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 A080043 #4 Aug 16 2015 12:03:56
%S A080043 1,4,21,100,465,2164,10053,46708,216993,1008100,4683381,21757828,
%T A080043 101081457,469599316,2181641637,10135364500,47086382913,218751625156,
%U A080043 1016265649365,4721317472932,21934066839825,101900219778100
%N A080043 a(n)=floor((2+sqrt(7))^n).
%H A080043 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4, 4, -4, -3).
%F A080043 G.f.: g(t)=(1+t^2+4t^3)/(1-4t-4t^2+4t^3+3t^4) a(n)=A080042(n)-(1+(-1)^n)/2
%F A080043 a(0)=1, a(1)=4, a(2)=21, a(3)=100, a(n)=4*a(n-1)+4*a(n-2)- 4*a(n-3)- 3*a(n-4). - _Harvey P. Dale_, Aug 11 2015
%t A080043 CoefficientList[Series[(1+t^2+4t^3)/(1-4t-4t^2+4t^3+3t^4), {t, 0, 25}], t]
%t A080043 With[{c=2+Sqrt[7]},Floor[c^Range[0,30]]] (* or *) LinearRecurrence[{4,4,-4,-3},{1,4,21,100},30]
%K A080043 easy,nonn
%O A080043 0,2
%A A080043 Mario Catalani (mario.catalani(AT)unito.it), Jan 21 2003