Showing that $\sum^\infty_{n=1}\left(\sin\left(\frac{1}{n}\right)-\ln\left(1+\frac{1}{\sqrt n}\right)\right)$ diverges

57 Views Asked by At

Separately, I can show that both parts of the sum diverge, but that doesn't help me very much. I don't see any test which could come in handy. I know both sides of the sum have popular Taylor forms, so I can partially represent them as such around $x=0$, but my question then what do I do with the remainder?

1

There are 1 best solutions below

0
On BEST ANSWER

$$\begin{split}\sin\left(\frac 1 n\right)-\ln\left(1+\frac 1 {\sqrt n}\right)&=\left[\frac 1 n +\mathcal O\left(\frac 1 {n^3}\right)\right]-\left[\frac 1 {\sqrt n}-\frac 1 {2n}+\mathcal O\left(\frac 1 {n\sqrt n}\right)\right]\\ &=-\frac 1 {\sqrt n}+\frac 3 {2n}+\mathcal O\left(\frac 1 {n\sqrt n}\right)\\ &\sim -\frac 1 {\sqrt n}\end{split}$$