User:Leigh.Samphier/Topology/Nagata-Smirnov Metrization Theorem/Lemma 1

Theorem
Let $T = \struct {S, \tau}$ be a topological space.

Let $\BB = \ds \bigcup_{n \mathop \in \N} \BB_n$ be a $\sigma$-locally finite basis of $T$ where $\BB_n$ is locally finite set of subsets for each $n \in \N$.

Let $I = \set{\tuple{B, n} : B \in \BB, B \in \BB_n}$.

For each $\tuple{B, n} \in I$, let $f_{\tuple{B, n}}:S \to \closedint 0 1$:
 * $B = \set{x \in S : \map {f_{\tuple{B, n}}} x \ne 0}$

Then:
 * $\forall x \in S$ and $n \in \N$:
 * the generalized sum $\ds \sum_{B \in \BB_n} \map {f_{\tuple{B, n}}^2} x$ converges