A Purely Algebraic Summation Method

Abstract

It is mathematical folklore that 1 + 2 + 3 + 4 + ... = --1/12. This result is usually achieved using elaborate analytical methods, such as zeta function regularization or Ramanujan summation. However, in its notebooks, Ramanujan has also provided a very simple derivation which relied instead on algebraic manipulations. Recently, a video from Numberphile has presented a similar derivation of the result (provoking lots of discussions and debates about the meaning of such an equality). But this derivation, simple as it is, is usually considered as less rigorous than those using more elaborate analytical methods. However, this derivation is indeed perfectly rigourous, and in this article, we will define a general algebraic construction which we will use as a framework for expressing this derivation and, more generally, for providing a new summation method.