# How to study the irrational numbers with a high school math background?

Recently a friend posed the question "can the product of two irrational numbers be rational?" We the trivial answers like for example $\sqrt{2}\sqrt{8} = 4$. I have become somewhat obsessed with the question and I would like to ask if anyone would have an idea on what field(s) of mathematics that one could pursue in order to reason and investigate this question further?

-
Excuse the question, but what is there to "investigate" in this question further?? – DonAntonio Mar 24 '14 at 17:50
I suggest getting Ivan Niven's book Numbers: Rational and Irrational. – Dave L. Renfro Mar 24 '14 at 18:06
The set of irrational numbers is simply nowhere near closed under multiplication. If you give me any irrational numbers $x_1,\dots,x_k$, either their product is already rational, or I can give you an irrational number $y$ such that $x_1\cdots x_ky$ is rational (for example, $y=1/x_1\cdots x_k$). The question is like asking "can the concatenation of two non-words be a word?". – Greg Martin Mar 24 '14 at 18:23
@DonAntonio I think the OP is curious about mathematics and asking how they can educate themselves to get to the point where they can conclude there is nothing to investigate. (This is a much better question in my opinion than the standard copy and paste of homework along with the instruction to show our working neatly.) – TooTone Mar 24 '14 at 20:04
@TooTone Exactly! I'm merely interested in looking in to the matter myself and seeing what I find. Unfortunately I didn't now exactly where to start with my somewhat limited understanding of mathematics (which I hope to improve on) – Tephra Mar 24 '14 at 22:15

• Galois Theory by Ian Stewart. It's surprisingly accessible given the field, but you will need to know linear algebra (and I believe nothing else past high school math). It's worth mentioning for its relevance to the topic, and for the fact that you can get to it relatively soon in your mathematical career, but it's too difficult for a high school math background. Possibly cool to just flip through and admire that if you use lots and lots of complicated polynomials, you can in fact prove $\pi^2$ is irrational.