# Overlapping interval topology

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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 $[-1,b)$ and $(a,1]$ with $a < 0 < b$. 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 T0 space that is not a T1 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 (and thus countable).