**Problem**Does the following equality hold for every graph ?

The crossing number of a graph is the minimum number of edge crossings in any drawing of in the plane. In the *pairwise crossing number* , we minimize the number of pairs of edges that cross.

Obviously we have .

The problem was first posed by Pach and Tóth in~[PT], who first spotted the possibility that the pairwise crossing number might be different from the crossing number. They proved for graphs with pairwise crossing number , which was later improved by Valtr~[V05] to and by Tóth~[T08] to .

## Bibliography

*[PT] János Pach, Géza Tóth, Which crossing number is it anyway?, Journal of Combinatorial Theory Series B 80 (2000), no. 2, 225--246. MathSciNet

[V05] Pavel Valtr, On the pair-crossing number, Combinatorial and computational geometry, 52 (2005), 569--575. MathSciNet

[T08] Géza Tóth, Note on the pair-crossing number and the odd-crossing number, Discrete Comput. Geom., 39 (2008), no. 4, 791--799. MathSciNet

* indicates original appearance(s) of problem.