Let (an)n∈N be a monotone sequence of positive real numbers, such that lim inf Does it always follow that \sum_{n = 0}^\infty a_n < \infty
It is easy to show that \sum_{n=0}^\infty a_n < \infty implies \liminf_{n \to \infty} na_n = 0, even if (a_n)_{n \in \mathbb N} is not a monotone sequence. Moreover, one can easily come up with plenty of non-mononote sequences (a_n)_{n \in \mathbb N}, satisfying \liminf_{n \to \infty} na_n = 0, but \sum_{n=0}^\infty a_n = \infty. It is also easy to see that the existence of some \epsilon > 0, such that \limsup_{n \to \infty} n^{1+\epsilon} a_n < \infty imples the convergence of \sum_{n = 0}^\infty a_n, again regardless of whether (a_n)_{n \in \mathbb N} is monotone or not. However, I am stuck at this particular question. If there exists a counterexample, it should, more or less, be a sequence (a_n)_{n \in \mathbb N} converging to 0, but simultaneously slower than any sequence of the form n^{-(1+\epsilon)} for all \epsilon > 0 and faster than than n^{-1}.
Answer
You can take \frac 1{n \log(n)}, for example. The sum diverges. You can also take \frac 1{n(\log (n))^2}, for which the sum converges.
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