[再寄小读者之数学篇](2015-06-24 Series)

(AMM. Problems and Solutions. 2015. 03) Let $\sed{a_n}$ be a monotone decreasing sequence of real numbers that converges to $0$. Prove that $$\bex \vsm{n}a_n<\infty \eex$$ if and only if $a_n=O(1/\ln n)$ and $\dps{\vsm{n}(a_n-a_{n+1}) \ln n<\infty}$. 

 

必要性好办, 但是充分条件怎么办呢?