Recall the classical Riemann zeta function:
and the Dirichlet L-functions for a character :
defined for . These functions can be analytically continued to the entire complex plane (except a pole at in the case of ). In particular, the values at non positive integers carry great arithmetic significance and enjoy many properties.
For instance, is an algebraic integer and in fact equal to where are the generalized Bernoulli numbers. Moreover, these values satisfy p-adic congruences and integrality properties such as the Kummer congruence (and generalizations to the L-functions).
The standard proof of these proceeds by showing that satisfies a functional equation that relates to and then computing the values for integral using analytic techniques (such as Fourier analysis).
Proving the p-adic properties is then by working directly with the definition of the generalized Bernoulli numbers instead of the L-functions. However, the L-functions are clearly the fundamental object here and it would be nice to have a way to directly work with the values at negative integers (without using the functional equation).
One might be tempted to extend the series definition to the negative integers and say:
For instance, consider the following (bogus) computation:
. . .
and let us “sum” the columns first:
which, remarkably enough, is the right generating function for ! We have exchanged the summation over two divergent summations and ended up with the right answer.
In fact, it is possible to rigorously justify this procedure of divergent summation and moreover, one can use it to prove a lot of arithmetic properties of these values rather easily (like the Kummer congruence). I learnt the basic method from some lecture notes of Prof. Akshay Venkatesh here: Section 3, Analytic Class Number formula and L-functions.
I then discovered that one could use these techniques to compute the explicit values (along the outline above) and prove some more stuff. I wrote this up in an article (that also explains the basic technique and should be (almost) self contained) here: Divergent Series and Dirichlet L-functions.