Neighbourhood of a point edit

If ${\displaystyle X}$  is a topological space and ${\displaystyle p}$  is a point in ${\displaystyle X,}$  then a neighbourhood^[1] of ${\displaystyle p}$  is a subset ${\displaystyle V}$  of ${\displaystyle X}$  that includes an open set ${\displaystyle U}$  containing ${\displaystyle p}$ ,

This is equivalent to the point ${\displaystyle p\in X}$  belonging to the topological interior of ${\displaystyle V}$  in ${\displaystyle X.}$ 

The neighbourhood ${\displaystyle V}$  need not be an open subset of ${\displaystyle X.}$  When ${\displaystyle V}$  is open (resp. closed, compact, etc.) in ${\displaystyle X,}$  it is called an open neighbourhood^[2] (resp. closed neighbourhood, compact neighbourhood, etc.). Some authors^[3] require neighbourhoods to be open, so it is important to note their 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 closed 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 edit

If ${\displaystyle S}$  is a subset of a topological space ${\displaystyle X}$ , then a neighbourhood of ${\displaystyle S}$  is a set ${\displaystyle V}$  that includes an open set ${\displaystyle U}$  containing ${\displaystyle S}$ ,

In a metric space ${\displaystyle M=(X,d),}$  a set ${\displaystyle V}$  is a neighbourhood of a point ${\displaystyle p}$  if there exists an open ball with center ${\displaystyle p}$  and radius ${\displaystyle r>0,}$  such that

${\displaystyle V}$  is called a uniform neighbourhood of a set ${\displaystyle S}$  if there exists a positive number ${\displaystyle r}$  such that for all elements ${\displaystyle p}$  of ${\displaystyle S,}$ 

Under the same condition, for ${\displaystyle r>0,}$  the ${\displaystyle r}$ -neighbourhood ${\displaystyle S_{r}}$  of a set ${\displaystyle S}$  is the set of all points in ${\displaystyle X}$  that are at distance less than ${\displaystyle r}$  from ${\displaystyle S}$  (or equivalently, ${\displaystyle S_{r}}$  is the union of all the open balls of radius ${\displaystyle r}$  that are centered at a point in ${\displaystyle S}$ ):

It directly follows that an ${\displaystyle r}$ -neighbourhood is a uniform neighbourhood, and that a set is a uniform neighbourhood if and only if it contains an ${\displaystyle r}$ -neighbourhood for some value of ${\displaystyle r.}$ 

Given the set of real numbers ${\displaystyle \mathbb {R} }$  with the usual Euclidean metric and a subset ${\displaystyle V}$  defined as

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 ${\displaystyle X}$  is the assignment of a filter ${\displaystyle N(x)}$  of subsets of ${\displaystyle X}$  to each ${\displaystyle x}$  in ${\displaystyle X,}$  such that

1. the point ${\displaystyle x}$  is an element of each ${\displaystyle U}$  in ${\displaystyle N(x)}$ 
2. each ${\displaystyle U}$  in ${\displaystyle N(x)}$  contains some ${\displaystyle V}$  in ${\displaystyle N(x)}$  such that for each ${\displaystyle y}$  in ${\displaystyle V,}$  ${\displaystyle U}$  is in ${\displaystyle N(y).}$ 

One can show that both definitions are compatible, that is, 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 ${\displaystyle S=(X,\Phi ),}$  ${\displaystyle V}$  is called a uniform neighbourhood of ${\displaystyle P}$  if there exists an entourage ${\displaystyle U\in \Phi }$  such that ${\displaystyle V}$  contains all points of ${\displaystyle X}$  that are ${\displaystyle U}$ -close to some point of ${\displaystyle P;}$  that is, ${\displaystyle U[x]\subseteq V}$  for all ${\displaystyle x\in P.}$ 

A deleted neighbourhood of a point ${\displaystyle p}$  (sometimes called a punctured neighbourhood) is a neighbourhood of ${\displaystyle p,}$  without ${\displaystyle \{p\}.}$  For instance, the interval ${\displaystyle (-1,1)=\{y:-1<y<1\}}$  is a neighbourhood of ${\displaystyle p=0}$  in the real line, so the set ${\displaystyle (-1,0)\cup (0,1)=(-1,1)\setminus \{0\}}$  is a deleted neighbourhood of ${\displaystyle 0.}$  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).^[4]

• Isolated point – Point of a subset S around which there are no other points of S
• Neighbourhood system – (for a point x) collection of all neighborhoods for the point xPages displaying wikidata descriptions as a fallback
• Region (mathematics) – Connected open subset of a topological spacePages displaying short descriptions of redirect targets
• Tubular neighbourhood – neighborhood of a submanifold homeomorphic to that submanifold’s normal bundlePages displaying wikidata descriptions as a fallback

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• Kaplansky, Irving (2001). Set Theory and Metric Spaces. American Mathematical Society. ISBN 0-8218-2694-8.
• Kelley, John L. (1975). General topology. New York: Springer-Verlag. ISBN 0-387-90125-6.
• Willard, Stephen (2004) [1970]. General Topology. Mineola, N.Y.: Dover Publications. ISBN 978-0-486-43479-7. OCLC 115240.