# Show that $S=\mathbb{Q} \cap [0,2]$ is not compact

Question: Let $S$ be the set of rational numbers in the interval $[0, 2]$. Using the definition of compactness, show that $S$ is not compact.

Using the definition of closeness/sequential-compactness, it is easy to show that $S$ is not closed/sequential-compact, and thus it is not compact.

But I am trying to show non-compactness of $S$ by a counterexample using purely definition of compactness, but it is not possible; since every covering example for $A=[0, 2]$ seems to be the same for $S=\mathbb{Q} \cap A$.

I highly appreciate some guidance.

EDIT Answers in here are either relevant to Topology or using non-closeness as an intermediate.

• Try an open cover consisting of sets of the form $(S\cap[0,\sqrt2-1/n))\cup(S\cap(\sqrt2+1/n,2])$. – David Mitra Jan 31 '15 at 10:05
• @David Mitra: Thank you very much. Just a bit question: Is it $(-0.1,\sqrt2-1/n)\cup(\sqrt2+1/{(2n)},2.1)$? $[]\rightarrow ()$ because it must be (?) open intervals, and $n\rightarrow 2n$ because of definition of interval? – L.G. Jan 31 '15 at 10:17
• You can work with those sets also. Probably better than what I suggested (where I thought of the ambient space as $S$ with the subspace topology). – David Mitra Jan 31 '15 at 10:26
• "every covering example for $A=[0, 2]$ seems to be the same for $S=\mathbb{Q} \cap A$." This is true but one should start from some covering of $S=\mathbb{Q} \cap A$. Since some them are not covering $A$, this makes all the difference. – Did Jan 31 '15 at 10:48
• You might want to be a bit clearer about which definition of compactness you want us to use. It takes a bit of effort to determine that. – robjohn Jan 31 '15 at 15:32

Although there is nothing wrong with proofs that do, I tried not to reference irrational numbers in the following proof.

Define a Sequence of Squares Converging to $\boldsymbol{2}$

Consider the sequence $$a_1=1\quad\text{and}\quad a_{n+1}=\frac{a_n}2+\frac1{a_n}\tag{1}$$ If $a_n\in[1,2]$, then $(1)$ implies $a_{n+1}\in[1,2]$.

Each $a_n$ is in $\mathbb{Q}\cap[0,2]$ and \begin{align} a_{n+1}^2-2 &=\frac{a_n^2}4-1+\frac1{a_n^2}\\ &=\left(\frac{a_n^2-2}{2a_n}\right)^2\tag{2} \end{align} For $x\gt0$, $\dfrac{x^2-2}{2x}=\dfrac x2-\dfrac1x$ is monotonically increasing. Therefore, for $a_n\in[1,2]$ we have $$-\frac12\le\frac{a_n^2-2}{2a_n}\le\frac12\tag{3}$$ Combining $(2)$ and $(3)$ gives \begin{align} \left|a_{n+1}^2-2\right| &=\left(\frac{a_n^2-2}{2a_n}\right)^2\\ &=\left|\frac{a_n^2-2}{2a_n}\right|\frac1{2a_n}\left|a_n^2-2\right|\\ &\le\frac14\left|a_n^2-2\right|\tag{4} \end{align} which implies $$\lim_{n\to\infty}|a_n^2-2|=0\tag{5}$$

Define an Infinite Cover of $\boldsymbol{\mathbb{Q}\cap[0,2]}$

Consider the open sets $$U_n=\left\{x\in\mathbb{Q}\cap[0,2]:|x^2-2|\gt\frac1n\right\}\tag{6}$$

According to this answer there is no rational number so that $x^2=2$; therefore, we have $$\bigcup_{n=1}^\infty U_n=\mathbb{Q}\cap[0,2]\tag{7}$$ Since $\bigcup\limits_{n=1}^mU_n=U_m$, the union of any finite collection of $U_n$ is another $U_n$. $(5)$ guarantees that no matter how big we choose $n$, there is an $a_{k_n}\not\in U_n$. Since $a_{k_n}\in\mathbb{Q}\cap[0,2]$, there is no $U_n$ that contains all of $\mathbb{Q}\cap[0,2]$.

That is, no finite subset of $\{U_n\}$ can cover all of $\mathbb{Q}\cap[0,2]$. Therefore, $$\mathbb{Q}\cap[0,2]\text{ is not compact.}\tag{8}$$

• I have updated the answer a bit to use the desired definition of compactness. – robjohn Jan 31 '15 at 18:10

Disclaimer: Somehow I've missed the comment of David Mitra; as my answer is essentially the same, I'm making this CW. I'm not deleting because here $U_i\cap U_j = \varnothing$, so peraps it is more clear that no finite cover exists (all the sets $U_n$ have to be taken).

Take any strictly decreasing sequence $(a_n)_{n=1,2,\ldots}$ of non-rational numbers that tends to $\sqrt{2}$ with $a_1 < 3$, for example

$$a_n = \sqrt{2}+\frac{1}{n+2015}$$

Now consider a sequence of open sets:

\begin{align} U_0 &= (-1,\sqrt{2})\\ U_1 &= (a_1,3)\\ U_n &= (a_n,a_{n-1}) &\text{ for } n \geq 2 \end{align}

Obviously $\mathbb{Q}\cap [0,2] \subset \bigcup_{k=1}^{\infty} U_k$, but no finite cover exists.

I hope this helps $\ddot\smile$

A compact set has to be bounded and closed. We can easily see that closedness is violated here. For example, take the famous Leibniz formula $$\frac{\pi}{4}= 1-\frac13+\frac15-\frac17+\frac19-\cdots$$ The sequence of partial sums are all rational numbers in between 0 and 2, however their limit point is not rational, so the set $S$ is not closed.