Optimal open-locating-dominating sets in infinite triangular grids
  An open-locating-dominating set (OLD-set) is a subset of vertices of a graph such that every vertex in the graph has at least one neighbor in the set and no two vertices in the graph have the same set of neighbors in the set. This is an analogue to the well-studied identifying code in the literature. In this paper, we prove that the optimal density of the OLD-set for the infinite triangular grid is $4/13$. 
Rex Kincaid and Allison Oldham and Gexin Yu
math.CO