Neighbourhood system

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In topology and related areas of mathematics, the neighbourhood system, complete system of neighbourhoods,[1] or neighbourhood filter 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

such that


That is, for any neighbourhood we can find a neighbourhood in the neighbourhood basis that is contained in .

Conversely, as with any filter base, the local basis allows to get back the corresponding neighbourhood filter as .[2]


  • 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.


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,

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 space is a topological group or the topology is defined by a pseudometric.

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

See also[edit]


  1. ^ Mendelson, Bert (1990) [1975]. Introduction to Topology (Third ed.). Dover. p. 41. ISBN 0-486-66352-3. 
  2. ^ Willard, Stephen (1970). General Topology. Addison-Wesley Publishing.  (See Chapter 2, Section 4)