Problem 1117 (difficulty: 6/10)

Let \(\displaystyle \sum a_{\varphi(n)}\) be a rearrangment of the conditionally convergent series \(\displaystyle \sum a_n\). What can be the set of limit points of the set of the partial sums \(\displaystyle \sum\limits_{k=1}^na_{\varphi(k)}\)?

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