How to find the interval of p such that the series is convergent?

197 Views Asked by At

The question is as following:

For what values of $p$ is the series convergent? $$ \sum_{n=2}^\infty (-1)^{n-1}\frac{(\ln n)^p}{n^2}$$

What I have done so far:

  1. Take Absolute Value (Test for Absolute Convergent)

  2. Using Integration By Parts repeatedly (Do Integral Test) $$\int_{2}^\infty\frac{(\ln x)^p}{x^2}dx= \lim_{t\to\infty}\left[(\ln x)^p(-\frac{1}{x})+p(\ln x)^{p-1}(- \frac{1}{x})+p(p-1)(\ln x)^{p-2}(- \frac{1}{x})+...+p!(- \frac{1}{x})\right]_{2}^{t}$$

I found that the value of $p$ seems have no effect on the convergence of the series and my answer for this question is $[0,\infty)$ but when I tested it with WolframAlpha, I found it when $p=n$, it is not convergent.

And thus I know I may have made some mistakes.
So can anyone point out which part did I do it wrongly? It would be really appreciated :D
Thanks for you help!

1

There are 1 best solutions below

2
On

It is always convergent by comparison, indeed $$\frac{(\ln n)^p}{n^2} = o \left(\frac 1{n^{\frac32}}\right)$$