Identifying codes in line graphs

An identifying code of a graph is a subset of its vertices such that every vertex of the graph is uniquely identified by the set of its neighbours within the code. We study the edge-identifying code problem, i.e. the identifying code problem in line graphs. If γID(G) denotes the size of a minimum identifying code of an identifiable graph G, we show that the usual bound γID(G) ≥ ⌈log2(n+1)⌉, where n denotes the order of G, can be improved to Θ( √ n) in the class of line graphs. Moreover, this bound is tight. We also prove that the upper bound γID(L(G)) ≤ 2|V (G)|−5, where L(G) is the line graph of G, holds (with two exceptions). This implies that a conjecture of R. Klasing, A. Kosowski, A. Raspaud and the first author holds for a subclass of line graphs. Finally, we show that the edge-identifying code problem is NP-complete, even for the class of planar bipartite graphs of maximum degree 3 and arbitrarily large girth.