# How to analyze convergence and sum of a telescopic series? I can't find a generic form

Hello I would like to know what is the generic form to calculate a telescopic series.

http://www.dummies.com/how-to/content/how-to-analyze-a-telescoping-series.html

here uses a sum that startes at $n=1$, but what if it started at $n=2$ or $n=3$?

the $n$ index has any influence over the sum? I think yes, but no one explains this.. my series is:

$$\sum_{n=1}^\infty\frac{4}{(n+1)(n+2)}$$

how do I solve this really step by step for new people to those series

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If you give us a particular sum, and tell us if how you want us to tweak it, then we might find it easier to write an answer. – user21436 Feb 2 '12 at 11:41

If a telescoping sum starts at $n=m$, then $$\sum_{n=m}^{N}\left( a_{n}-a_{n+1}\right) =a_{m}-a_{N+1}$$

and the telescoping series is thus $$\begin{eqnarray*} \sum_{n=m}^{\infty }\left( a_{n}-a_{n+1}\right) &=&\lim_{N\rightarrow \infty }\sum_{n=m}^{N}\left( a_{n}-a_{n+1}\right) \\ &=&a_{m}-\lim_{N\rightarrow \infty }a_{N+1}=a_{m}-\lim_{N\rightarrow \infty }a_{N+1} \\ &=&a_{m}-\lim_{N\rightarrow \infty }a_{N}. \end{eqnarray*}$$

Of course the series converges if and only if there exists $\lim_{N\rightarrow \infty }a_{N}.$

The case $m=1$ is $$\begin{eqnarray*} \sum_{n=1}^{N}\left( a_{n}-a_{n+1}\right) &=&a_{1}-a_{N+1} \\ \sum_{n=1}^{\infty }\left( a_{n}-a_{n+1}\right) &=&a_{1}-\lim_{N\rightarrow \infty }a_{N}. \end{eqnarray*}$$ The difficult part is to write a series $\sum_{n=m}^{\infty}u_n$ in the form $\sum_{n=m}^{\infty}\left( a_{n}-a_{n+1}\right)$, when possible. Concerning Wilf-Zeilberger method (also called "creative telescoping") see answers to the question Mathematical Telescoping.

Example: write $\frac{2n+1}{n^{2}\left( n+1\right) ^{2}}=\frac{1}{n^{2}}- \frac{1}{\left( n+1\right) ^{2}}$ to evaluate $$\begin{eqnarray*} \sum_{n=1}^{\infty }\frac{2n+1}{n^{2}\left( n+1\right) ^{2}} &=&\sum_{n=1}^{\infty }\left( \frac{1}{n^{2}}-\frac{1}{\left( n+1\right) ^{2} }\right) \\ &=&1-\lim_{N\rightarrow \infty }\frac{1}{N^{2}}=1-0=1. \end{eqnarray*}$$

Added in response to the edited question. The standard technique is to expand $\frac{4}{(n+1)(n+2)}$ into partial fractions. Write $$\begin{eqnarray*} \frac{4}{(n+1)(n+2)} &=&\frac{A}{n+1}+\frac{B}{n+2} \\ &=&\frac{(n+2)A+(n+1)B}{(n+1)(n+2)} \\ &=&\frac{\left( A+B\right) n+2A+B}{(n+1)(n+2)} \end{eqnarray*}$$

and find the constants $A$ and $B$. The following system must hold $$\left\{ \begin{array}{c} A+B=0 \\ 2A+B=4 \end{array} \right. \Leftrightarrow \left\{ \begin{array}{c} A=4 \\ B=-4. \end{array} \right.$$

So $$\frac{4}{(n+1)(n+2)}=\frac{4}{n+1}-\frac{4}{n+2}$$

and, since $a_{n}=\frac{4}{n+1}$, you get $$\begin{eqnarray*} \sum_{n=1}^{\infty }\frac{4}{(n+1)(n+2)} &=&\sum_{n=1}^{\infty }\left( \frac{ 4}{n+1}-\frac{4}{n+2}\right) \\ &=&\frac{4}{1+1}-\lim_{n\rightarrow \infty }\frac{4}{n+1} \\ &=&2-0=2. \end{eqnarray*}$$

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thanks for your answer but I think I'm miss understand some steps and the problem is that I only have left 3 hours to the exam. So I've updated my question with a specific example. A step-by-step example would be really much appreciated thanks – Totty.js Feb 2 '12 at 14:22
@Totty: Do you need additional details? – Américo Tavares Feb 2 '12 at 16:36
no, that was ok. thanks :D – Totty.js Feb 10 '12 at 9:13

To add to Américo Tavares's answer, for more general telescoping series:

$$\sum_{n=m}^{N}\left( a_{n}-a_{n+k}\right) =(a_{m}+a_{m+1}+\cdots+a_{m+k-1})-(a_{N+1}+a_{N+2}+\cdots+a_{N+k})$$

$$\begin{eqnarray*} \sum_{n=m}^{\infty }\left( a_{n}-a_{n+k}\right) &=&\lim_{N\rightarrow \infty }\sum_{n=m}^{N}\left( a_{n}-a_{n+k}\right) \\ &=&\lim_{N\rightarrow \infty }((a_{m}+a_{m+1}+\cdots+a_{m+k-1})-(a_{N+1}+a_{N+2}+\cdots+a_{N+k}))\\ &=&(a_{m}+a_{m+1}+\cdots+a_{m+k-1})-\lim_{N\rightarrow \infty }(a_{N+1}+a_{N+2}+\cdots+a_{N+k})\\ &=& a_{m}+a_{m+1}+\cdots+a_{m+k-1}-k\lim_{N\rightarrow\infty}a_N. \end{eqnarray*}$$

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I think that if the series is convergent it evaluates to $$\sum_{m=1}^{\infty }\left( a_{n}-a_{n+k}\right) =a_{m}+a_{m+1}+\ldots +a_{m+k-1}-k\lim_{N\rightarrow \infty }a_{N}.$$ – Américo Tavares Feb 2 '12 at 12:45
Ah, in fact you're correct - editing to make the correction. Thanks! (I should know better than to try to do math at 6a!) – Aru Ray Feb 2 '12 at 13:42