# Local homeomorphism

In mathematics, more specifically topology, a local homeomorphism is intuitively a function, f, between topological spaces that preserves local structure.

## Formal definition

Let X and Y be topological spaces. A function $f : X \to Y\,$ is a local homeomorphism[1] if for every point x in X there exists an open set U containing x, such that the image $f(U)$ is open in Y and the restriction $f|_U : U\to f(U)\,$ is a homeomorphism.

## Examples

By definition, every homeomorphism is also a local homeomorphism.

If U is an open subset of Y equipped with the subspace topology, then the inclusion map i : UY is a local homeomorphism. Openness is essential here: the inclusion map of a non-open subset of Y never yields a local homeomorphism.

Every covering map is a local homeomorphism; in particular, the universal cover p : CY of a space Y is a local homeomorphism. In certain situations, the converse is true: if X and Y are locally compact spaces and p : XY is a proper local homeomorphism, then p is a covering map.

Let f : S1S1 be the map that wraps the circle around itself n times (i.e. has winding number n). This is a local homeomorphism for all non-zero n, but a homeomorphism only in the cases where it is bijective, i.e. n = 1 or -1.

It is shown in complex analysis that a complex analytic function f gives a local homeomorphism precisely when the derivative f ′(z) is non-zero for all z in the domain of f. The function f(z) = zn on an open disk around 0 is not a local homeomorphism at 0 when n is at least 2. In that case 0 is a point of "ramification" (intuitively, n sheets come together there).

## Properties

Every local homeomorphism is a continuous and open map. A bijective local homeomorphism is therefore a homeomorphism.

A local homeomorphism f : XY preserves "local" topological properties:

If f : XY is a local homeomorphism and U is an open subset of X, then the restriction f|U is also a local homeomorphism.

If f : XY and g : YZ are local homeomorphisms, then the composition gf : XZ is also a local homeomorphism.

The local homeomorphisms with codomain Y stand in a natural 1-1 correspondence with the sheaves of sets on Y. Furthermore, every continuous map with codomain Y gives rise to a uniquely defined local homeomorphism with codomain Y in a natural way. All of this is explained in detail in the article on sheaves.