Let \(X\) be a topological space and \(p\in X\) then a collection \(\mathcal{B}_p\) of neighbourhoods of \(p\) is called a neighbourhood basis for \(X\) at \(p\) if every neighbourhood contains some \(B\in \mathcal{B}_p\).
Let \(X\) be a topological space and \(p\in X\) then a collection \(\mathcal{B}_p\) of neighbourhoods of \(p\) is called a neighbourhood basis for \(X\) at \(p\) if every neighbourhood contains some \(B\in \mathcal{B}_p\).