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E.g.f. 3x(1+x-x^2)/(1-x).
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%I #11 Apr 18 2017 07:03:57

%S 0,3,12,18,72,360,2160,15120,120960,1088640,10886400,119750400,

%T 1437004800,18681062400,261534873600,3923023104000,62768369664000,

%U 1067062284288000,19207121117184000,364935301226496000

%N E.g.f. 3x(1+x-x^2)/(1-x).

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=583">Encyclopedia of Combinatorial Structures 583</a>

%F E.g.f.: 3*(-x+x^2-1)*x/(-1+x)

%F Recurrence: {a(0)=0, (-1-n)*a(n)+a(n+1)=0, a(1)=3, a(3)=18, a(2)=12}

%F 3*n!, n>2.

%p spec := [S,{S=Prod(Union(Z,Sequence(Z)),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