# Every uncountable subset $A$ of $\mathbb{R^n}$ has cardinality of the continuum?

Every uncountable subset $A$ of $\mathbb{R^n}$ has cardinality of the continuum?

Proof: We know that an uncountable subset has uncountable many points that are limit points ( there is a proof in thie link: Uncountably many points of an uncountable set in a second countable are limit points )

Let $B=\{ x \in A : x \text{is not limit point of A}\}$. If $B$ is uncountable then there is a point $y\in B$ which is a limit point of $B$ this implies that $y$ is a limit point of $A$, but this is a contradiction because if $y\in B$ then $y$ is a limt point of $A$

Then $B$ is at most countable. Let $P=A\setminus B$ then $P$ is perfect so its cardinality is at least the continuum $c$

So $c \le \text{card}(P) \le \text{card}(A) \le \text{card}(\mathbb{R^n} ) = c$ therefore $A$ has cardinality of the continuum.

But I think this proof is incorrect due to tha fact that I´ve seen this "theorem" but with an extra hypothesis: $A$ must be closed... so I don´t know where is my mistake.

I would really appreciate if ypu can help me with this problem.

• Why is $P$ perfect? You may test your argument with $A=\mathbb{R}\setminus\mathbb{Q}$. – Sangchul Lee Jul 4 '18 at 4:01
• And interestingly enough, every Borel subset of $\mathbb{R}^n$ is either (at most) countable or has cardinality of the continuum. In particular, this claim is true for any open/closed subsets. – Sangchul Lee Jul 4 '18 at 4:08

You claim $P$ is perfect and this is in general false. The fact that $B$ is at most countable shows that $|A \setminus B| = |A|$. Perfect sets are closed. $B \setminus A$ is merely a crowded subset, not a perfect subset.
Recall $P$ is perfect when $P=P'$ where $P'$ is the set of limit points, and whenever $A' \subseteq A$, $A$ is closed, for any $A$. So perfect implies closed. And there is no reason $A \setminus B$ would be closed.
• So if $A$ is closed then $A \setminus B$ would be perfect right? every point of $A \setminus B$ would be a limit point of $A \setminus B$? – user128422 Jul 4 '18 at 4:35
• @user128422 yes as $B$ is open in $A$. But then it’s ok. An uncountable closed set has size continuum – Henno Brandsma Jul 4 '18 at 4:40
• @user128422 It not not true in general that that set would be perfect. $A - B$ is the set of all limit points of $A$, which is normally denoted $A'$. This is not necessarily perfect, even if $A$ is closed. For example, take $A = \{0\} \cup \{\frac{1}{n} : n \in \mathbb{N} \}$. Then $A' = \{0\}$, which is not perfect. – user571438 Jul 4 '18 at 5:07
• So how can we use the fact that $A$ is also uncountable to prove this? – user128422 Jul 4 '18 at 5:43
• You can use the Cantor Bendixson Theorem, which states that every closed subset of $\mathbb R$ can be written uniquely as a disjoint union of a perfect set and a countable set. From here, the proof you had above will work. The proof of this is nontrivial. One method I know is to use transfinite recursion to essentially iterate the process of $A \mapsto A'$. Alternatively, we say $x \in A$ is a condensation point of $A$ if every open neighborhood of $x$ intersects with $A$ in an uncountable set. Then let $\text{kernel}(A)$ be the condensation points of $A$. That's your perfect set. – user571438 Jul 4 '18 at 6:14