# One and One and One Make Three

Every once in a while, a bit of esoteric mathematics drifts into more popular view and leaves poor souls like me wondering, “Why?”

Why is this piece of gee-whizzery being waved about, when the popularized “explanation” of it is so warped as to be misleading? Is the goal of “popularizing mathematics” just to inflate the reader’s ego—the intended result being, “Look what I understand!,” or, worse, “Look at what those [snort] professional mathematicians are saying, and how obviously wrong it is.”

Today’s instalment (noticed by my friend Dr. SkySkull): the glib assertion going around that

$$1 + 2 + 3 + 4 + 5 + \cdots = -\frac{1}{12}.$$

Sigh.

It’s like an Upbuzzdomeworthy headline: These scientists added together all the counting numbers. You’ll never guess what happened next!

“This crazy calculation is actually used in physics,” we are solemnly assured.

Sigh.

The physics side of the story is, roughly, “Sometimes you’re doing a calculation and it looks like you’ll have to add up $$1+2+3+4+\cdots$$  and so on forever. Then you look more carefully and realize that you shouldn’t—something you neglected matters. It turns out that you can swap in $$-1/12$$ for the corrected calculation and get a good first stab at the answer. More specifically, swapping in $$-1/12$$ tells you the part of the answer which doesn’t depend on the particular details of the extra effect you originally neglected.”

For an example of this being done, see David Tong’s notes on quantum field theory, chapter 2, page 27. For the story as explained by a mathematician, see Terry Tao’s “The Euler-Maclaurin formula, Bernoulli numbers, the zeta function, and real-variable analytic continuation.” As that title might hint, these do presume a certain level of background knowledge, but that’s kind of the point. This is an instance where the result itself requires at least moderate expertise to understand, unlike, say, the four-colour theorem, where the premise and the result are pretty easy to set out, and it’s the stuff in between which is much harder to follow.

ADDENDUM (19 January 2014): I’ve heard the argument in favour of this gee-whizzery that it “gets people excited about mathematics.” So what? A large number of people are misinformed; a tiny fraction of that population goes on to learn more and realize that they were, essentially, lied to. Getting people interested in mathematics is a laudable goal, but you need to pick your teaser-trailer examples more carefully.

And I see Terry Tao has weighed in himself with a clear note and some charming terminology.