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 A382999 #11 Apr 12 2025 11:21:14
%S A382999 1,-1,6,-8,20,-30,70,-136,270,-500,1034,-2088,4108,-8134,16440,-32912,
%T A382999 65552,-130878,262162,-524800,1048740,-2096138,4194326,-8390976,
%U A382999 16777300,-33550348,67109418,-134225840,268435484,-536855640,1073741854,-2147516704,4294969404
%N A382999 a(n) = Sum_{d|n} phi(n/d) * (-2)^(d-1).
%F A382999 a(n) = Sum_{k=1..n} (-2)^(gcd(n,k) - 1).
%F A382999 G.f.: Sum_{k>=1} phi(k) * x^k / (1 + 2*x^k).
%o A382999 (PARI) a(n) = sumdiv(n, d, eulerphi(n/d)*(-2)^(d-1));
%Y A382999 Column k=2 of A382995.
%Y A382999 Cf. A000010, A034738, A074763.
%K A382999 sign
%O A382999 1,3
%A A382999 _Seiichi Manyama_, Apr 12 2025