# Sum converging to 0

by Bipolarity
Tags: converging
 P: 783 Does there exist a sequence of real nonzero numbers whose sum converges to 0? I would think there isn't, but I'm interested in people's opinions and arguments. For any nonzero m, a series of nonzero numbers whose sum converges to m can easily be constructed using the formula: $\sum ^{\infty}_{n=1}m(0.5)^{n}$ But that is for nonzero m, what if you wanted to construct a series whose sum converged to 0? BiP
 P: 350 1+-1+.5+-5+.25+-.25+.125+-.125+......
P: 783
 Quote by Vargo 1+-1+.5+-5+.25+-.25+.125+-.125+......
Can you find an explicit representation for that seqence (i.e. with sigma notation) ?

BiP

 P: 783 Sum converging to 0 Eureka!!! I believe I found it! $$\sum^{\infty}_{n=1} (-1)^{n+1} (\frac{1}{2})^{ \frac{2n-3+(-1)^{n+1}}{4}}$$ I believe it converges to 0, but can anyone verify this? BiP
 P: 686 If $\sum ^{\infty}_{n=1}m(0.5)^{n}$ converges to m, then shouldn't $m-\sum ^{\infty}_{n=1}m(0.5)^{n}$ converge to 0?
P: 783
 Quote by Edgardo If $\sum ^{\infty}_{n=1}m(0.5)^{n}$ converges to m, then shouldn't $m-\sum ^{\infty}_{n=1}m(0.5)^{n}$ converge to 0?
Yes, but $m-\sum ^{\infty}_{n=1}m(0.5)^{n}$ is not a series... unless you can express it as one with nonzero terms.

BiP
 HW Helper P: 2,264 Who cares about expressing it as one with nonzero terms? A series is a series is a series. This is simple How about $$\sum_{n=0}^\infty \frac{(\pi)^{2n+1}}{(2n+1)!} (-1)^n$$
P: 783
 Quote by lurflurf Who cares about expressing it as one with nonzero terms? A series is a series is a series.
The problem requires it.

BiP
Homework
 Quote by Bipolarity Yes, but $m-\sum ^{\infty}_{n=1}m(0.5)^{n}$ is not a series... unless you can express it as one with nonzero terms.
 P: 295 How about taking a sequence $(a_x)_x$ which satisfies $\displaystyle \lim_{x\to\infty}a_x = 0$ and then using the series $\displaystyle \sum_{x=0}^{\infty} (-1)^x b_x$, where the sequence $b_x$ is defined as $b_{2x} = b_{2x+1} = a_x$?
 HW Helper P: 2,264 So you dislike the pi example and the usual example $$\sum_{k=0}^\infty a_k b_x$$ where a_k is a sequence of positive numbers tending to zero and B_k is any sequence of -1 and 1 such that the series tends to zero. What about any number of obvious examples such as $$\sum_{k=0}^\infty (2k-1)\left(\frac{1}{3}\right)^k$$