# Neighbourhood system

In topology and related areas of mathematics, the neighbourhood system or neighbourhood filter ${\displaystyle {\mathcal {V}}(x)}$ for a point x is the collection of all neighbourhoods for the point x.

A neighbourhood basis or local basis for a point x is a filter base of the neighbourhood filter, i.e. a subset

${\displaystyle {\mathcal {B}}(x)\subset {\mathcal {V}}(x)}$

such that

${\displaystyle \forall V\in {\mathcal {V}}(x)\quad \exists B\in {\mathcal {B}}(x){\mbox{ with }}B\subset V}$.

That is, for any neighbourhood ${\displaystyle V}$ we can find a neighbourhood ${\displaystyle B}$ in the neighbourhood basis that is contained in ${\displaystyle V}$.

Conversely, as with any filter base, the local basis allows to get back the corresponding neighbourhood filter as ${\displaystyle {\mathcal {V}}(x)=\left\{V\supset B~:~B\in {\mathcal {B}}(x)\right\}}$.[1]

## Examples

• Trivially the neighbourhood system for a point is also a neighbourhood basis for the point.
${\displaystyle \left\{\mu \in {\mathcal {M}}(E):|\mu f_{i}-\nu f_{i}|<\varepsilon _{i},i=1,\ldots ,n\right\}}$
where ${\displaystyle f_{i}}$ are continuous bounded functions from E to the real numbers.

## Properties

In a semi normed space, that is a vector space with the topology induced by a semi norm, all neighbourhood systems can be constructed by translation of the neighbourhood system for the point 0,

${\displaystyle {\mathcal {V}}(x)={\mathcal {V}}(0)+x.}$

This is because, by assumption, vector addition is separate continuous in the induced topology. Therefore the topology is determined by its neighbourhood system at the origin. More generally, this remains true whenever the topology is defined by a translation invariant metric or pseudometric.

Every neighbourhood system for a non empty set A is a filter called the neighbourhood filter for A.