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 A052631 #16 May 15 2019 02:59:02
%S A052631 0,1,4,30,288,3480,50400,851760,16450560,357436800,8629286400,
%T A052631 229162348800,6638962176000,208362342988800,7042436719718400,
%U A052631 255029193619200000,9851119008546816000,404305986955014144000,17569457946995834880000,805912049524456562688000
%N A052631 a(n) = n!*Pell(n) (or n!*A000129(n)).
%H A052631 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=577">Encyclopedia of Combinatorial Structures 577</a>
%F A052631 E.g.f.: -x/(-1 + 2*x + x^2).
%F A052631 Recurrence: {a(1)=1, a(0)=0, (-2-n^2-3*n)*a(n)+(-4-2*n)*a(n+1)+a(n+2)}.
%F A052631 Sum((-1/4)*(-1+_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^2))*n!.
%p A052631 spec := [S,{S=Prod(Z,Sequence(Union(Z,Z,Prod(Z,Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%p A052631 with(combstruct):ZL:=[T,{T=Union(Z,Prod(Epsilon,Z,T),Prod(T,Z,Epsilon),Prod(T,Z,Z))},labeled]:seq(count(ZL,size=i),i=0..19); # _Zerinvary Lajos_, Dec 16 2007
%K A052631 easy,nonn
%O A052631 0,3
%A A052631 encyclopedia(AT)pommard.inria.fr, Jan 25 2000