A364211 a(n) = (1/(4*n)) * Sum_{d|n} 5^(n/d-1) * phi(5*d).
1, 3, 9, 33, 126, 527, 2233, 9783, 43409, 195378, 887785, 4069297, 18780049, 87194199, 406901134, 1907353533, 8975758273, 42385547227, 200773540297, 953674414158, 4541306270097, 21674416725855, 103660251783289, 496705375169547, 2384185791015751, 11462431696965147, 55189485903168409
Offset: 1
Keywords
Programs
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Mathematica
a[n_] := DivisorSum[n, 5^(n/#-1)*EulerPhi[5*#]/(4*n) &]; Array[a, 25] (* Amiram Eldar, Jul 14 2023 *)
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PARI
a(n) = sumdiv(n, d, 5^(n/d-1)*eulerphi(5*d))/(4*n);
Formula
G.f.: (-1/4) * Sum_{k>0} phi(5*k) * log(1-5*x^k)/(5*k).