# Prove that the product of a rational and irrational number is irrational

Could you please confirm if this proof is correct?

Theorem: If $q \neq 0$ is rational and $y$ is irrational, then $qy$ is irrational.

Proof: Proof by contradiction, we assume that $qy$ is rational. Therefore $qy=\frac{a}{b}$ for integers $a$, $b \neq 0$. Since $q$ is rational, we have $\frac{x}{z}y=\frac{a}{b}$ for integers $x \neq 0$, $z \neq 0$. Therefore, $xy = a$, and $y=\frac{a}{x}$. Since both $a$ and $x$ are integers, $y$ is rational, leading to a contradiction.

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Looks good but what happened to the $z$ and $b$ in the line $xy = a$? –  Eric O. Korman Jun 13 '11 at 12:59
It's surely not quite correct. For example, you missed factors of $1/z$ and $1/b$ in evaluating or "simplifying" $xy/z=a/b$. Otherwise the logic is OK. –  Luboš Motl Jun 13 '11 at 13:01
You just get the relation $y=\frac{za}{xb}$ which is a ratio of integers and therefore rational. –  Beni Bogosel Jun 13 '11 at 13:17
@Eric: Hm, I assumed that if $\frac{a}{b}=\frac{c}{d}$ then $a=c$. Which is not true... –  persepolis Jun 13 '11 at 13:24
Minor point: this is not a proof by contradiction, you prove that qy is irrational by proving that it is not rational, this is just the definition of being irrational. –  Guillaume Brunerie Oct 28 '11 at 16:11

It's wrong. You wrote $\frac{x}{z}y = \frac{a}{b}$. That is correct. Then you said "Therefore $xy = a$. That is wrong.

You need to solve $\frac{x}{z}y = \frac{a}{b}$ for $y$. You get $y = \frac{a}{b} \cdot \frac{z}{x}$.

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You can directly divide by $q$ assuming the fact that $q \neq 0$.

Suppose $qy$ is rational then, you have $qy = \frac{m}{n}$ for some $n \neq 0$. This says that $y = \frac{m}{nq}$ which says that $\text{y is rational}$ contradiction.

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As I mention here frequently, this ubiquitous property is simply an instance of complementary view of the subgroup property, i.e.

THEOREM $\$ A nonempty subset $\rm\:S\:$ of abelian group $\rm\:G\:$ comprises a subgroup $\rm\iff\ S\ + \ \bar S\ =\ \bar S\$ where $\rm\: \bar S\:$ is the complement of $\rm\:S\:$ in $\rm\:G$

Instances of this are ubiquitous in concrete number systems, e.g.

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A group theoretic proof: You know that if $G$ is a group and $H\neq G$ is one of its subgroups then $h \in H$ and $y \in G\setminus H$ implies that $hy \in G\setminus H$. Proof: suppose $hy \in H$. You know that $h^{-1} \in H$, and therefore $y=h^{-1}(hy) \in H$. Contradiction.

In our case, we have the group $(\Bbb{R}^*,\cdot)$ and its proper subgroup $(\Bbb{Q}^*,\cdot)$. By the arguments above $q \in \Bbb{Q}^*$ and $y \in \Bbb{R}\setminus \Bbb{Q}$ implies $qy \in \Bbb{R}\setminus \Bbb{Q}$.

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Shouldn't it be $\mathbb{R}^{\ast} \setminus\mathbb{Q}^{*}$ –  user9413 Jun 13 '11 at 13:29
yes, but you can see that $\Bbb{R}\setminus \Bbb{Q}=\Bbb{R}^*\setminus \Bbb{Q}^*$. –  Beni Bogosel Jun 13 '11 at 13:59
You are right, great answer :) –  user9413 Jun 13 '11 at 14:02

Let's see how we can modify your argument to make it perfect.

First of all, a minor picky point. You wrote $$qy=\frac{a}{b} \qquad\text{where a and b are integers, with b \ne 0}$$

So far, fine. Then come your $x$ and $z$. For completeness, you should have said "Let $x$, $z$ be integers such that $q=\frac{x}{z}$. Note that neither $x$ nor $z$ is $0$." Basically, you did not say what connection $x/z$ had with $q$, though admittedly any reasonable person would know what you meant. By the way, I probably would have chosen the letters $c$ and $d$ instead of $x$ and $z$.

Now for the non-picky point. You reached $$\frac{x}{z}y=\frac{a}{b}$$ From that you should have concluded directly that $$y=\frac{za}{xb}$$ which ends things, since $za$ and $xb$ are integers.

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I don't think it correct. It seems like a good idea to indicate both x as an integer, and z as a non-zero integer. Then you also want to "solve for" y, which as Eric points out, you didn't quite do.

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