On plane subgraphs of complete topological drawings

Topological drawings are representations of graphs in the plane, where vertices are represented by points, and edges by simple curves connecting the points. A drawing is simple if two edges intersect at most in a single point, either at a common endpoint or at a proper crossing. In this paper we study properties of maximal plane subgraphs of simple drawings D-n of the complete graph K-n on n vertices. Our main structural result is that maximal plane subgraphs are 2-connected and what we call essentially 3-edge-connected. Besides, any maximal plane subgraph contains at least inverted right perpendicular3n/2inverted left perpendicular edges. We also address the problem of obtaining a plane subgraph of D-n with the maximum number of edges, proving that this problem is NP-complete. However, given a plane spanning connected subgraph of D-n, a maximum plane augmentation of this subgraph can be found in O(n(3)) time. As a side result, we also show that the problem of finding a largest compatible plane straight-line graph of two labeled point sets is NP-complete.