Few days ago I found something very interesting on 9gag:

When I first read this post, I was amazed because I never thought about this obvious fact!!

There are lots of interesting comments, but here is a proof from the comments:

…. Infinite x zero (as a limit) is indefinite. But infinite x zero (as a number) is zero. So lim( 0 x exp (x²) ) = 0 while lim ( f(X) x exp(X) ) with f(X)->0 is indefinite …

Though the statement made in the post is very vague and can lead to different opinions, like what about doing the product with surreal numbers, but we can safely avoid this by considering the product of real numbers only.

Now an immediate question should be (since every positive real number has a negative counterpart):

Is the sum of all real numbers zero?

In my opinion the answer should be “no”. As of now I don’t have a concrete proof but the intuition is:

Sum of a convergent series is the limit of partial sums, and for real numbers due to lack of starting point we can’t define a partial sum. Hence we can’t compute the limit of this sum and the sum of series of real numbers doesn’t exist.

Moreover, since the sum of all “positive” real numbers is not a finite value (i.e. the series of positive real numbers is divergent) we conclude that we can’t rearrange the terms in series of “all” real numbers (Riemann Rearrangement Theorem). Thus the sum of real numbers can only be conditionally convergent. So, my above argument should work. Please let me know if you find a flaw in these reasonings.

Also I found following interesting answer on Quora:

The real numbers are uncountably infinite, and the standard notions of summation are only defined for countably many terms.

**Note: ***Since we are dealing with infinite product and sum, we can’t argue using algebra of real numbers (like commutativity etc.).*

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I liked it. It does seem “convincing” also on a first read !!! But, I feel one needs to devote special time to thinking about such stuff…,which I have not yet done.

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First of all, if sum of real numbers is convergent, only then it can be conditionally convergent. But how can we say that sum of real numbers is convergent?

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This is exactly the doubt I have expressed. Using this argument I am not able to rule out the possibility of the sum of real numbers being convergent. But for sure, if the sum converges then it has to converge conditionally i.e. we can’t rearrange to prove it to be convergent (pairing positive real numbers and their negative counterparts).

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