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%I #13 May 10 2024 09:25:53
%S 1,1,3,2,5,3,6,4,9,5,10,6,12,6,15,8,16,9,18,10,18,10,22,12,25,12,27,
%T 12,28,15,30,16,30,16,30,18,36,18,36,20,40,18,42,20,45,22,46,24,42,25,
%U 48,24,52,27,50,24,54,28,58,30,60,30,54,32,60,30,66,32,66,30,70,36,72,36,75,36,60,36
%N a(n) = phi(15 * n)/8.
%F G.f.: Sum_{k>=1} mu(15 * k) * x^k / (1 - x^k)^2, where mu() is the Moebius function (A008683).
%F Multiplicative with a(p^e) = p^e if p = 3 or 5, and (p-1)*p^(e-1) otherwise.
%F Sum_{k=1..n} a(k) ~ (225/(64*Pi^2)) * n^2. - _Amiram Eldar_, May 10 2024
%t a[n_] := EulerPhi[15 * n]/8; Array[a, 100] (* _Amiram Eldar_, May 10 2024 *)
%o (PARI) a(n) = eulerphi(15*n)/8;
%o (PARI) my(N=80, x='x+O('x^N)); Vec(sum(k=1, N, moebius(15*k)*x^k/(1-x^k)^2))
%Y Column k=15 of A372673.
%Y Cf. A008683.
%K nonn,mult,easy
%O 1,3
%A _Seiichi Manyama_, May 10 2024