 Typical choices of the groups used for voltage graphs include the two-element group ℤ2 (for defining the bipartite double cover of a graph), free groups (for defining the universal cover of a graph), "d"-dimensional integer lattices ℤ"d" (viewed as a group under vector addition, for defining periodic structures in "d"-dimensional Euclidean space), and finite cyclic groups ℤ"n" for "n" > 2.When Π is a cyclic group, the voltage graph may be called a "cyclic-voltage graph".Formal definition of a Π-voltage graph, for a given group Π