In mathematics, and particularly general topology, the half-disk topology is an example of a topology given to the set , given by all points in the plane such that . The set can be termed the closed upper half plane. To give the set a topology means to say which subsets of are "open", and to do so in a way that the following axioms are met: The union of open sets is an open set. The finite intersection of open sets is an open set. The set and the empty set are open sets. Construction We consider to consist of the open upper half plane , given by all points in the plane such that ; and the x-axis , given by all points in the plane such that . Clearly is given by the union . The open upper half plane has a topology given by the Euclidean metric topology. We extend the topology on to a topology on by adding some additional open sets. These extra sets are of the form , where is a point on the line and is an open, with respect to the Euclidean metric topology, neighbourhood of in the plane. See also List of topologies References General topology Topological spaces