Topological Neighborhoods
Definition
Given a topological space $(X, \mathcal{T} )$, a subset $N$ of $X$ is called a neighborhood (or topological neighborhood) of a point $a \in X$ if $N$ contains an open set that contains $a$.
Given a topological space $(X, \mathcal{T} )$, a subset $N$ of $X$ is called a neighborhood (or topological neighborhood) of a point $a \in X$ if $N$ contains an open set that contains $a$.