Lower bounds on the maximum number of non-crossing acyclic graphs

This paper is a contribution to the problem of counting geometric graphs on point sets. More concretely, we look at the maximum numbers of non-crossing spanning trees and forests. We show that the so-called double chain point configuration of NN points has Ω(12.52N)Ω(12.52N) non-crossing spanning trees and Ω(13.61N)Ω(13.61N) non-crossing forests. This improves the previous lower bounds on the maximum number of non-crossing spanning trees and of non-crossing forests among all sets of NN points in general position given by Dumitrescu, Schulz, Sheffer and Tóth (2013). Our analysis relies on the tools of analytic combinatorics, which enable us to count certain families of forests on points in convex position, and to estimate their average number of components. A new upper bound of O(22.12N)O(22.12N) for the number of non-crossing spanning trees of the double chain is also obtained.