In the mathematics of graph drawing, the crossing number inequality or crossing lemma gives a lower bound on the minimum number of edge crossings in a plane drawing of a given graph, as a function of the number of edges and vertices of the graph. It states that, for graphs where the number  of edges is sufficiently larger than the number  of vertices, the crossing number is at least proportional to .

It has applications in VLSI design and combinatorial geometry,
and was discovered independently by Ajtai, Chvátal, Newborn, and Szemerédi
and by Leighton.

Statement and history
The crossing number inequality states that, for an undirected simple 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 .

It is important to distinguish between the crossing number  and the pairwise crossing number . As noted by , the pairwise crossing number  refers to the minimum number of pairs of edges that each determine one crossing, whereas the crossing number simply refers to the minimum number of crossings. (This distinction is necessary since some authors assume that in a proper drawing no two edges cross more than once.)

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. (In fact we have  for ).

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  (since we assumed that ), 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

References

Topological graph theory
Inequalities
Articles containing proofs
Graph drawing