In this post we discuss a proof for the infinitude of primes that uses topology (due to Furstenberg, see [2]). This is an interesting proof that gives a topological view point to a very familiar and basic mathematical fact. It is an elegant proof that is worthy to be considered as straight from “The Book” by Paul Erdos (at least “The Approximate Book”). Specifically it is one of the six proofs for the infinitude of primes found in [1].

Let be the set of all integers and be the set of all prime numbers. The key to the proof is to define a topological space on such that the assumption that is finite will contradict some fact about this topological space.

____________________________________________________________________

**Defining the Topological Space**

For any with , define . Let be the set of all possible . We show that is a base for a topology on the space .

We can visualize by thinking being in the center and the other points of are generated by adding integer multiples of to the center (in both the positive and negative directions). A useful observation is that we can make any point in the center and we can still generate the same set . Specifically, for each , we have .

We now show that is a base for a topology on . Clearly is a cover of . Next we show that if , then there is some with .

Let . Based on the observation made above, we have and . So we have . Observe that .

Let denote the space with the topology generated by the base . We need the following facts:

- Every non-empty open subset of is infinite.
- Every is a closed set in

The first bullet point is clear since every non-empty open set would have to contain one , which is infinite. To see that is closed, let . Observe that:

Thus the complement of is open is .

____________________________________________________________________

**The Infinitude of Primes**

We now tie this topological space to the prime numbers. Recall that denotes the set of all prime numbers. Also recall that any integer not equaled to 1 or -1 has a prime divisor. Thus we have the following:

If is finite, then the left-hand side of the above equation is a closed set (being the union of finitely many closed sets). This implies that is an open set in the space , which is impossible since every non-empty open set in this space is infinite. Thus must be infinite.

____________________________________________________________________

**Some Comments About This Topological Space**

The topological space generated by the base has nice topological properties. It is Hausdorff. It is regular since the base consists of open and closed sets. It is a separable metric space since it has a countable base. The space is non-discrete at every point. Thus it differs from the usual topology on the integers.

____________________________________________________________________

**Reference**

- Aigner, M., Gunter, M.Z.
*Proofs from THE BOOK, third edition*, Springer-Verlag, Berlin, 2004. - Furstenberg, H.,
*On the infinitude of primes*, Amer. Math. Monthly,**62**, 353, 1955.

____________________________________________________________________