The Riemann zeta function and prime numbers

In a previous post I defined the famous Riemann zeta function,

\displaystyle \displaystyle \zeta(s) = \sum_{n \geq 1} \frac{1}{n^s}.

Today I want to give you a glimpse of what it has to do with prime numbers—which is a big part of why it is so famous.

Consider the infinite product

\displaystyle \left(\frac{1}{1-2^{-s}}\right)\left(\frac{1}{1-3^{-s}}\right)\left(\frac{1}{1-5^{-s}}\right)\left(\frac{1}{1-7^{-s}}\right) \dots \left(\frac{1}{1-p^{-s}}\right) \dots

where each sequential factor has the next prime number raised to the -s power. Using big-Pi notation, we can write this infinite product more concisely as

\displaystyle \prod_{p \text{ prime}} \frac{1}{1 - p^{-s}}

(The big \Pi means a \Piroduct just like a big \Sigma means a \Sigmaum.)

Now let’s do a bit of algebra. First, recall that the infinite geometric series 1 + r + r^2 + r^3 + \dots has the sum

\displaystyle 1 + r + r^2 + r^3 + \dots = \frac{1}{1-r},

as long as r < 1. (For some hints on how to derive this formula if you haven’t seen it before, see this blog post or this one.) Of course, 1/(1-p^{-s}) is of this form, with r = p^{-s}. Note that p^{-s} = 1/p^s which is less than 1 as long as s > 0, so the geometric series formula applies, and we have

\displaystyle \prod_p \frac{1}{1 - p^{-s}} = \prod_p (1 + p^{-s} + p^{-2s} + p^{-3s} + \dots)

(From now on I’ll just write \prod_p instead of \prod_{p\text{ prime}}.) That is,

\displaystyle (1 + 2^{-s} + 2^{-2s} + \dots)(1 + 3^{-s} + 3^{-2s} + \dots)(1 + 5^{-s} + 5^{-2s} + \dots) \dots

So this is an infinite product of infinite sums! But you’re not scared of a little infinity, are you? Good, I thought not. Now, what would happen if we “multiplied out” this infinite product of infinite sums? Note that every term in the result would come from picking one term of the form p^{-ks} from each of the factors, one for each prime p, and multiplying them. (Though infinitely many of the choices have to be 1 = p^{-0s} if we are to get a finite term as a result.) For example, one way to choose terms would be

\displaystyle 1 \cdot 3^{-2s} \cdot 5^{-s} \cdot 1 \cdot 13^{-3s} \cdot 1 \cdot 1 \cdot \dots

which would give us (3^2 \cdot 5 \cdot 13)^{-s} = 585^{-s}. In fact, because of the Fundamental Theorem of Arithmetic (every positive integer has a distinct prime factorization), each choice gives us the prime factorization of a different positive integer, and conversely, every positive integer shows up exactly once. That is, after multiplying everything out, we get one term of the form n^{-s} for each positive integer n:

\displaystyle \prod_p \frac{1}{1 - p^{-s}} = \sum_{n \geq 1} n^{-s} = \sum_{n \geq 1} \frac{1}{n^s}

But that’s just our old friend \zeta(s)! So in fact,

\displaystyle \zeta(s) = \prod_p \frac{1}{1 - p^{-s}}

turns out to be an equivalent way to write the Riemann zeta function.

We can now use this in a really cute proof that there are infinitely many primes. Consider \zeta(1), where we substitute 1 for s. In our original definition of \zeta, we get

\displaystyle \zeta(1) = \sum_{n \geq 1} \frac{1}{n} = 1 + 1/2 + 1/3 + 1/4 + 1/5 + \dots

This is known as the harmonic series, and it is a well-known fact that it diverges, that is, as you keep adding up more and more terms of the series, the sum keeps getting bigger and bigger without bound. Put another way, pick any number you like—a hundred, a million, a trillion—and eventually, if you keep adding long enough, the sum 1 + 1/2 + 1/3 + 1/4 + 1/5 + \dots will become bigger than your chosen number. (Though you may have to wait a very long time—the harmonic series diverges rather slowly indeed!) One way to prove this is to note that the series 1 + 1/2 + 1/3 + 1/4 + 1/5 + \dots is greater than

\displaystyle 1 + 1/2 + (1/4 + 1/4) + (1/8 + 1/8 + 1/8 + 1/8) + (1/16 + \dots) + \dots

(the original series is greater than this because we only made some of its terms smaller—I changed the 1/3 into 1/4, and then changed 1/5 through 1/7 into 1/8, and then 1/9 through 1/15 into 1/16, and so on). But this new smaller series is equal to 1 + 1/2 + 1/2 + 1/2 + 1/2 + \dots which will clearly get arbitrarily large. So the harmonic series, which is larger, must diverge as well.

So, \zeta(1) diverges. But what happens if we plug s = 1 into the other expression for \zeta(s)? We get

\displaystyle \prod_p \frac{1}{1 - p^{-1}} = \prod_p \frac{1}{1 - 1/p} = \prod_p \frac{p}{p-1}.

If there were only finitely many primes, this would be a finite product of some fractions and would thus have some definite, finite value—but we know it has to diverge! Thus there must be infinitely many primes.

I will note one other thing—when I was writing up some notes for this post I was initially confused by the fact that if we set, say, s = 2, we already know that \zeta(2) = \pi^2/6; but now we also know that

\displaystyle \zeta(2) = \prod_p \frac{p^2}{p^2 - 1} = \frac{4}{3} \cdot \frac{9}{8} \cdot \frac{25}{24} \cdot \frac{49}{48} \cdot \frac{121}{120} \dots

But this is an infinite product of fractions which are all bigger than 1! How could it converge? …well, my intuition was just playing tricks with me. Although I have lots of practice thinking about infinite sums that converge, I am just not used to thinking about infinite products that converge. But in the end it is not really any more surprising than the fact that an infinite sum can converge even though all its terms are positive: as long as the fractions are getting smaller quickly enough, such an infinite product certainly can converge, and in fact it does. Using a computer confirms that the more terms of this product we include, the closer the product gets to \pi^2 / 6 \approx 1.64493\dots

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About Brent

Assistant Professor of Computer Science at Hendrix College. Functional programmer, mathematician, teacher, pianist, follower of Jesus.
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15 Responses to The Riemann zeta function and prime numbers

  1. Anon says:

    From the last computation you can also note that the fact that \pi^2/6 is irrational implies that there are infinitely many primes (because a finite product of rational numbers is rational).

    • Brent says:

      Ah, good point! Though I have to ask — how do you know that \pi^2/6 is irrational?

      • sn0wleopard says:

        Let’s assume that x = pi^2/6 is rational. Then, pi = sqrt(6x), which is merely irrational, not transcendental. So, it seems that we’ve reached a contradiction 🙂

        • sn0wleopard says:

          P.S.: The above actually proves a stronger result: pi^2/6 must be transcendental.

        • Brent says:

          Indeed, so this depends on knowing \pi is transcendental — a rather big result! Using the transcendence of \pi to prove the infinitude of the primes is like dropping an anvil on a mosquito. (Which can be rather fun, of course.) (Though to be sure, I don’t know whether the proof of the transcendence of \pi already depends on the infinitude of primes…)

  2. Naren Sundar says:

    I haven’t thought about infinite products converging before but I was just wondering if it is possible to analyze convergence of infinite products by taking their `log` version and looking at the infinite sum instead?

    • Brent says:

      Good question! I am not an expert in this area, but I can’t think of any reason why this wouldn’t work.

    • Indeed, this is generally the way that the convergence of infinite products is handled. By definition (or, at least, by the usual definition), an infinite product \prod_n a_n is said to converge if and only if the corresponding sum \sum_n \log(a_n) converges. Note that this implies (for example) that the infinite product \prod_n 0 = 0 does NOT converge. In general, in order for an infinite product to converge, it cannot “converge” to either 0 or infinity!

  3. Curtis Bechtel says:

    I think you meant to say ‘greater than’ when you compared the harmonic series to 1 + 1/2 + (1/4 + 1/4) + …

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