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%I #12 Jan 06 2024 04:00:02
%S 1,2,10,29,82,236,730,2216,6571,19604,59050,177410,531442,1593596,
%T 4783060,14351123,43046722,129133838,387420490,1162281098,3486785140,
%U 10460294156,31381059610,94143358424,282429536563,847288078004,2541865834900,7625599078610
%N a(n) = (-1)^n * [x^n] Sum_{k>=1} x^(k-1)/(1+3*x^k).
%F a(n) = Sum_{k=0..n} (-1)^(n-k)*if(mod(n+1, k+1)=0, 1, 0)*3^k.
%t a[n_] := Sum[(-1)^(n - k) * If[Mod[n + 1, k + 1] == 0, 1, 0] * 3^k, {k, 0, n}]; Table[a[n], {n, 0, 25}] (* _James C. McMahon_, Jan 01 2024 *)
%Y Cf. A051731, A081295, A048272, A101562, A101563.
%K easy,nonn
%O 0,2
%A _Paul Barry_, Dec 07 2004
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