summation - Sum of divergent series

I saw a lot of article in Math SE like Why does 1+2+3+⋯=−1/12? and S=1+10+100+100+10000+…=−1/9? How is that and lot of others. Also I saw this one of Ramanujan summation but I do not get the contradiction.

I do not want to explain how the sum of such series is calculated since I read these articles but I want an explanation of the logic of these series.

• Are these a contradictory results?


• Where is the logic behind such series?


• How come the sum of infinite positive numbers is equal to a negative one?


• Is the problem with infinity $\infty$?


• If someone uses this result then this someone can create a lot of absurd results ($1=0$), how to explain this please?

I appreciate your help. Thanks.

Answer

L. Euler explained his assumptions about infinite series - convergent or divergent - with the following idea (just paraphrasing, don't have the article at hand, but you can look at the Euler-archives the treatize "De series divergentibus"): The evaluation of an infinite series is different from a finite sum. But always when we want to assign a value for such a series we should do it in the sense, that it is the result of an infinitely applied arithmetic operation - so that the geometric series (to which we meanwhile assign a value) occurs as result of the infinite formal long-division $s(x) = {1 \over 1-x } \to s(x) = 1 + x + x^2 + ... $ and then insert the value for $x$ in the finite rational formula.

Possibly this is meant in a sense, that similarly we can discuss infinite periodic continued fractions as representations of finite expressions like $\sqrt{1+x}$ and others. It is "compatible" somehow to an axiom, that we require for number theory that we can have a closed-form representation for general infinitely repeated (symbolic) algebraic operation. (in the german translation of E247 this occurs in §11 and §12)

From this, I think, for instance Euler-summation and other manipulations on infinite (convergent and divergent) series by L. Euler can be nicely understood.

[update] The Euler-archives seem to have moved to MAA; the original links, for instance //www.eulerarchive.com/ is taken over by some completely unrelated commercials. A seemingly valid link to Ed Sandifer's column "How Euler did it", however only accessible via internal MAA-access is this (but I think via webarchive.org one can still access the former existent openly available pages)

[update 2]: here is a currently valid link to Ed Sandifer's article