In topology and related areas of mathematics, the neighbourhood system, complete system of neighbourhoods, or neighbourhood filter for a point x is the collection of all neighbourhoods of 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
That is, for any neighbourhood we can find a neighbourhood in the neighbourhood basis that is contained in .
- Trivially the neighbourhood system for a point is also a neighbourhood basis for the point.
- Given a space X with the indiscrete topology the neighbourhood system for any point x only contains the whole space,
- In a metric space, for any point x, the sequence of open balls around x with radius 1/n form a countable neighbourhood basis . This means every metric space is first-countable.
- In the weak topology on the space of measures on a space E, a neighbourhood base about is given by
- where are continuous bounded functions from E to the real numbers.
This is because, by assumption, vector addition is separately continuous in the induced topology. Therefore, the topology is determined by its neighbourhood system at the origin. More generally, this remains true whenever the space is a topological group or the topology is defined by a pseudometric.