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 A108480 #14 Sep 05 2025 10:28:57
%S A108480 1,1,3,13,35,117,379,1197,3859,12357,39563,126845,406371,1302101,
%T A108480 4172443,13369293,42838835,137266917,439837739,1409354397,4515934339,
%U A108480 14470215157,46366299963,148569565165,476055153491,1525403341701
%N A108480 Expansion of (1-x-2*x^2)/(1-2*x-3*x^2-4*x^3+4*x^4).
%H A108480 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,3,4,-4).
%F A108480 a(n) = 2*a(n-1) + 3*a(n-2) + 4*a(n-3) - 4*a(n-4).
%F A108480 a(n) = Sum_{k=0..floor(n/2)} C(2*(n-k), 2k) * 2^k.
%F A108480 a(n) ~ (1+sqrt((4*sqrt(2)-1)/31)) * (1+2*sqrt(2)+sqrt(1+4*sqrt(2)))^n/2^(n+2). - _Vaclav Kotesovec_, Jul 24 2013
%t A108480 CoefficientList[Series[(1-x-2*x^2)/(1-2*x-3*x^2-4*x^3+4*x^4), {x, 0, 20}], x] (* _Vaclav Kotesovec_, Jul 24 2013 *)
%t A108480 LinearRecurrence[{2,3,4,-4},{1,1,3,13},30] (* _Harvey P. Dale_, Aug 29 2023 *)
%Y A108480 Cf. A001541, A387622, A387623.
%K A108480 easy,nonn,changed
%O A108480 0,3
%A A108480 _Paul Barry_, Jun 04 2005