This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A373276 #11 May 29 2024 13:46:37
%S A373276 1,2,10,23,82,236,730,2156,6571,19604,59050,176918,531442,1593596,
%T A373276 4783060,14346689,43046722,129133838,387420490,1162241726,3486785140,
%U A373276 10460294156,31381059610,94143003584,282429536563,847288078004,2541865834900,7625595889958
%N A373276 a(n) = Sum_{d|n} (-1)^(d-1) * 3^(n/d-1).
%F A373276 G.f.: 1/3 * Sum_{k>=1} (3*x)^k / (1 + x^k).
%F A373276 If p is an odd prime, a(p) = 1 + 3^(p-1).
%o A373276 (PARI) a(n) = sumdiv(n, d, (-1)^(d-1)*3^(n/d-1));
%o A373276 (PARI) my(N=30, x='x+O('x^N)); Vec(sum(k=1, N, (3*x)^k/(1+x^k))/3)
%Y A373276 Cf. A048272, A373275.
%Y A373276 Cf. A321386, A357051.
%K A373276 nonn,easy
%O A373276 1,2
%A A373276 _Seiichi Manyama_, May 29 2024