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 A117353 #5 Jun 13 2015 00:52:04
%S A117353 1,2,5,9,18,33,65,126,253,505,1018,2041,4097,8198,16405,32793,65570,
%T A117353 131089,262145,524238,1048493,2097017,4194186,8388521,16777281,
%U A117353 33554646,67109317,134218265,268436018,536871105,1073741505,2147482398
%N A117353 Expansion of (1-x)/(1-3x+x^2+4x^3-4x^4).
%C A117353 Diagonal sums of number triangle A117352.
%H A117353 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,-1,-4,4).
%F A117353 G.f.: (1-x)/((1-2x)(1-x-x^2+2x^3)); a(n)=3a(n-1)-a(n-2)-4a(n-3)+4a(n-4); a(n)=sum{k=0..floor(n/2), sum{j=0..n-2k, C(j-k,j)C(n-2k,j)}}.
%K A117353 easy,nonn
%O A117353 0,2
%A A117353 _Paul Barry_, Mar 09 2006