Lindelöf's lemma

In mathematics, Lindelöf's lemma is a simple but useful lemma in topology on the real line, named for the Finnish mathematician Ernst Leonard Lindelöf.

Statement of the lemma
Let the real line have its standard topology. Then every open subset of the real line is a countable union of open intervals.

Generalized Statement
Lindelöf's lemma is also known as the statement that every open cover in a second-countable space has a countable subcover (Kelley 1955:49). This means that every second-countable space is also a Lindelöf space.

Proof of the generalized statement
Let $$B$$ be a countable basis of $$X$$. Consider an open cover, $$\mathcal{F} = \bigcup_{\alpha} U_{\alpha}$$. To get prepared for the following deduction, we define two sets for convenience, $$B_{\alpha} := \left \{ \beta \in B: \beta \subset U_{\alpha} \right \}$$, $$B':= \bigcup_{\alpha} B_{\alpha}$$.

A straight-forward but essential observation is that, $$U_{\alpha} = \bigcup_{\beta \in B_{\alpha}} \beta$$ which is from the definition of base. Therefore, we can get that,

$$\mathcal{F} = \bigcup_{\alpha} U_{\alpha} = \bigcup_{\alpha} \bigcup_{\beta \in B_{\alpha}} \beta = \bigcup_{\beta \in B'} \beta$$

where $$B' \subset B$$, and is therefore at most countable. Next, by construction, for each $$\beta\in B'$$ there is some $$\delta_{\beta}$$ such that $$\beta\in U_{\delta_{\beta}}$$. We can therefore write

$$\mathcal{F} = \bigcup_{\beta\in B'} U_{\delta_{\beta}}$$

completing the proof.