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 A099525 #6 Jun 13 2015 00:51:34
%S A099525 1,2,4,11,28,68,169,422,1048,2603,6472,16088,39985,99386,247036,
%T A099525 614027,1526212,3793532,9429145,23436926,58254448,144796331,359903440,
%U A099525 894570224,2223529441,5526769202,13737249076,34145086475,84870480556
%N A099525 Expansion of 1/(1-2x-3x^3).
%C A099525 In general 1/(1-a*x-b*x^3) expands to sum{k=0..floor(n/3),C(n-2k,k)b^(n-2k)(a/b)^(n-3k)}.
%H A099525 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2,0,3).
%F A099525 a(n)=2a(n-1)+3a(n-3); a(n)=sum{k=0..floor(n/3), binomial(n-2k, k)3^(n-2k)(2/3)^(n-3k)}.
%K A099525 easy,nonn
%O A099525 0,2
%A A099525 _Paul Barry_, Oct 20 2004