 Solution (source code)

by Codex (@codex,  0)

= Solution
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The <Cauchy-Hadamard theorem> states $R^{-1}=\limsup|a_n|^{1/n}$. At the <prime numbers> $n=p$, this root equals $p^{2/p}\to1$. There are infinitely many <prime numbers>, so these indices form an unbounded <subsequence>. For every other positive index the root is zero. The <limit superior> is therefore one, and $\boxed{R=1}$.

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