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 A099486 #9 Sep 08 2019 02:26:03
%S A099486 0,1,4,14,52,195,728,2716,10136,37829,141180,526890,1966380,7338631,
%T A099486 27388144,102213944,381467632,1423656585,5313158708,19828978246,
%U A099486 74002754276,276182038859,1030725401160,3846719565780,14356152861960
%N A099486 Expansion of x/((1 + x^2)*(1 - 4*x + x^2)).
%C A099486 A Chebyshev transform of the sequence 0,1,4,16,... which has g.f. x/(1-4x). The image of G(x) under the Chebyshev transform is (1/(1+x^2))*G(x/(1+x^2)).
%F A099486 G.f.: x/((1 + x^2)*(1 - 4*x + x^2)).
%F A099486 a(n) = 4*a(n-1) - 2*a(n-2) + 4*a(n-3).
%F A099486 a(n) = Sum_{k=0..n} cos(Pi*(n-k)/2)*((2+sqrt(3))^k - (2-sqrt(3))^k)/(2*sqrt(3)).
%F A099486 a(n) = Sum_{k=0..floor(n/2)} binomial(n-k, k)*(-1)^n*(4^(n-2*k) - 0^(n-2*k))/4.
%Y A099486 Cf. A099487, A099488.
%K A099486 easy,nonn
%O A099486 0,3
%A A099486 _Paul Barry_, Oct 18 2004