Blockers for non-crossing spanning trees in complete geometric graphs
  In this paper we present a complete characterization of the smallest sets that block all the simple spanning trees (SSTs) in a complete geometric graph. We also show that if a subgraph is a blocker for all SSTs of diameter at most 4, then it must block all simple spanning subgraphs, and in particular, all SSTs. For convex geometric graphs, we obtain an even stronger result: being a blocker for all SSTs of diameter at most 3 is already sufficient for blocking all simple spanning subgraphs. 
Chaya Keller and Micha A. Perles and Eduardo Rivera-Campo and Virginia   Urrutia-Galicia
math.CO