Fürstenberg's proof ([1], [2]) that there are infinitely many primes is an amusing and beautiful blend of elementary number theory and point-set topology.

Consider the arithmetic progression topology on the positive integers, where a basis of open sets is given by subsets of the form $U_{a,b}=\{n\in\Z^+|n\equiv b\mod a\}$ . Arithmetic progressions themselves are by definition open, and in fact clopen, since

Consider the set $U=\cup_p U_{p,0}$ , where the union runs over all primes $p$ . Then the complement of $U$ in $\Z^+$ is the single element $\{1\}$ , which is clearly not an open set (every open set is infinite in this topology). Thus $U$ is not closed, but since we have written $U$ as a union of closed sets and a finite union of closed sets is again closed, this implies that there must be infinitely many terms appearing in that union, i.e. that there must be infinitely many distinct primes.

Bibliography

1

Furstenberg, Harry, On the infinitude of primes, American Mathematical Monthly, Vol. 62, 1955, p. 353.

2

Ribenboim, Paulo. The New Book of Prime Number Records. Springer, 1996. p. 10