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Recently, sources for mathematical infotainment, for example numberphile, have given some information on the interpretation of divergent series as real numbers, for example

$\sum_{i=0}^\infty i = -{1 \over 12}$

This equation in particular is said to have some importance in modern physics, but, being infotainment, there is not much detail beyond that.

As an IT Major, I am intrigued by the implications of this, and also the mathematical backgrounds, especially since this equality is also used in the expansion of the domain of the Riemann-Zeta function.

But how does this work? Where does this equation come from, and how can we think of it in more intuitive terms?

marked as duplicate by Matthew Conroy, Emily, Jean-Sébastien, mrf, Ian Mateus Apr 11 '14 at 18:58

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 3
    This has been asked at least twenty times before. Try looking for those questions. For what it's worth, the equality is false. – Git Gud Apr 11 '14 at 17:32
  • @MatthewConroy wow, I wonder why this didn't come up in my search... Thanks for the link. – Andreas Grapentin Apr 11 '14 at 17:33
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  • @AndreasGrapentin It's a little hard to search for, I think. For instance, searching for "-1/12" yields lot of stuff that doesn't have -1/12 in it. – Matthew Conroy Apr 11 '14 at 17:36
  • Yes, this question has already been asked many times here, an unfortunately some people are pretty hostile to it now, but no worries. Definitely check out their answers! – abnry Apr 11 '14 at 18:04
up vote 3 down vote accepted

Basically, the video is very disingenuous as they never define what they mean by "=."

This series does not converge to -1/12, period. Now, the result does have meaning, but it is not literally that the sum of all naturals is -1/12. The methods they use to show the equality are invalid under the normal meanings of series convergence.

What makes me dislike this video is when the people explaining it essentially say it is wrong to say that this sum tends to infinity. This is not true. It tends to infinity under normal definitions. They are the ones using the new rules which they did not explain to the viewer.

This is how you get lots of shares and likes on YouTube.

Usually we do not assign a value to divergent series. The assertion that these kinds of series are divergent means that these kinds of series do not have sums. Now if we want to assign a sum (in some sense) to divergent series, what these sums "should" be and how they should be calculated? Obviously we are going out of customary mathematics. We are extending it to "illegal" cases, and it is not a surprise that we get odd results. G.H. Hardy's "Divergent Series" is a good reference. You can find more discussion on this subject there.

I also found this video very helpful.

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