Rational Numbers are F-Sigma Set in Real Line
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Theorem
Let $\struct {\R, \tau}$ be the real number line considered asa topological space with the usual (Euclidean) topology.
Then:
- $\Q$ is an $F_\sigma$ set in $\struct {\R, \tau}$.
Proof
Define the set of subsets of $\R$ as:
- $\FF := \set {\set x: x \in \Q}$
By Closed Real Interval is Closed Set:
- $\forall x \in \Q: \closedint x x = \set x$ is closed (in topological sense)
Then:
- $\forall A \in \FF: A$ is closed
By Cardinality of Set of Singletons:
- $\card \FF = \card \Q$
where $\card \FF$ denotes the cardinality of $\FF$.
By Rational Numbers are Countably Infinite:
- $\Q$ is countable.
Therefore by Set is Countable if Cardinality equals Cardinality of Countable Set:
- $\FF$ is countable.
By Union of Set of Singletons:
- $\bigcup \FF = \Q$
Thus, by definition, $\Q$ is an $F_\sigma$ set.
$\blacksquare$
Sources
- Mizar article TOPGEN_4:45