# The Prime Bet

Let's say you sit in a pub, minding your own business, when all of a sudden a stranger walks up to you and offers you a bet:

We'll choose two positive integers at random. If they have any divisor in common (other than $1$) I'll pay you a dollar, else you'll pay me a dollar. Are you in?

Apart from the question what kind of establishments you frequent, you should be wondering: is this a good bet for you?

When two integers have no divisors in common except the trivial divisor $1$ we say they are coprime or relatively prime. $6$ and $9$ have the common divisor $3$, so they are not coprime, whilst $8$ and $9$ only have the trivial common divisor $1$, so they are coprime.

This makes you start thinking: "As numbers grow bigger, aren't there a lot of divisors out there? After all, half the numbers are even, so if we hit two even numbers, they'll have the factor $2$ in common and I'll win. And then there's $3$, $5$, $7$, ... Seems like a good deal!" Continue reading The Prime Bet

# Primes from a Different World

Every textbook on number theory will begin with a treatise on prime numbers; every treatise on prime numbers will begin by emphasising their importance as building blocks or atoms of our number system: every integer can be expressed as a product of prime numbers in one way and one way only. Six is two times three and there is no other way to decompose it.1 Euclid proved this over two thousand years ago and it is so fundamental (hence the name fundamental theorem of arithmetic) to our thinking about numbers that we take it for granted. It is not!

There are numbers that behave very much like the integers but have a different structure. One rather simple example are the Gaussian integers (usually denoted $\mathbb Z[i]$) which look just like complex numbers $z=x+iy$, except that $x$ and $y$ are restricted to integer values. They live in the complex plane, but exclusively on a discrete grid amongst their continuous cousins.

1. Yes, you can write 12 as 3*4 or 2*6, but you can continue either way and eventually reach the unambiguous 2*2*3.