Clique-Based Separators for Geometric Intersection Graphs.

Let F be a set of n objects in the plane and let G ^×(F) be its intersection graph. A balanced clique-based separator of G ^×(F) is a set S consisting of cliques whose removal partitions G ^×(F) into components of size at most δn, for some fixed constant δ < 1. The weight of a clique-based separator is defined as ^P _C∈S log(|C| + 1). Recently De Berg et al. (SICOMP 2020) proved that if S consists of convex fat objects, then G ^×(F) admits a balanced clique-based separator of weight O(√n). We extend this result in several directions, obtaining the following results. Map graphs admit a balanced clique-based separator of weight O(√n), which is tight in the worst case. Intersection graphs of pseudo-disks admit a balanced clique-based separator of weight O(n ^2/3 log n). If the pseudo-disks are polygonal and of total complexity O(n) then the weight of the separator improves to O(√n log n). Intersection graphs of geodesic disks inside a simple polygon admit a balanced clique-based separator of weight O(n ^2/3 log n). Visibility-restricted unit-disk graphs in a polygonal domain with r reflex vertices admit a balanced clique-based separator of weight O(√n + r log(n/r)), which is tight in the worst case. These results immediately imply sub-exponential algorithms for Maximum Independent Set (and, hence, Vertex Cover), for Feedback Vertex Set, and for q-Coloring for constant q in these graph classes.