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 A106855 #6 Jun 13 2015 00:51:49
%S A106855 1,0,1,-3,1,-6,10,-9,28,-39,55,-123,172,-288,541,-804,1405,-2427,3817,
%T A106855 -6642,11098,-18093,31024,-51387,85303,-144459,239464,-400368,672841,
%U A106855 -1118760,1873945,-3137283,5230225,-8759118,14642074,-24449793,40919428,-68376015,114268807,-191134299,319396852
%N A106855 Expansion of 1/(1-x^2(1-3x)).
%C A106855 Diagonal sums of Riordan array (1,x(1-3x)).
%H A106855 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,1,-3).
%F A106855 G.f.: 1/(1-x^2+3x^3); a(n)=a(n-2)-3a(n-3); a(n)=sum{k=0..floor(n/2), (-1)^n*binomial(k, n-2k)*3^(n-2k)}.
%Y A106855 Cf. A052947, A106852.
%K A106855 easy,sign
%O A106855 0,4
%A A106855 _Paul Barry_, May 08 2005