# Locally decomposable spaces

## Idea

Local decomposability is a sort of separation axiom, like a weak sort of regularity, that is trivial in classical mathematics, but interesting in constructive mathematics.

## Definition

A topological space $X$ is locally decomposable if for any open set $U\subseteq X$ and point $x\in X$, there exists an open set $V$ with $x\in V$ such that for all $y\in X$ we have either $y\in U$ or $y\notin V$. If excluded middle holds, of course, we can take $V = U$.

For point-set apartness spaces, which are equivalent to certain topological spaces, the condition can be rephrased as: if $x\bowtie A$, then there is a set $B$ such that $x\bowtie B$ and for all $y$ we have either $y\bowtie A$ or $y\in B$.

For uniform spaces, the notion of uniform regularity is really a notion of “uniform local decomposability”; but since in the uniform case it is sufficient to imply full regularity, we generally call it “uniform regularity” instead. For quasi-uniform spaces this is no longer true (since after all, there are non-regular quasi-uniform spaces classically), so we should speak of uniform local decomposability instead.

• Douglas Bridges, Peter Schuster, and Luminita Vita, Apartness, Topology, and Uniformity: a Constructive View, doi