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%I #18 Jun 03 2022 19:17:04
%S 1,4,26,240,2904,43680,786960,16531200,396789120,10713669120,
%T 321413702400,10606692096000,381841394457600,14891820610867200,
%U 625456552834713600,28145546185236480000,1350986237814140928000,68900298484208615424000,3720616124549638938624000
%N a(n) = (3^(n+1)-1)*n!/2.
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=520">Encyclopedia of Combinatorial Structures 520</a>
%F E.g.f.: 1/(-1+x)/(-1+3*x).
%F Recurrence: {a(0)=1, a(1)=4, (3*n^2+9*n+6)*a(n)+(-4*n-8)*a(n+1)+a(n+2)=0.}.
%F a(n) = (-1/2+1/2*3^(n+1))*n!.
%F a(n)=n!*A003462(n+1). - _R. J. Mathar_, Jun 03 2022
%p spec := [S,{S=Prod(Sequence(Z),Sequence(Union(Z,Z,Z)))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%K easy,nonn
%O 0,2
%A encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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