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 A052739 #19 Jan 30 2020 21:29:14
%S A052739 0,0,2,24,288,4800,103680,2741760,85800960,3100446720,127037030400,
%T A052739 5819550105600,294727768473600,16350861400473600,986127353590579200,
%U A052739 64238655955009536000,4495021381191204864000,336249161369543245824000
%N A052739 E.g.f. (1-sqrt(1-4x-4x^2))/2 -x*(1+x).
%H A052739 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=695">Encyclopedia of Combinatorial Structures 695</a>
%F A052739 D-finite with recurrence: a(1)=0; a(2)=2; a(3)=24; a(4)=288; (-4*n^2+4)*a(n) +(-4*n-2)*a(n+1) +a(n+2)=0.
%F A052739 a(n) = A052727(n), n>2.- _R. J. Mathar_, Jan 26 2020
%p A052739 spec := [S,{B=Prod(Z,Z),S=Prod(C,C),C=Union(B,S,Z)},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052739 With[{nn=20},CoefficientList[Series[(1-Sqrt[1-4x-4x^2])/2-x(1+x),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Jun 19 2018 *)
%K A052739 easy,nonn
%O A052739 0,3
%A A052739 encyclopedia(AT)pommard.inria.fr, Jan 25 2000