An undirected graph G = (V, E) consists of a set of vertices V and a set of edges E consisting of unordered pairs of vertices. In a directed graph, E is considered as a set of ordered vertex pairs. Below, it will be convenient for simplicity of the presentation to focus on undirected graphs. Undirected graphs are equivalent to symmetric directed graphs, i.e., xy ∈ E(G) ⇐⇒ yx ∈ E(G), where xy and yx denotes directed edges between the vertices x, y ∈ V . Where necessary, we will write V (G) and E(G) to refer to the vertex and edge sets of G. We define the edge neighborhood of a vertex