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 A101551 #16 Dec 30 2023 22:05:37
%S A101551 0,0,0,0,1,3,6,10,15,21,29,42,66,111,192,330,554,906,1452,2303,3651,
%T A101551 5826,9382,15225,24807,40431,65748,106584,172321,278184,448980,725140,
%U A101551 1172412,1897380,3072365,4975551,8055918,13038606,21096027,34125561
%N A101551 a(n) = C(n-2,2)+C(n-5,5)+...+C(n-(3*floor((n-3)/3)+2),3*floor((n-3)/3)+2).
%H A101551 Vincenzo Librandi, <a href="/A101551/b101551.txt">Table of n, a(n) for n = 0..1000</a>
%H A101551 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1,0,0,1)
%F A101551 G.f.: x^4/((1-x)^3-x^6) = -x^4/ ((x^2+x-1)*(x^4-x^3+2*x^2-2*x+1)).
%F A101551 a(n) = Sum_{k=0..n} if(mod(k+1, 3)=0, C(n-k, k), 0).
%F A101551 a(n+2) = Sum_{k=0..floor(n/6)} binomial(n-3k, 3k+2). - _Paul Barry_, Jan 13 2005
%t A101551 CoefficientList[Series[x^4/((1-x)^3-x^6),{x,0,50}],x] (* _Vincenzo Librandi_, Jul 08 2012 *)
%t A101551 LinearRecurrence[{3,-3,1,0,0,1},{0,0,0,0,1,3},40] (* _Harvey P. Dale_, Feb 20 2014 *)
%Y A101551 Cf. A024490, A101552.
%K A101551 easy,nonn
%O A101551 0,6
%A A101551 _Paul Barry_, Dec 06 2004