limit points of uncountable subsets of R^n
Proposition. Let be an -dimensional, real normed space and let . If is uncountable, then there exists limit point
of in .
Proof. For any let
i.e. is a closed ball centered in with radius . Assume, that for any the set
is finite. Then would be at most countable. Contradiction
, since is uncountable. Thus, there exists such that is infinite
. But and since is compact
(and is infinite), then there exists limit point of in . This completes
the proof.
Corollary. If is uncountable, then there exist infinitely many limit points of in .
Proof. Assume, that there are finitely many limit points of , namely . For define
Briefly speaking, is a complement of a union of closed balls centered at with radii . Of course since there are finitely many limit points. Let
Assume, that is countable for every . Then
would be at most countable (of course under assumption of Axiom of Choice
). Contradiction. Thus, there is such that is uncountable. Then (due to proposition) there is a limit point of . Note, that
for some . Thus is different from any . Contradiction, since is also a limit point of .