

The same article below, that proves that $ cr(K_{11})=100 $ and $ cr(K_{12})=150 $, states that $ 0.8594 \cdot Z(n)\leq cr(K_n) \leq Z(n) $.

true upto n=12

The conjecture was recently verified for n=11 and 12 (The Crossing Number of $ K_{11} $ Is 100 by Shengjun Pan and R. Bruce Richter).

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