Bernoulli numbers, Riemann zeta, and strange sums

In the previous post, we looked at sums of the first n consecutive powers, i.e. sums of the form

S_p(n) = \sum_{k=1}^n k^p

where p was a positive integer. Here we look at what happens when we let p be a negative integer and we let n go to infinity. We’ll learn more about Bernoulli numbers and we’ll see what is meant by apparently absurd statements such as 1 + 2 + 3 + … = -1/12.

If p < -1, then the limit as n goes to infinity of Sp(n) is ζ(-p). That is, for s > 1, the Riemann zeta function ζ(s) is defined by

\zeta(s) = \sum_{n=1}^\infty \frac{1}{n^s}

We don’t have to limit ourselves to real numbers s > 1; the definition holds for complex numbers s with real part greater than 1. That’ll be important below.

When s is a positive even number, there’s a formula for ζ(s) in terms of the Bernoulli numbers:

\zeta(2n) = (-1)^{n-1} 2^{2n-1} \pi^{2n} \frac{B_{2n}}{(2n)!}

 

The best-known special case of this formula is that

1 + 1/4 + 1/9 + 1/16 + … = π2 / 6.

It’s a famous open problem to find a closed-form expression for ζ(3) or any other odd argument.

The formula relating the zeta function and Bernoulli tells us a couple things about the Bernoulli numbers. First, for n ≥ 1 the Bernoulli numbers with index 2n alternate sign. Second, by looking at the sum defining ζ(2n) we can see that it is approximately 1 for large n. This tells us that for large n, |B2n| is approximately (2n)! / 22n-1 π2n.

We said above that the sum defining the Riemann zeta function is valid for complex numbers s with real part greater than 1. There is a unique analytic extension of the zeta function to the rest of the complex plane, except at s = 1. The zeta function is defined, for example, at negative integers, but the sum defining zeta in the half-plane Re(s) > 1 is NOT valid.

You may have seen the equation

1 + 2 + 3 + … = -1/12.

This is an abuse of notation. The sum on the left clearly diverges to infinity. But if the sum defining ζ(s) for Re(s) > 1 were valid for s = -1 (which it is not) then the left side would equal ζ(-1). The analytic continuation of ζ is valid at -1, and in fact ζ(-1) = -1/12. So the equation above is true if you interpret the left side, not as an ordinary sum, but as a way of writing ζ(-1). The same approach could be used to make sense of similar equations such as

12 + 22 + 32 + … = 0

and

13 + 23 + 33 + … = 1/120.

2 thoughts on “Bernoulli numbers, Riemann zeta, and strange sums

  1. I think it is really a disservice to non-mathematicians to be presenting the sum of the integers equal -1/12 “result” because the context to understand it is specialized and esoteric and just is not accessible to the layman. Tube channels like Numberphile shouldn’t try and I can’t help but think the motives behind such context-dropping is bad, i.e., to impress and/or intimidate others or to destroy math (nihilism). (To be clear, that doesn’t apply to this blog 🙂

  2. I can’t disagree with dmfdmf more. I really doubt that non-mathematicians are among John’s core followers and for us aspiring mathematicians the Numberphile video on this topic was engaging and stimulated further study. Thanks for your spin, John.

    PS – Here’s an article on the topic in “Physics Today”, about as mainstream as you can get http://scitation.aip.org/content/aip/magazine/physicstoday/news/10.1063/PT.5.8029?utm_medium=email&utm_source=Physics+Today&utm_campaign=5144944_Physics+Today%3a+The+year+in+physics+2014&dm_i=1Y69,329V4,HPHZYI,B01MG,1

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