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 A378681 #7 Dec 06 2024 11:29:53
%S A378681 2,1,2,6,3,6,3,3,6,2,1,3,6,3,15,30,30,15,5,10,30,15,15,15,30,10,5,15,
%T A378681 30,30,15,30,210,210,70,35,105,105,210,210,105,35,70,210,105,21,21,42,
%U A378681 14,70,210,105,105,210,210,210,105,35,70,210,210,105,105,210
%N A378681 a(n) = denominator(Sum_{k=1..n} 1/P_2(k)), where P_2(k) = A087040(k) is the second largest prime dividing the k-th composite number.
%C A378681 See A378680 for more details.
%H A378681 Amiram Eldar, <a href="/A378681/b378681.txt">Table of n, a(n) for n = 1..1000</a>
%t A378681 p2[c_] := Module[{f = FactorInteger[c]}, If[f[[-1, 2]] > 1, f[[-1, 1]], f[[-2, 1]]]]; Denominator@ Accumulate[Table[1/p2[c], {c, Select[Range[50], CompositeQ]}]]
%o A378681 (PARI) lista(nmax) = {my(s = 0); forcomposite(n = 1, nmax, f = factor(n); s += if(f[#f~, 2] > 1, 1/f[#f~, 1], 1/f[#f~ - 1, 1]); print1(denominator(s), ", "));}
%Y A378681 Cf. A087039, A087040, A378680 (numerators).
%K A378681 nonn,easy,frac
%O A378681 1,1
%A A378681 _Amiram Eldar_, Dec 03 2024