 Solution

Let $S_N={\sum }_{k=1}^N{a}_k$, with $S_0=0$. The indices in the $n$th dyadic block have one sign, soA convergent sequence of partial sums must be a Cauchy sequence: for every $\epsilon >0$, all sufficiently late pairs of partial sums differ by less than $\epsilon$. The displayed differences violate this necessary condition with $\epsilon =1/4$, at arbitrarily late endpoints. Thus the series diverges, even though its individual terms tend to zero. This dyadic-block alternation of reciprocal terms has growing block lengths; the fixed-length block argument of the previous part does not apply.