title: Half-disk topology
text: # Half-disk topology
  
  In mathematics, and particularly general topology, the half-disk topology is an example of a topology given to the set $X$, given by all points $(x,y)$ in the plane such that $y\geq 0$. The set $X$ can be termed the closed upper half plane.
  
  To give the set $X$ a topology means to say which subsets of $X$ 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 $X$ and the empty set $\emptyset$ are open sets.
  
  ## Construction
  
  We consider $X$ to consist of the open upper half plane $P$, given by all points $(x,y)$ in the plane such that $y>0$; and the x-axis $L$, given by all points $(x,y)$ in the plane such that $y=0$. Clearly $X$ is given by the union $P\cup L$. The open upper half plane $P$ has a topology given by the Euclidean metric topology. We extend the topology on $P$ to a topology on $X=P\cup L$ by adding some additional open sets. These extra sets are of the form ${(x,0)}\cup (P\cap U)$, where $(x,0)$ is a point on the line $L$ and $U$ is a neighbourhood of $(x,0)$ in the plane, open with respect to the Euclidean metric (defining the disk radius).
  
  