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 A052715 #21 May 28 2022 04:01:34
%S A052715 0,0,0,0,24,480,10080,241920,6652800,207567360,7264857600,
%T A052715 282291609600,12067966310400,563171761152000,28496491114291200,
%U A052715 1554354060779520000,90929712555601920000,5679609738088366080000
%N A052715 Expansion of e.g.f. (1-2*x-sqrt(1-4*x))/2 - x*(1-2*x-sqrt(1-4*x)) - x^2.
%H A052715 G. C. Greubel, <a href="/A052715/b052715.txt">Table of n, a(n) for n = 0..350</a>
%H A052715 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=671">Encyclopedia of Combinatorial Structures 671</a>
%F A052715 D-finite with recurrence: a(1)=0; a(2)=0; a(3)=0; a(4)=24; 4*(-n-3+2*n^2)*a(n) +2*(-1-3*n)*a(n+1) +a(n+2) =0.
%F A052715 a(n) = n!*A002057(n-4). - _R. J. Mathar_, Oct 18 2013
%p A052715 spec := [S,{B=Union(Z,C),C=Prod(B,B),S=Prod(C,C)},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052715 With[{nn=20},CoefficientList[Series[(1-2x-Sqrt[1-4x])/2-x(1-2x- Sqrt[ 1-4x])- x^2,{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Jul 18 2016 *)
%o A052715 (Magma) [n le 3 select 0 else 2*(n-3)*Factorial(n-1)*Catalan(n-2): n in [0..30]]; // _G. C. Greubel_, May 27 2022
%o A052715 (SageMath) [0]+[4*factorial(n-1)*binomial(2*n-5, n-4) for n in (1..30)] # _G. C. Greubel_, May 27 2022
%Y A052715 Cf. A000108, A002057.
%K A052715 easy,nonn
%O A052715 0,5
%A A052715 encyclopedia(AT)pommard.inria.fr, Jan 25 2000