# Homework Help: Closed and Open Subsets of a Metric Space

1. Feb 7, 2013

### gajohnson

1. The problem statement, all variables and given/known data

Let $X$ be an infinite set. For $p\in X$ and $q\in X$,

$d(p,q)=1$ for $p\neq q$ and $d(p,q)=0$ for $p=q$

Prove that this is a metric. Find all open subsets of $X$ with this metric. Find all closed subsets of $X$ with this metric.

2. Relevant equations

NA

3. The attempt at a solution

I showed easily that this is indeed a metric.

On the second part of the question, it seems to be the case that all subsets $\left\{x\right\}$ for all $x\in X$ are open because choosing a radius less than 1 gives a neighborhood around $x$ which only contains $x$ itself.

But then any subset of $X$ should be open, shouldn't it? Because each point of that subset can be shown to be an interior point using the logic above.

Similarly, there should be no closed subsets. Each point in a subset of $X$ obviously has a neighborhood which contains only that point.

Any ideas? Thanks!

2. Feb 7, 2013

### micromass

Correct. All sets are open.

You might want to rethink this.

3. Feb 7, 2013

### gajohnson

OK, so every subset of $X$ contains no limit points, so every subset of $X$ must be closed.

...so every subset of $X$ is both open and closed?

4. Feb 7, 2013

### micromass

That is correct!

5. Feb 7, 2013

### gajohnson

Excellent. Many thanks to you!