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 A366562 #13 Jun 23 2025 22:59:39
%S A366562 1,0,-2,0,-4,0,-6,0,-6,0,-10,0,-12,0,8,0,-16,0,-18,0,12,0,-22,0,-20,0,
%T A366562 -18,0,-28,0,-30,0,20,0,24,0,-36,0,24,0,-40,0,-42,0,24,0,-46,0,-42,0,
%U A366562 32,0,-52,0,40,0,36,0,-58,0,-60,0,36,0,48,0,-66,0,44,0,-70,0,-72,0
%N A366562 a(n) = Sum_{k=1..n} A366561(n,k)*A023900(k)/n.
%F A366562 a(n) = Sum_{k=1..n} A366561(n,k)*A023900(k)/n.
%F A366562 Conjecture: a(n) = [Mod[n, 2] = 1]*A000010(n)*(-1)^A001221(n).
%F A366562 Conjectures from _Ridouane Oudra_, Jun 17 2025: (Start)
%F A366562 a(n) = (-1)^omega(n)*(2*phi(n) - phi(2*n)), where omega = A001221.
%F A366562 a(n) = (-1)^omega(n)*Sum_{d|n} mu(n/d)*A000265(d).
%F A366562 a(2*n) = 0.
%F A366562 a(2*n+1) = A076479(2*n+1)*phi(2*n+1). (End)
%t A366562 nn = 74; f = x^2 - y^2; g[n_] := DivisorSum[n, MoebiusMu[#] # &]; Table[Sum[Sum[Sum[If[GCD[f, n] == k, 1, 0]*g[k]/n, {x, 1, n}], {y, 1, n}], {k, 1, n}], {n, 1, nn}]
%Y A366562 Cf. A000010, A001221, A366561, A366563.
%Y A366562 Cf. A000265, A076479, A008683.
%K A366562 sign
%O A366562 1,3
%A A366562 _Mats Granvik_, Oct 13 2023