Equivalence of Definitions of Closed Set in Normed Vector Space

Theorem
Let $V = \struct {X, \norm {\, \cdot \,}}$ be a normed vector space.

Let $F \subseteq X$.

Proof
Let $\sequence {x_n}_{n \mathop \in \N}$ be a sequence in $F$ with a limit point $x$ in $X$.

Definition 1 implies Definition 2
$x \notin F$.

Since $F$ is closed, $X \setminus F$ is open.

Let $\map {B_\epsilon} x := \set {y \in X : \norm {y - x} < \epsilon}$ be an open ball.

Then there exists $\map {B_\epsilon} x$, which belongs to $X \setminus F$:


 * $\forall x \in X \setminus F: \exists \epsilon \in \R_{>0}: \map {B_\epsilon} x \subset X \setminus F$

$\sequence {x_n}_{n \mathop \in \N}$ converges to $x$.

Therefore:


 * $\displaystyle \forall \epsilon \in \R_{> 0} : \exists N \in \N : \forall n \in N : n > N \implies \norm {x_n - x} < \epsilon$

Hence:


 * $\norm {x_{N + 1} - x} < \epsilon$.

Then we have:


 * $x_{N + 1} \in \map {B_\epsilon} x \subset X \setminus F$

But at the same time:


 * $x_{N + 1} \in F$

and we reached a contradiction.

Definition 2 implies Definition 1
Suppose, $x \in F$.

$X \setminus F$ is closed.

Then:


 * $\neg \paren {\forall x \in X \setminus F : \exists \epsilon \in \R_{> 0} : \map {B_\epsilon} x \subseteq X \setminus F}$

By Assertion of Existence, this is equivalent to:


 * $\exists x \in X \setminus F : \forall \epsilon \in \R_{> 0} : \map {B_\epsilon} x \cap F \ne \empty$

Set $\displaystyle \epsilon = \frac 1 n$ with $n \in \N$.

Then:


 * $\exists n \in \N : x_n \in \map {B_\epsilon} x \cap F$

So there is a sequence $\sequence {x_n}_{n \mathop \in \N}$ such that:


 * $\displaystyle \forall n \in \N : n > N \implies \norm {x - x_n} < \frac 1 n$

and $x_n$ converges to $x$.

Therefore, $x \notin F$, and we reached a contradiction.