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 A383000 #10 Apr 12 2025 11:21:39
%S A383000 1,-2,11,-28,85,-238,735,-2216,6585,-19610,59059,-177428,531453,
%T A383000 -1593606,4783175,-14351152,43046737,-129134082,387420507,-1162281100,
%U A383000 3486785925,-10460294174,31381059631,-94143360856,282429536825,-847288078026,2541865841523,-7625599078020,22876792454989
%N A383000 a(n) = Sum_{d|n} phi(n/d) * (-3)^(d-1).
%F A383000 a(n) = Sum_{k=1..n} (-3)^(gcd(n,k) - 1).
%F A383000 G.f.: Sum_{k>=1} phi(k) * x^k / (1 + 3*x^k).
%o A383000 (PARI) a(n) = sumdiv(n, d, eulerphi(n/d)*(-3)^(d-1));
%Y A383000 Column k=3 of A382995.
%Y A383000 Cf. A000010, A034754, A343465.
%K A383000 sign
%O A383000 1,2
%A A383000 _Seiichi Manyama_, Apr 12 2025