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 A108484 #17 Aug 03 2024 13:24:15
%S A108484 1,1,4,19,55,220,793,2845,10480,37963,138259,503608,1831969,6669865,
%T A108484 24276892,88362451,321640831,1170726484,4261339801,15510894949,
%U A108484 56458080328,205502135851,748007984827,2722677076336,9910284168961
%N A108484 a(n) = Sum_{k=0..floor(n/2)} binomial(2n-2k,2k) * 3^(n-k).
%C A108484 In general, Sum_{k=0..floor(n/2)} C(2n-2k,2k)a^k*b^(n-k) has expansion (1-bx-abx^2)/(1-2bx-(2ab-b^2)x^2-2ab^2*x^3+(ab)^2*x^4).
%H A108484 Seiichi Manyama, <a href="/A108484/b108484.txt">Table of n, a(n) for n = 0..1000</a>
%H A108484 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,5,6,-9).
%F A108484 G.f.: (1-x-3x^2)/(1-2x-5x^2-6x^3+9x^4).
%F A108484 a(n) = 2a(n-1)+5a(n-2)+6a(n-3)-9a(n-4).
%Y A108484 Cf. A108485, A108486, A108487.
%K A108484 easy,nonn
%O A108484 0,3
%A A108484 _Paul Barry_, Jun 04 2005