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A359203
a(n) = Sum_{d|n} (n/d) * 3^(n-d).
2
1, 7, 28, 127, 406, 1756, 5104, 20575, 61237, 230122, 649540, 2579932, 6908734, 26044984, 74578888, 269985151, 731794258, 2799670555, 7360989292, 27392181562, 75948764752, 268482753172, 721764371008, 2742292424188, 7078172334031, 25701091008418, 71173405454680
OFFSET
1,2
FORMULA
G.f.: Sum_{k>=1} x^k/(1 - (3 * x)^k)^2.
MATHEMATICA
a[n_] := DivisorSum[n, 3^(n-#)*n/# &]; Array[a, 27] (* Amiram Eldar, Aug 23 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, n/d*3^(n-d));
(PARI) my(N=30, x='x+O('x^N)); Vec(sum(k=1, N, x^k/(1-(3*x)^k)^2))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Dec 20 2022
STATUS
approved