In mathematics, more specifically general topology, the double origin topology is an example of a topology given to the plane R<sup>2</sup> with an extra point, say 0*, added. In this case, the double origin topology gives a topology on the set , where â denotes the disjoint union.
Given a point x belonging to X, such that and , the neighbourhoods of x are those given by the standard metric topology on We define a countably infinite basis of neighbourhoods about the point 0 and about the additional point 0*. For the point 0, the basis, indexed by n, is defined to be:
In a similar way, the basis of neighbourhoods of 0* is defined to be:
The space }, along with the double origin topology is an example of a Hausdorff space, although it is not completely Hausdorff. Since it has a base of regular open sets, it is an example of a semiregular space that is not regular. In terms of compactness, the space }, along with the double origin topology fails to be either compact, paracompact or locally compact, however, X is second countable. Finally, it is an example of an arc connected space.