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 A099215 #8 Jun 13 2015 00:51:32
%S A099215 1,2,4,11,34,104,313,938,2812,8435,25306,75920,227761,683282,2049844,
%T A099215 6149531,18448594,55345784,166037353,498112058,1494336172,4483008515,
%U A099215 13449025546,40347076640,121041229921,363123689762,1089371069284
%N A099215 a(n)=4a(n-1)-4a(n-2)+3a(n-3).
%H A099215 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (4,-4,3).
%F A099215 G.f.: (1-2x)/((1-2x)^2-3x^3); a(n)=sum{k=0..floor(n/3), binomial(n-k, 2k)3^k*2^(n-3k)}.
%F A099215 a{x}=1/7(-Cos[(Pi*x)/3]+Cosh[x*Log[3]]+3*Sqrt[3]*Sin[(Pi*x)/3]+Sinh[x*Log[3]]) - _Harvey P. Dale_, Mar 02 2013
%t A099215 LinearRecurrence[{4,-4,3},{1,2,4},30] (* _Harvey P. Dale_, Mar 02 2013 *)
%Y A099215 Cf. A099214.
%K A099215 easy,nonn
%O A099215 0,2
%A A099215 _Paul Barry_, Oct 06 2004