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%I #12 Apr 18 2017 07:03:55
%S 1,2,16,180,2688,50160,1123200,29342880,876072960,29425939200,
%T 1098191001600,45083550758400,2019049224192000,97957525739673600,
%U 5118154482618777600,286518188551795200000,17108806991486386176000
%N E.g.f. (1-x)^2/(1-4x+x^2).
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=551">Encyclopedia of Combinatorial Structures 551</a>
%F E.g.f.: (-1+x)^2/(1-4*x+x^2)
%F Recurrence: {a(0)=1, a(1)=2, (n^2+3*n+2)*a(n)+(-4*n-8)*a(n+1)+a(n+2)=0, a(2)=16}
%F Sum(-1/3*(-1+2*_alpha)*_alpha^(-1-n), _alpha=RootOf(1-4*_Z+_Z^2))*n!
%p spec := [S,{S=Sequence(Prod(Sequence(Z),Sequence(Z),Union(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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