A364210 - OEIS

A364210

a(n) = (1/(2*n)) * Sum_{d|n} 3^(n/d-1) * phi(3*d).

1, 2, 4, 8, 17, 44, 105, 278, 733, 1978, 5369, 14792, 40881, 113934, 318884, 896948, 2532161, 7174862, 20390553, 58114072, 166037460, 475473286, 1364393897, 3922640132, 11297181473, 32588043882, 94143179560, 272342824320, 788854912241, 2287679406940, 6641649422409

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..31.

FORMULA

G.f.: (-1/2) * Sum_{k>0} phi(3*k) * log(1-3*x^k)/(3*k).

MATHEMATICA

a[n_] := DivisorSum[n, 3^(n/#-1)*EulerPhi[3*#]/(2*n) &]; Array[a, 30] (* Amiram Eldar, Jul 14 2023 *)

PROG

(PARI) a(n) = sumdiv(n, d, 3^(n/d-1)*eulerphi(3*d))/(2*n);

CROSSREFS