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 A124281 #5 Jun 13 2015 00:52:16
%S A124281 1,1,3,3,9,11,29,39,93,135,301,463,981,1575,3213,5327,10565,17943,
%T A124281 34845,60255,115189,201895,381453,675375,1264869,2256503,4198397,
%U A124281 7532415,13945941,25126983,46350829,83777743,154117317,279225111
%N A124281 Expansion of 1/(1-x-2*x^2+2*x^3-2*x^4).
%H A124281 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,-2,2).
%F A124281 a(n)=sum{k=0..floor(n/2), sum{j=0..n-2k, C(j,n-2k-j)C(k,n-2k-j)*2^k}}
%Y A124281 Cf. A124280.
%K A124281 easy,nonn
%O A124281 0,3
%A A124281 _Paul Barry_, Oct 24 2006