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G.f.: 1 + 5 * Sum_{k>=1} mu(k) * x^k / (1 - 5*x^k). - Ilya Gutkovskiy, Apr 14 2021
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seq (a(n), n=0..30); # Alois P. Heinz, Oct 21 2012
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a[0] = 1; a[n_] := Sum[MoebiusMu[d]*5^(n/d), {d, Divisors[n]}]; Table[a[n], {n, 0, 23}] (* Jean-Fran�ois Alcover, Mar 11 2014 *)
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Alois P. Heinz, <a href="/A054720/b054720.txt">Table of n, a(n) for n = 0..450</a>