# Definition:Local Basis

## Contents

## Definition

Let $T = \struct {S, \tau}$ be a topological space.

Let $x$ be an element of $S$.

### Local Basis for Open Sets

A **local basis** at $x$ is a set $\BB$ of open neighborhoods of $x$ such that:

- $\forall U \in \tau: x \in U \implies \exists H \in \BB: H \subseteq U$

That is, such that every open neighborhood of $x$ also contains some set in $\BB$.

### Neighborhood Basis of Open Sets

A **local basis** at $x$ is a set $\BB$ of open neighborhoods of $x$ such that every neighborhood of $x$ contains a set in $\BB$.

That is, a **local basis** at $x$ is a neighborhood basis of $x$ consisting of open sets.

## Also defined as

Some more modern sources suggest that in order to be a **local basis**, the neighborhoods of which the set $\mathcal B$ consists do not need to be open.

Such a structure is referred to on $\mathsf{Pr} \infty \mathsf{fWiki}$ as a neighborhood basis.

## Also known as

A **local basis** is also known as a **neighborhood basis**, but that term is used on $\mathsf{Pr} \infty \mathsf{fWiki}$ for a weaker notion.