The crossing number inequality states that, for an undirectedsimple graph with vertices and edges such that, the crossing number obeys the inequality The constant is the best known to date, and is due to Ackerman. For earlier results with weaker constants see and. The constant can be lowered to, but at the expense of replacing with the worse constant of.
Applications
The motivation of Leighton in studying crossing numbers was for applications to VLSI design in theoretical computer science. Later, realized that this inequality yielded very simple proofs of some important theorems in incidence geometry. For instance, the Szemerédi–Trotter theorem, an upper bound on the number of incidences that are possible between given numbers of points and lines in the plane, follows by constructing a graph whose vertices are the points and whose edges are the segments of lines between incident points. If there were more incidences than the Szemerédi–Trotter bound, this graph would necessarily have more crossings than the total number of pairs of lines, an impossibility. The inequality can also be used to prove Beck's theorem, that if a finite point set does not have a linear number of collinear points, then it determines a quadratic number of distinct lines. Similarly, Tamal Dey used it to prove upper bounds on geometric k-sets.
Proof
We first give a preliminary estimate: for any graph with vertices and edges, we have To prove this, consider a diagram of which has exactly crossings. Each of these crossings can be removed by removing an edge from. Thus we can find a graph with at least edges and vertices with no crossings, and is thus a planar graph. But from Euler's formula we must then have, and the claim follows.. To obtain the actual crossing number inequality, we now use a probabilistic argument. We let be a probability parameter to be chosen later, and construct a random subgraph of by allowing each vertex of to lie in independently with probability, and allowing an edge of to lie in if and only if its two vertices were chosen to lie in. Let and denote the number of edges, vertices and crossings of, respectively. Since is a subgraph of, the diagram of contains a diagram of. By the preliminary crossing number inequality, we have Taking expectations we obtain Since each of the vertices in had a probability of being in, we have. Similarly, each of the edges in has a probability of remaining in since both endpoints need to stay in, therefore. Finally, every crossing in the diagram of has a probability of remaining in, since every crossing involves four vertices. To see this consider a diagram of with crossings. We may assume that any two edges in this diagram with a common vertex are disjoint, otherwise we could interchange the intersecting parts of the two edges and reduce the crossing number by one. Thus every crossing in this diagram involves four distinct vertices of. Therefore, and we have Now if we set , we obtain after some algebra A slight refinement of this argument allows one to replace by for.
Variations
For graphs with girth larger than and, demonstrated an improvement of this inequality to Adiprasito showed a generalization to higher dimensions: If is a simplicial complex that is mapped piecewise-linearly to, and it has -dimensional faces and -dimensional faces such that, then the number of pairwise intersecting -dimensional faces is at least
