[Math] convergent but not absolutely convergence series problem which has zero sum

real-analysissequences-and-series

It is given that the series $\sum_{n=1}^\infty a_n$ is convergent, but not absolutely and $\sum_{n=1}^\infty a_n=0$. Denote by $S_k$ the partial sum $\sum_{n=1}^k a_n$ , $k=1,2,\dots$ Then,
(a) $S_k=0$ for infinitely many $k$;
(b) $S_k>0$ for infinitely many $k$ , $S_k<0$ for infinitely many $k$;
(c) it is possible that $S_k>0$ for all $k$;
(d) it is possible that $S_k>0$ for all but finite number of values of $k$.

I am completely stuck on it. How can I solve this problem? Please help.

Best Answer

It helps to reformulate the assumptions in terms of $S_k$. We are told that

  • $S_k\to 0$
  • $\sum |S_{k+1}-S_k| =\infty$

and nothing else. Of course, there is nothing here that implies $S_k$ being zero, or positive for infinitely many values of $k$. The examples $S_k=(2+(-1)^k)/k$ and $S_k=(-2+(-1)^k)/k$ take care of all four parts, confirming Did's answer in a comment: "(a) and (b) are false in general while (c) and (d) hold".

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