Finding a Maximum Clique in a Disk Graph

A disk graph is an intersection graph of disks in the Euclidean plane, where the disks correspond to the vertices of the graph and a pair of vertices are adjacent if and only if their corresponding disks intersect. The problem of determining the time complexity of computing a maximum clique in a disk graph is a long-standing open question. The problem is known to be open even when the radii of all the disks are in the interval $[1,(1+\varepsilon)]$, where $\varepsilon>0$. However, the maximum clique problem is known to be APX-hard for the intersection graphs of many other convex objects such as intersection graphs of ellipses, triangles, and a combination of unit disks and axis-parallel rectangles. Furthermore, there exists an $O(n^3\log n)$-time algorithm to compute a maximum clique for unit disks. Here we obtain the following results. - We give an algorithm to compute a maximum clique in a unit disk graph in $O(n^{2.5}\log n)$-time, which improves the previously best known running time of $O(n^3\log n)$ [Eppstein '09]. - We extend a widely used `co-2-subdivision approach' to prove that computing a maximum clique in a combination of unit disks and axis-parallel rectangles is NP-hard to approximate within $4448/4449 \approx 0.9997$. The use of a `co-2-subdivision approach' was previously thought to be unlikely in this setting [Bonnet et al. '20]. Our result improves the previously known inapproximability factor of $7633010347/7633010348\approx 0.9999$. - We show that the parameter minimum lens width of the disk arrangement may be used to make progress in the case when disk radii are in $[1,(1+\varepsilon)]$. For example, if the minimum lens width is at least $0.265$ and $\varepsilon\le 0.0001$, which still allows for non-Helly triples in the arrangement, then one can find a maximum clique in polynomial time.