Neighborhood

The neighborhood of a point is a notion which has slightly different meanings in different contexts.

General topology

Let $X$ be a topology, and let $x$ be an element of $X$ . We say that a set $N \subset X$ is a neighborhood of $x$ if there exists some open set $S$ for which $x \in S \subset N$ .

Metric spaces

Let $X$ be a metric space, and let $x$ be an element of $X$ . A neighborhood $N$ of $x$ is the set of points $a$ in $X$ such that $d(a,x)<r$ , for some positive real $r$ specific to $N$ . The real $r$ is called the radius of $N$ . This neighborhood is sometimes denoted $N_r(x)$ . In metric spaces, neighborhoods are also called open balls.

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Neighborhood

General topology

Metric spaces