 Solution

A series ${\sum }_{n=1}^{\infty }{u}_n$ is a convergent series if its partial sums $S_N={\sum }_{n=1}^N{u}_n$ have a finite limit $S$ as $N\to \infty$. The number $S$ is the sum of the series. Explicitly, for every $\epsilon >0$ there is an integer $N_0$ such that $|S_N-S|<\epsilon$ whenever $N\ge N_0$. The condition $u_n\to 0$ alone is insufficient: the harmonic series has terms tending to zero but diverges.