In topology and related areas of mathematics, a **neighbourhood** (or **neighborhood**) 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 the point belonging to the topological interior of in

The neighbourhood need *not* be an open subset but when is open in then it is called an **open neighbourhood**.^{ [1] } Some authors have been known to require neighbourhoods to 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.

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 V is a neighbourhood of S if and only if it is a neighbourhood of all the points in S. Furthermore, V is a neighbourhood of S if and only if S is a subset of the interior of V. A neighbourhood of S that is also an open set is called an **open neighbourhood** of S. 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 (or equivalently, _{$r$} is the union of all the open balls of radius that are centred at a point in ):

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 .

Given the set of real numbers with the usual Euclidean metric and a subset defined as

then is a neighbourhood for the set of natural numbers, but is *not* a uniform neighbourhood of this set.

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

- the point is an element of each in
- 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.

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 .

A **deleted neighbourhood** of a point (sometimes called a **punctured neighbourhood**) 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 and in the definition of limit points (among other things).

In mathematics, more specifically in general topology, **compactness** is a property that generalizes the notion of a subset of Euclidean space being closed and bounded. Examples include a closed interval, a rectangle, or a finite set of points. This notion is defined for more general topological spaces than Euclidean space in various ways.

In mathematics, a **continuous function** is a function that does not have any abrupt changes in value, known as discontinuities. More precisely, a function is continuous if arbitrarily small changes in its output can be assured by restricting to sufficiently small changes in its input. If not continuous, a function is said to be *discontinuous*. Up until the 19th century, mathematicians largely relied on intuitive notions of continuity, during which attempts such as the epsilon–delta definition were made to formalize it.

In topology and related branches of mathematics, a **connected space** is a topological space that cannot be represented as the union of two or more disjoint non-empty open subsets. Connectedness is one of the principal topological properties that are used to distinguish topological spaces.

In topology and related branches of mathematics, a **Hausdorff space**, **separated space** or **T _{2} space** is a topological space where for any two distinct points there exist neighbourhoods of each which are disjoint from each other. Of the many separation axioms that can be imposed on a topological space, the "Hausdorff condition" (T

In mathematics, a **filter** is a special subset of a partially ordered set. Filters appear in order and lattice theory, but can also be found in topology, from where they originate. The dual notion of a filter is an order ideal.

In mathematics, a **topological space** is, roughly speaking, a geometrical space in which *closeness* is defined but, generally, cannot be measured by a numeric distance. More specifically, a topological space is a set of points, along with a set of neighbourhoods for each point, satisfying a set of axioms relating points and neighbourhoods.

This is a glossary of some terms used in the branch of mathematics known as topology. Although there is no absolute distinction between different areas of topology, the focus here is on general topology. The following definitions are also fundamental to algebraic topology, differential topology and geometric topology.

In the mathematical field of topology, a **uniform space** is a set with a **uniform structure**. Uniform spaces are topological spaces with additional structure that is used to define uniform properties such as completeness, uniform continuity and uniform convergence. Uniform spaces generalize metric spaces and topological groups, but the concept is designed to formulate the weakest axioms needed for most proofs in analysis.

In mathematics, **open sets** are a generalization of open intervals in the real line. In a metric space—that is, when a distance is defined—open sets are the sets that, with every point P, contain all points that are sufficiently near to P.

In mathematics, a **topological vector space** is one of the basic structures investigated in functional analysis. A topological vector space is a vector space which is also a topological space, this implies that vector space operations be continuous functions. More specifically, its topological space has a uniform topological structure, allowing a notion of uniform convergence.

In geometry, topology, and related branches of mathematics, a **closed set** is a set whose complement is an open set. In a topological space, a closed set can be defined as a set which contains all its limit points. In a complete metric space, a closed set is a set which is closed under the limit operation. This should not be confused with a closed manifold.

In the mathematical field of topology, the **Alexandroff extension** is a way to extend a noncompact topological space by adjoining a single point in such a way that the resulting space is compact. It is named after the Russian mathematician Pavel Alexandroff. More precisely, let *X* be a topological space. Then the Alexandroff extension of *X* is a certain compact space *X** together with an open embedding *c* : *X* → *X** such that the complement of *X* in *X** consists of a single point, typically denoted ∞. The map *c* is a Hausdorff compactification if and only if *X* is a locally compact, noncompact Hausdorff space. For such spaces the Alexandroff extension is called the **one-point compactification** or **Alexandroff compactification**. The advantages of the Alexandroff compactification lie in its simple, often geometrically meaningful structure and the fact that it is in a precise sense minimal among all compactifications; the disadvantage lies in the fact that it only gives a Hausdorff compactification on the class of locally compact, noncompact Hausdorff spaces, unlike the Stone–Čech compactification which exists for any topological space, a much larger class of spaces.

In mathematics, a **base** or **basis** for the topology τ of a topological space (*X*, τ) is a family *B* of open subsets of *X* such that every open set is equal to a union of some sub-family of *B*. For example, the set of all open intervals in the real number line is a basis for the Euclidean topology on because every open interval is an open set, and also every open subset of can be written as a union of some family of open intervals.

In topology and related branches of mathematics, a **T _{1} space** is a topological space in which, for every pair of distinct points, each has a neighborhood not containing the other point. An

In mathematics, **general topology** is the branch of topology that deals with the basic set-theoretic definitions and constructions used in topology. It is the foundation of most other branches of topology, including differential topology, geometric topology, and algebraic topology. Another name for general topology is **point-set topology**.

In functional analysis and related areas of mathematics, **locally convex topological vector spaces** (**LCTVS**) or **locally convex spaces** are examples of topological vector spaces (TVS) that generalize normed spaces. They can be defined as topological vector spaces whose topology is generated by translations of balanced, absorbent, convex sets. Alternatively they can be defined as a vector space with a family of seminorms, and a topology can be defined in terms of that family. Although in general such spaces are not necessarily normable, the existence of a convex local base for the zero vector is strong enough for the Hahn–Banach theorem to hold, yielding a sufficiently rich theory of continuous linear functionals.

In mathematics, a topological space is usually defined in terms of open sets. However, there are many equivalent **characterizations of the category of topological spaces**. Each of these definitions provides a new way of thinking about topological concepts, and many of these have led to further lines of inquiry and generalisation.

In topology and other branches of mathematics, a topological space *X* is **locally connected** if every point admits a neighbourhood basis consisting entirely of open, connected sets.

In topology and related areas of mathematics, a subset *A* of a topological space *X* is called **dense** if every point *x* in *X* either belongs to *A* or is a limit point of *A*; that is, the closure of *A* constitutes the whole set *X*. Informally, for every point in *X*, the point is either in *A* or arbitrarily "close" to a member of *A* — for instance, the rational numbers are a dense subset of the real numbers because every real number either is a rational number or has a rational number arbitrarily close to it.

In functional analysis and related areas of mathematics, a **complete topological vector space** is a topological vector space (TVS) with the property that whenever points get progressively closer to each other, then there exists some point towards which they all get closer to. The notion of "points that get progressively closer" is made rigorous by *Cauchy nets* or *Cauchy filters*, which are generalizations of *Cauchy sequences*, while "point towards which they all get closer to" means that this net or filter converges to Unlike the notion of completeness for metric spaces, which it generalizes, the notion of completeness for TVSs does not depend on any metric and is defined for *all* TVSs, including those that are not metrizable or Hausdorff.

- ↑ Dixmier, Jacques (1984).
*General Topology*. Undergraduate Texts in Mathematics. Translated by Sterling K. Berberian. Springer. p. 6. ISBN 0-387-90972-9.According to this definition, an

*open neighborhood of*is nothing more than an open subset of that contains

- Kelley, John L. (1975).
*General topology*. New York: Springer-Verlag. ISBN 0-387-90125-6. - Bredon, Glen E. (1993).
*Topology and geometry*. New York: Springer-Verlag. ISBN 0-387-97926-3. - Kaplansky, Irving (2001).
*Set Theory and Metric Spaces*. American Mathematical Society. ISBN 0-8218-2694-8.

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