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 A364011 #25 Jul 02 2023 10:03:16
%S A364011 1,1,1,0,1,1,2,0,1,0,1,0,2,2,1,-1,1,1,2,-1,2,0,1,0,2,2,1,0,1,0,2,-1,1,
%T A364011 0,2,0,2,2,2,-2,1,2,2,-1,1,0,1,-1,3,1,1,0,1,1,2,0,2,0,1,-1,2,2,2,-2,2,
%U A364011 0,2,-1,1,0,1,0,2,2,2,0,2,2,2,-3,1,0,1,0,2,2,1,-2,1,0,4,-1,2,0
%N A364011 Expansion of Sum_{k>0} x^k / (1 + x^(3*k)).
%H A364011 Seiichi Manyama, <a href="/A364011/b364011.txt">Table of n, a(n) for n = 1..10000</a>
%F A364011 G.f.: Sum_{k>0} (-1)^(k-1) * x^(3*k-2) / (1 - x^(3*k-2)).
%F A364011 a(n) = -Sum_{d|n, n/d==1 (mod 3)} (-1)^(n/d) = -Sum_{d|n, d==1 (mod 3)} (-1)^d.
%t A364011 a[n_] := -DivisorSum[n, (-1)^(n/#) &, Mod[n/#, 3] == 1 &]; Array[a, 100] (* _Amiram Eldar_, Jul 01 2023 *)
%o A364011 (PARI) a(n) = -sumdiv(n, d, (d%3==1)*(-1)^d);
%Y A364011 Cf. A364012, A364013.
%Y A364011 Cf. A002654, A363037.
%K A364011 sign
%O A364011 1,7
%A A364011 _Seiichi Manyama_, Jul 01 2023