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 A052743 #15 Jan 30 2020 21:29:14
%S A052743 0,1,2,6,48,480,5040,65520,1048320,18869760,377395200,8462361600,
%T A052743 209802700800,5666588928000,165900288153600,5242466541312000,
%U A052743 177718177308672000,6430828699975680000,247490157968621568000
%N A052743 E.g.f. ( 1-x-sqrt(1-2*x+x^2-4*x^3) )/(2*x^2).
%H A052743 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=699">Encyclopedia of Combinatorial Structures 699</a>
%F A052743 D-finite with recurrence: a(1)=1; a(3)=6; a(2)=2; a(4)=48; (-56*n^2-26*n^3-12-46*n-4*n^4)*a(n) +(n^3+7*n^2+16*n+12)*a(n+1) +(-13*n-2*n^2-21)*a(n+2) +(n+5)*a(n+3) =0.
%p A052743 spec := [S,{B=Prod(C,C),S=Union(B,Z,C),C=Prod(S,Z)},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%K A052743 easy,nonn
%O A052743 0,3
%A A052743 encyclopedia(AT)pommard.inria.fr, Jan 25 2000