**Conjecture**Every string graph with edges has a separator of size .

Consider a graph with vertex weights . A separator in is a set of vertices such that there is a partition of the vertices with no edge going between and and with . The size of the separator is the number of vertices of .

The result holds for planar graphs (which form a subclass of string graphs) by the Lipton-Tarjan theorem~[LT]. The best known upper bound for string graphs is currently from~[FP].

Update: Solved by James Lee [Lee].

## Bibliography

*[FPT] Jacob Fox, János Pach, Csaba D. Tóth: Intersection patterns of curves, J. London Math. Soc., to appear

[FP] Jacob Fox, János Pach A separator theorem for string graphs and its applications, Combinatorics, Probability and Computing, to appear.

[LT] Richard J. Lipton, Robert Endre Tarjan: A separator theorem for planar graphs, SIAM Journal on Applied Mathematics 36 (1979), 177-–189.

[Lee] James R. Lee. Separators in region intersection graphs, 2016.

* indicates original appearance(s) of problem.