# Thread: Proving something is compact

1. ## Proving something is compact

Let $K \subset R^1$ consist of $0$ and the numbers $1/n$ for any $n = 1, 2, 3 ...$ Prove that $K$ is compact directly from the definition (w/o using the Heine-Borel Theorem).

Definition from the book:
A subset $K$ of a metric space $X$ is said to be compact if every open cover of $K$ contains a finite subcover. More explicitly, the requirement is that if $\{G_{\alpha}\}$ is an open cover of $K$, then there are finitely many indices $\alpha_1, ... \alpha_n$ such that $K \subset G_{\alpha_1} \bigcup ... \bigcup G_{\alpha_n}.$

2. I cannot resist first telling you the easy method:

Let $A = \{\frac{1}{n}|n\in\mathbb{N}\}\subset\mathbb{R}$. Then the closure of A is equal to your set $K$, whereas $K$ is closed. Clearly, $K$ is also bounded, which is why $K$ is compact.

Maybe you already thought of this.

The other (and in this case legal) way: Let $\{G_\alpha\}$ be an open cover of $K$. In particular, $0\in K$ is contained in some $G_\alpha$, say $0\in G_{\alpha_0}$.

An open subset of $\mathbb{R}$ is a disjoint union of open intervals. In particular, the point 0 is contained in an open interval inside of $G_{\alpha_0}$. Suppose this open interval has $\delta$ as its right end point. Then all points of $K$ with $\frac{1}{n}<\delta$ are also contained in this interval, i.e. all such points are contained in $G_{\alpha_0}$. But all but finitely many points $1,\frac{1}{2},\frac{1}{3},\ldots,\frac{1}{k}$ of $K$ satisfy $\frac{1}{n} < \delta$, and so all but finitely many points of $K$ are contained in $G_{\alpha_0}$.

Since we have a cover of the set $K$, the point $\frac{1}{j}$ is contained in some set $G_{\alpha_j}$, for each $j=1,2,\ldots,k$. But then the cover

$\{G_{\alpha_0}, G_{\alpha_1},G_{\alpha_2},\ldots,G_{\alpha_k}\}$

is a finite subcover of $\{G_\alpha\}$.