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 A099917 #6 Oct 01 2023 20:13:07
%S A099917 1,0,2,-1,1,-2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0,2,
%T A099917 -1,1,-2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0,2,-1,1,
%U A099917 -2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0,2,-1,1,-2,0,-1,0,1,0
%N A099917 Expansion of (1+x^2)^2/(1+x^3+x^6).
%C A099917 The denominator is the 9th cyclotomic polynomial. The g.f. is a Chebyshev transform of that of (-1)^n*A052931(n) by the Chebyshev mapping g(x)->(1/(1+x^2))g(x/(1+x^2)). The reciprocal of the 9th cyclotomic polynomial A014018 is given by sum{k=0..n, A099917(n-k)(k/2+1)(-1)^(k/2)(1+(-1)^k)/2}.
%H A099917 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (0, 0, -1, 0, 0, -1).
%F A099917 a(n)=sum{k=0..floor(n/2), C(n-k, k)(-1)^k*sum{j=0..n-2k, C(j, n-2k-2j)3^k(-1/3)^(n-2k)}}; a(n)=sum{k=0..n, A014018(n-k)C(2, k/2)(1+(-1)^k)/2}.
%Y A099917 Cf. A099916.
%K A099917 easy,sign
%O A099917 0,3
%A A099917 _Paul Barry_, Oct 30 2004