Overlapping interval topology
# Overlapping interval topology

In mathematics, the overlapping interval topology is a topology which is used to illustrate various topological principles.

## Definition

Given the closed interval $[-1,1]$ of the real number line, the open sets of the topology are generated from the half-open intervals $(a,1]$ with $a<0$ and $[-1,b)$ with $b>0$. The topology therefore consists of intervals of the form $[-1,b)$, $(a,b)$, and $(a,1]$ with $a<0<b$, together with $[-1,1]$ itself and the empty set.

## Properties

Any two distinct points in $[-1,1]$ are topologically distinguishable under the overlapping interval topology as one can always find an open set containing one but not the other point. However, every non-empty open set contains the point 0 which can therefore not be separated from any other point in $[-1,1]$, making $[-1,1]$ with the overlapping interval topology an example of a $T_0$ space that is not a $T_1$ space.

The overlapping interval topology is second countable, with a countable basis being given by the intervals $[-1,s)$, $(r,s)$ and $(r,1]$ with $r<0<s$ and r and s rational.

