A covering of a topological space is a continuous map with special properties. Definition Let be a topological space. A covering of is a continuous map such that there exists a discrete space and for every an open neighborhood , such that and is a homeomorphism for every . Often, the notion of a covering is used for the covering space as well as for the map . The open sets are called sheets, which are uniquely determined up to a homeomorphism if is connected. For each the discrete subset is called the fiber of . The degree of a covering is the cardinality of the space . If is path-connected, then the covering is denoted as a path-connected covering. Examples For every topological space there exists the covering with , which is denoted as the trivial covering of The map with is a covering of the unit circle . The base of the covering is and the covering space is . For any point such that , the set is an open neighborhood of . The preimage of under is and the sheets of the covering are for The fiber of is Another covering of the unit circle is the map with for some For an open neighborhood of an , one has: . A map which is a local homeomorphism but not a covering of the unit circle is with . There is a sheet of an open neighborhood of , which is not mapped homeomorphically onto . Properties Local homeomorphism Since a covering maps each of the disjoint open sets of homeomorphically onto it is a local homeomorphism, i.e. is a continuous map and for every there exists an open neighborhood of , such that is a homeomorphism. It follows that the covering space and the base space locally share the same properties. If is a connected and non-orientable manifold, then there is a covering of degree , whereby is a connected and orientable manifold. If is a connected Lie group, then there is a covering which is also a Lie group homomorphism and is a Lie group. If is a graph, then it follows for a covering that is also a graph. If is a connected manifold, then there is a covering , whereby is a connected and simply connected manifold. If is a connected Riemann surface, then there is a covering which is also a