# An elementary way to show any bounded subset of $\Bbb{R}^k$ is totally bounded

I'm trying to show that any subset bounded of $\Bbb{R}^k$ is totally bounded.

Here is what I did:

(1)A subset of a totally bounded Set is bounded:

Proof: Let $X$ be a totally bounded subset and $Y\subset X$ then there exists an $\epsilon /2$-net $\{x_1,x_2,..,x_n\}$ and $X\subset \displaystyle\bigcup_{i=1}^n B(x_i,\epsilon/2)$ . Let $\{x_1,x_2,..,x_m\}$ be the points whose balls contain $Y$ $(m\le n)$. Now $\forall i \in \{1,..,m\} \exists q_i \in A \cap B(x_i,\epsilon/2)$ and $B(x_i,\epsilon/2)\subset B(q_i,\epsilon)$. We have for every $x\in B(x_i,\epsilon/2)$ $$d(x,q_i)\le d(x,x_i)+d(x_i,q_i) < \frac \epsilon 2 + \frac \epsilon 2 =\epsilon$$

Hence $Y \subset \displaystyle\bigcup_{i=1}^m B(x_i,\epsilon/2) \subset \bigcup_{i=1}^m B(q_i,\epsilon)$ and $q_i \in Y$ for all $i$ hence $Y$ is totally bounded

Back to the problem:

Let $A \subset \Bbb{R}^k$ be a bounded set then $A \subset B(0,R)$ for some $R$ then $A \subset [-R,R] \times [-R,R] \times ...\times [-R,R]$ then $A$ is a subset of a compact set by the Heine-Borel Theorem which is also a totally bounded set, hence by (1) $A$ is totally bounded.

I'm trying to do this problem by only using (1) without invoking the Heine-Borel theorem, can anyone tell me how that can be done? (Is the proof of (1) right for that matter?)

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Your proof of (1) is incorrect. There's no reason that the $x_i$ are in $Y$. You have to work a little bit harder. – JSchlather Feb 23 '13 at 13:08
@JSchlather Thanks for pointing that out I overlooked it. Is it correct now? I made sure the points are in $Y$ – user10444 Feb 23 '13 at 13:36
@user10444 Note that what I wrote is essentially Emanuele's take on the problem. – Pedro Tamaroff Mar 2 '13 at 16:22

## 2 Answers

I'll be systematic here, I think it can help.

D Let $S$ be any subset of $\Bbb R^n$. Given $\epsilon >0$, we say that $N$ is an $\epsilon$-net for $S$ if the set of open balls

$$B_\epsilon(N)=\{B(x,\epsilon):x\in N\}$$

covers $S$. That is, the set of open balls of radius $\epsilon$ centered at the points of $N$ cover $S$.

D We say a subset $S$ of $\Bbb R^n$ is precompact or totally bounded if for every $\epsilon >0$; there exists a finite $\epsilon$-net for $S$.

T Let $S$ be bounded in $\Bbb R^n$. Then $S$ is precompact.

P Boundedness implies $S$ is contained in some closed ball $B$. But each of these balls contain but a finite number of elements of the form

$${\bf x}_{\ell,{\bf k}}= \left(\frac{k_1}{2^\ell},\dots,\frac{k_n}{2^\ell}\right)$$ for $k_i,\ell \in \Bbb Z\;\;;\ell \geq 0$, a fixed number, while the $k_i$ varies independently through the integers. But then, given $\epsilon >0$, we can take $\ell$ sufficiently large so that $\frac 1 {2^\ell}<\epsilon$, and the set of such points ${\bf x}_{\ell,k}$ contained in $B$ will be a finite $\epsilon$-net for $S$.

NOTE Observe the proof simply relies in producing what we usually think a net is: we show the intersection of our ball with the grid of "mesh" $1/2^\ell$ is finite, and then show that this intersection is an $\epsilon$-net (since we make $2^{-\ell}$ small) of the underlying set $S$ inside $B$. Note that we use the $k$ in the denominator the eventually "get out" of the open ball (since it is bounded, some natural will make $k/2^\ell$ "leave" the ball, no matter how small $2^{-\ell}$ is.)

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If $B$ is bounded in $\mathbb R^n$ it is contained in some cube $[-R,R]^n$. Then for any $\epsilon>0$ just consider all balls of radius $\epsilon$ centered in the set $$(\frac \epsilon 2\mathbb Z)^n \cap [-R,R]^n$$ these balls are a finite number (less then $(4R/\epsilon+1)^n$) and cover the whole cube $[-R,R]$.

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Could you please clarify your notations? What is $(\frac \epsilon 2\mathbb Z)^n$ – user10444 Feb 23 '13 at 13:38
@user10444 $\newcommand{\eps}{\varepsilon}$ In general for a set of real numbers $A$ and a real number $c$ we can define $cA=\{cx \mid x \in A\}$ in your case $\eps \mathbb Z=\{\eps z \mid z \in \mathbb Z\}$. – JSchlather Feb 23 '13 at 13:47
I'm sorry but I can't seem to understand this at all, what is the center of each ball? Why is in contained in B(the center that is)? – user10444 Feb 23 '13 at 13:52
@user10444 You don't need to be as careful now that you've proven your lemma, you just need to show each bounded set is contained in a totally bounded set. – JSchlather Feb 23 '13 at 13:59
@Emanuele Paolini What is the radius of these balls? – user10444 Feb 23 '13 at 15:28