First-countable

Definition (First-countable)

(X,T)(X,\mathscr{T}) is first-countable if, for every xXx\in X, there exists a sequence of neighbourhoods {Uj}jN\{ \mathcal{U}_{j} \}_{j\in \mathbb{N}} of xx such that, for any neighbourhood U\mathcal{U} of xx, there exists jNj\in \mathbb{N} with UjU\mathcal{U}_{j}\subset \mathcal{U}.

Remark

Looking at the definition of Second-countable we can see that this weakens that notion by saying each point xXx\in X has a countable Neighbourhood Basis i.e. the countable collection of neighbourhoods {Uj}jN\{ \mathcal{U}_{j} \}_{j\in \mathbb{N}} forms a neighbourhood basis in the sense that for any other neighbourhood U\mathcal{U} of xx we can find an index that points us to something in the basis s.t. UjU\mathcal{U}_{j}\subset \mathcal{U}.

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