In topology and related areas of mathematics, a neighbourhood is one of the basic concepts in a topological space. It is closely related to the concepts of open set and interior. Intuitively speaking, a neighbourhood of a point is a set of points containing that point where one can move some amount in any direction away from that point without leaving the set.
If is a topological space and is a point in, a neighbourhood of is a subset of that includes an open set containing, This is also equivalent to being in the interior of. The neighbourhood need not be an open set itself. If is open it is called an . Some mathematicians require that neighbourhoods be open, so it is important to note conventions. A set that is a neighbourhood of each of its points is open since it can be expressed as the union of open sets containing each of its points. A rectangle, as illustrated in the figure, is not a neighbourhood of all its points; points on the edges or corners of the rectangle are not contained in any open set that is contained within the rectangle. The collection of all neighbourhoods of a point is called the neighbourhood system at the point.
Neighbourhood of a set
If is a subset of topological space then a neighbourhood of is a set that includes an open set containing. It follows that a set is a neighbourhood of if and only if it is a neighbourhood of all the points in. Furthermore, it follows that is a neighbourhood of iff is a subset of the interior of. The neighbourhood of a point is just a special case of this definition.
In a metric space, a set is a neighbourhood of a point if there exists an open ball with centre and radius, such that is contained in. is called uniform neighbourhood of a set if there exists a positive number such that for all elements of, is contained in. For the -neighbourhood of a set is the set of all points in that are at distance less than from : It directly follows that an -neighbourhood is a uniform neighbourhood, and that a set is a uniform neighbourhood if and only if it contains an -neighbourhood for some value of.
The above definition is useful if the notion of open set is already defined. There is an alternative way to define a topology, by first defining the neighbourhood system, and then open sets as those sets containing a neighbourhood of each of their points. A neighbourhood system on is the assignment of a filter of subsets of to each in, such that
each in contains some in such that for each in, is in.
One can show that both definitions are compatible, i.e. the topology obtained from the neighbourhood system defined using open sets is the original one, and vice versa when starting out from a neighbourhood system.
Uniform neighbourhoods
In a uniform space, is called a uniform neighbourhood of if there exists an entourage such that contains all points of that are -close to some point of ; that is, for all.
Deleted neighbourhood
A deleted neighbourhood of a point is a neighbourhood of, without. For instance, the interval is a neighbourhood of in the real line, so the set is a deleted neighbourhood of. A deleted neighbourhood of a given point is not in fact a neighbourhood of the point. The concept of deleted neighbourhood occurs in the definition of the limit of a function.
