# Excluded point topology

In mathematics, the excluded point topology is a topology where exclusion of a particular point defines openness. Formally, let X be any set and pX. The collection

$T=\{S\subseteq X:p\notin S{\text{ or }}S=X\}$ of subsets of X is then the excluded point topology on X. There are a variety of cases which are individually named:

• If X has two points we call it the Sierpiński space. This case is somewhat special and is handled separately.
• If X is finite (with at least 3 points) we call the topology on X the finite excluded point topology
• If X is countably infinite we call the topology on X the countable excluded point topology
• If X is uncountable we call the topology on X the uncountable excluded point topology

A generalization / related topology is the open extension topology. That is if $X\backslash \{p\}$ has the discrete topology then the open extension topology will be the excluded point topology.

This topology is used to provide interesting examples and counterexamples. A space with the excluded point topology is connected, since the only open set containing the excluded point is X itself and hence X cannot be written as disjoint union of two proper open subsets.