<s>[INST] Convergence of $ \sum_{n=1} ^\infty \frac {1}{n(1+\frac {1} {2}+\frac {1} {3}+ \cdots+\frac {1} {n} )}$ Convergence of $$ \sum_{n=1} ^\infty \dfrac {1}{n(1+\frac {1} {2}+\frac {1} {3}+ \cdots+\frac {1} {n} )}$$
Attempt: I believe not a nice attempt: $ n(1+\frac {1} {2}+\frac {1} {3}+ \cdots+\frac {1} {n} ) \leq n( 1+1+\cdots+1 )$ 
$\implies n(1+\frac {1} {2}+\frac {1} {3}+ \cdots+\frac {1} {n} ) \leq n^2$
$\implies \dfrac {1}{ n(1+\frac {1} {2}+\frac {1} {3}+ \cdots+\frac {1} {n} ) } \geq \dfrac {1}{n^2}$
I guess this result is of not much use. Can somebody please tell me a direction to move ahead in this problem?
Thank you very much for your help .
 [/INST] The other way to look at it is, denoting $\displaystyle H_n = \sum\limits_{k=1}^{n} \frac{1}{k}$
The tail of the series: $$\displaystyle \frac{\frac{1}{n+1}}{H_{n+1}}+\frac{\frac{1}{n+2}}{H_{n+2}}+\cdots +\frac{\frac{1}{n+r}}{H_{n+r}} \ge \frac{\sum\limits_{k=1}^{r}\frac{1}{n+k}}{H_{n+r}} = \frac{H_{n+r}-H_{n}}{H_{n+r}} = 1-\frac{H_n}{H_{n+r}}$$
Since, $\displaystyle \lim\limits_{r \to \infty} 1-\frac{H_n}{H_{n+r}} = 1$
The series diverges by Cauchy Criteria.
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