F-Sigma Sets Closed under Union
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Theorem
Let $T = \struct {S, \tau}$ be a topological space.
Let $F, F'$ be $F_\sigma$ sets of $T$.
Then their union $F \cup F'$ is also an $F_\sigma$ set of $T$.
Proof
By definition of $F_\sigma$ set, there exist sequences $\sequence {C_n}_{n \mathop \in \N}$ and $\sequence {C'_n}_{n \mathop \in \N}$ of closed sets of $T$ such that:
- $F = \ds \bigcup_{n \mathop \in \N} C_n$
- $F' = \ds \bigcup_{n \mathop \in \N} C'_n$
By General Self-Distributivity of Set Union, we have:
- $F \cup F' = \ds \bigcup_{n \mathop \in \N} \paren {C_n \cup C'_n}$
By Finite Union of Closed Sets is Closed in Topological Space, $C_n \cup C'_n$ is closed, for all $n \in \N$.
Thus $F \cup F'$ is seen to be an $F_\sigma$ set.
$\blacksquare$