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 A335417 #13 Jul 29 2020 06:17:39
%S A335417 6,15,20,15,4,35,360,630,420,1386,280,5005,280,1287,3080,3465,2520,
%T A335417 770,27720,9009,5720,16380,360360,10010,360360,3465,40040,25740,
%U A335417 360360,323323,1750320,765765,123760,3423420,720720,4849845,360360,45045
%N A335417 a(n) = denominator of Sum_{k=2..A335138(n)} abs(A309229(n, k))/k.
%t A335417 nn = 41; f[n_] := n; h[n_] := DivisorSum[n, MoebiusMu[#] # &]; A = Accumulate[Table[Table[h[GCD[n, k]], {k, 1, nn}], {n, 1, nn}]]; B = -Abs[A]; B[[All, 1]] = Table[f[n], {n, 1, nn}]; b = 1 + Total[Sign[1 + Sign[Accumulate[Transpose[B]]]]]; d = Table[Sum[Abs[A[[n, k]]]/k, {k, 2, b[[n]]}], {n, 1, nn}]; Denominator[d]
%Y A335417 Cf. A309229, A335138. See A335416 for numerators.
%K A335417 nonn,frac
%O A335417 1,1
%A A335417 _Mats Granvik_, Jun 09 2020