Is there any comprehensive list (books, online, …) of rules for manipulating infinite series (partial sums) to find convergence of a sum? Often authors use some "trick" to compute an infinite series. Following this trick is always a disclaimer, such as "adding infinite sequences is not the same as adding discrete values so the usual rules of algebra will not work."
Huh? Then how am I supposed to learn what I can and cannot do when all I have been shown is a trick that works in a particular case? For example,
\begin{align*}
\sum_{n=1}^{\infty}\frac{1}{n(n+1)}\rightarrow S_{n}&=\sum_{k=1}^{n}\frac{1}{k(k+1)}\\\
&=\frac{1}{1\cdot 2}+\frac{1}{2\cdot 3}+\frac{1}{3\cdot 4}+\frac{1}{n(n+1)}\\\
&=\; \sum_{k=1}^{n}(\frac{1}{k}-\frac{1}{k+1})\\
&=(1-\frac{1}{2})+(\frac{1}{2}-\frac{1}{3})+(\frac{1}{3}-\frac{1}{4})+\cdots+(\frac{1}{n}-\frac{1}{n+1})\\\
&=1-\frac{1}{n+1}\rightarrow \lim_{n\rightarrow \infty}\left(1-\frac{1}{n+1}\right)=1-0=1.
\end{align*}
OK, I see the author used partial fractions expansion followed by grouping like-terms. However, elsewhere I'll find that grouping like-terms will lead you to the wrong answer. Such is the case with this infinite sum:
$$
S_{n}=1-1+1-1+1+\cdots \stackrel{?}{\Longrightarrow} (1-1)+(1-1)+\cdots=0
$$
$$
S_{n}=1-1+1-1+1+\cdots\stackrel{?}{\Longrightarrow} 1+(-1+1)+(-1+1)+\cdots=1
$$
…So does this mean infinite series are not associative? Or are they? What are the invariant properties of infinite series that can be confidently used when manipulating an infinite series?
FOLLOW UP:
Thanks for the great answers so far, yet they have led me to question the practicality of writing in summation form. Is it not more useful to simply write the infinite sum as its corresponding infinite partial sum sequence?
E.g.: Why is the fourier series written as an infinite sum? I realize it can be used to create a continuous analog for many types of discontinuous functions, and yet I feel I gain nothing when I write out the fourier series to say, a solution to a partial differential equation, because Idk how to evaluate the output to a particular input since its defined by an infinite sum of sin's and cosines. (Besides finding the partial sum sequence) Is there not a more direct way?
Best Answer
I would suggest you read the book "An introduction to the Theory of Infinite Series" by Thomas Bromwich.
P.S.: There is a tale that goes something like Ramanujan was advised by British Mathematicians to read this book when he wanted to prove that the sum of a series of positive terms is negative ( I neither remember the series, nor Ramanujan's "sum" but all I could remember is it was divergent").
An online version, downloadable in many different formats, is here on the archive.com .
Read Comment below by Sivaram where he points out both the series and the sum!