Rearrangement of an absolutely convergent series

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Suppose I have the sequences $a_n$ such that $\sum_{n=1}^\infty a_n$ converges absolutely. Take the following rearrangement: $(a_1+a_3+....)+(a_2+a_4+....)$. The question is if that is a valid rearrangement. My problem is that I don't find a way to write this rearrangement as a function of the original arrangement.