On the chromatic number of disjointness graphs of curves

Abstract: Let $\omega(G)$ and $\chi(G)$ denote the clique number and chromatic number of a graph $G$, respectively. The {\em disjointness graph} of a family of curves (continuous arcs in the plane) is the graph whose vertices correspond to the curves and in which two vertices are joined by an edge if and only if the corresponding curves are disjoint. A curve is called {\em $x$-monotone} if every vertical line intersects it in at most one point. An $x$-monotone curve is {\em grounded} if its left endpoint lies on the $y$-axis. We prove that if $G$ is the disjointness graph of a family of grounded $x$-monotone curves such that $\omega(G)=k$, then $\chi(G)\leq \binom{k+1}{2}$. If we only require that every curve is $x$-monotone and intersects the $y$-axis, then we have $\chi(G)\leq \frac{k+1}{2}\binom{k+2}{3}$. Both of these bounds are best possible. The construction showing the tightness of the last result settles a 25 years old problem: it yields that there exist $K_k$-free disjointness graphs of $x$-monotone curves such that any proper coloring of them uses at least $\Omega(k^{4})$ colors. This matches the upper bound up to a constant factor.