Region Less One Point is Region

From ProofWiki
Jump to navigation Jump to search

Theorem

Let $M = \left({A, d}\right)$ be a dense-in-itself metric space.

Let $R \subseteq M$ be a region of $M$.

Let $x \in R$.

Then $R \setminus \left\{ {x}\right\}$ is also a region of $M$.


Proof

From the definition, a region is a non-empty, open, path-connected subset of $M$.


First, note that as $R$ is open it can not be a singleton from Finite Subspace of Dense-in-itself Metric Space is not Open.

Therefore $R \setminus \left\{{x}\right\}$ is not empty.


Next, we see that from Open Set Less One Point is Open that $R$ is open.


Now, let $\alpha, \beta \in R$.

As $R$ is path-connected, we can join $\alpha$ and $\beta$ with a path $\Gamma$.

If $x \notin \Gamma$, then $\Gamma$ is also a path in $R \setminus \left\{{x}\right\}$, and we are done.

If $x \in \Gamma$, then we consider the open $\epsilon$-ball $B_\epsilon \left({x}\right)$ of $x$ for some $\epsilon$ such that $B_\epsilon \left({x}\right) \subseteq R$.