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%I #13 Apr 18 2017 07:03:59
%S 1,2,8,48,408,4320,54720,806400,13587840,257644800,5428684800,
%T 125817753600,3181049625600,87128475033600,2570016024576000,
%U 81222270345216000,2738060898693120000,98070849049485312000
%N E.g.f. 1/(1-2x-x^4).
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=615">Encyclopedia of Combinatorial Structures 615</a>
%F E.g.f.: -1/(-1+2*x+x^4)
%F Recurrence: {a(0)=1, a(1)=2, a(2)=8, a(3)=48, (-n^4-35*n^2-50*n-24-10*n^3)*a(n)+(-2*n-8)*a(n+3)+a(n+4)=0}
%F Sum(1/86*(27+18*_alpha^3+12*_alpha^2+8*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^4))*n!
%F a(n) = n!*A008999(n). - _R. J. Mathar_, Nov 27 2011
%p spec := [S,{S=Sequence(Union(Z,Z,Prod(Z,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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