Covering graph

In the mathematical discipline of graph theory, a graph C is a covering graph of another graph G if there is a covering map from the vertex set of C to the vertex set of G. A covering map f is a surjection and a local isomorphism: the neighbourhood of a vertex v in C is mapped bijectively onto the neighbourhood of f(v) in G. In the mathematical discipline of graph theory, a graph C is a covering graph of another graph G if there is a covering map from the vertex set of C to the vertex set of G. A covering map f is a surjection and a local isomorphism: the neighbourhood of a vertex v in C is mapped bijectively onto the neighbourhood of f(v) in G. The term lift is often used as a synonym for a covering graph of a connected graph. In graph theory, a covering graph may also refer to a subgraph that contains either all edges (edge cover) or all vertexes (vertex cover). The combinatorial formulation of covering graphs is immediately generalized to the case of multigraphs. If we identify a multigraph with a 1-dimensional cell complex, a covering graph is nothing but a special example of covering spaces of topological spaces, so that the terminology in the theory of covering spaces is available; say covering transformation group, universal covering, abelian covering, and maximal abelian covering. Let G = (V1, E1) and C = (V2, E2) be two graphs, and let f: V2 → V1 be a surjection. Then f is a covering map from C to G if for each v ∈ V2, the restriction of f to the neighbourhood of v is a bijection onto the neighbourhood of f(v) in G. Put otherwise, f maps edges incident to v one-to-one onto edges incident to f(v). If there exists a covering map from C to G, then C is a covering graph, or a lift, of G. An h-lift is a lift such that the covering map f has the property that for every vertex v of G, its fiber f−1(v) has exactly h elements. In the following figure, the graph C is a covering graph of the graph H. The covering map f from C to H is indicated with the colours. For example, both blue vertices of C are mapped to the blue vertex of H. The map f is a surjection: each vertex of H has a preimage in C. Furthermore, f maps bijectively each neighbourhood of a vertex v in C onto the neighbourhood of the vertex f(v) in H. For example, let v be one of the purple vertices in C; it has two neighbours in C, a green vertex u and a blue vertex t. Similarly, let v′ be the purple vertex in H; it has two neighbours in H, the green vertex u′ and the blue vertex t′. The mapping f restricted to {t, u, v} is a bijection onto {t′, u′, v′}. This is illustrated in the following figure: