 The existence of a planar cover is a minor-closed graph property, and so can be characterized by finitely many forbidden minors, but the exact set of forbidden minors is not known.For the same reason, there exists a polynomial time algorithm for testing whether a given graph has a planar cover, but an explicit description of this algorithm is not known.A "covering map" from one graph "C" to another graph "H" may be described by a function "f" from the vertices of "C" onto the vertices of "H" that, for each vertex "v" of "C", gives a bijection between the neighbors of "v" and the neighbors of "f"("v").