Furstenberg's proof of the infinitude of primes

In number theory, Hillel Furstenberg's proof of the infinitude of primes is a celebrated topological proof that the integers contain infinitely many prime numbers. When examined closely, the proof is less a statement about topology than a statement about certain properties of arithmetic sequences.^[1] Unlike Euclid's classical proof, Furstenberg's proof is a proof by contradiction. The proof was published in 1955 in the American Mathematical Monthly while Furstenberg was still an undergraduate student at Yeshiva University.

Furstenberg's proof

Define a topology on the integers Z, called the evenly spaced integer topology, by declaring a subset U ⊆ Z to be an open set if and only if it is either the empty set, ∅, or it is a union of arithmetic sequences S(a, b) (for a ≠ 0), where

S(a, b) = \{ a n + b\, |\, n \in \mathbb{Z} \} = a \mathbb{Z} + b. \,

In other words, U is open if and only if every x ∈ U admits some non-zero integer a such that S(a, x) ⊆ U. The axioms for a topology are easily verified:

By definition, ∅ is open; Z is just the sequence S(1, 0), and so is open as well.
Any union of open sets is open: for any collection of open sets U_i and x in their union U, any of the numbers a_i for which S(a_i, x) ⊆ U_i also shows that S(a_i, x) ⊆ U.
The intersection of two (and hence finitely many) open sets is open: let U₁ and U₂ be open sets and let x ∈ U₁ ∩ U₂ (with numbers a₁ and a₂ establishing membership). Set a to be the lowest common multiple of a₁ and a₂. Then S(a, x) ⊆ S(a_i, x) ⊆ U_i.

This topology has two notable properties:

Since any non-empty open set contains an infinite sequence, a finite set cannot be open; put another way, the complement of a finite set cannot be a closed set.
The basis sets S(a, b) are both open and closed: they are open by definition, and we can write S(a, b) as the complement of an open set as follows:

S(a, b) = \mathbb{Z} \setminus \bigcup_{j = 1}^{a - 1} S(a, b + j).

The only integers that are not integer multiples of prime numbers are −1 and +1, i.e.

\mathbb{Z} \setminus \{ -1, + 1 \} = \bigcup_{p \mathrm{\, prime}} S(p, 0).

By the first property, the set on the left-hand side cannot be closed. On the other hand, by the second property, the sets S(p, 0) are closed. So, if there were only finitely many prime numbers, then the set on the right-hand side would be a finite union of closed sets, and hence closed. This would be a contradiction, so there must be infinitely many prime numbers.

Notes

↑ Mercer, Idris D. (2009). "On Furstenberg's Proof of the Infinitude of Primes". American Mathematical Monthly 116: 355–356. doi:10.4169/193009709X470218.

References

Aigner, Martin; Ziegler, Günter M. (1998). "Proofs from The Book". Berlin, New York: Springer-Verlag.
Furstenberg, Harry (1955). "On the infinitude of primes". American Mathematical Monthly (Mathematical Association of America) 62 (5): 353. doi:10.2307/2307043. JSTOR 2307043. MR 0068566

Furstenberg's proof of the infinitude of primes