Consider the sequence defined recursively as

Prove that .

**Solution**

**Lemma: **If is a sequence for which then

*Proof*: In Stolz theorem we set and .

It is easy to see that is is monotonically decreasing to zero. Moreover, an application of L’Hospital’s rule gives

Therefore

Now, due to the lemma we have and the result follows.

**Remark : **The asymptotic now follows to be .

**Problem: **Find what inequality should satisfy such that the series

converges.