%I A052687
%S A052687 1,2,6,18,96,480,3600,25200,241920,2177280,25401600,279417600,
%T A052687 3832012800,49816166400,784604620800,11769069312000,209227898880000,
%U A052687 3556874280960000,70426110763008000,1338096104497152000
%N A052687 A simple regular expression in a labeled universe.
%H A052687 INRIA Algorithms Project, <a href="http://algo.inria.fr/bin/encyclopedia?Search=ECSnb&argsearch=635">
               Encyclopedia of Combinatorial Structures 635</a>
%F A052687 E.g.f.: -(-x+x^3-1)/(-1+x)/(-1+x^2)
%F A052687 Recurrence: {a(0)=1, a(1)=2, a(2)=6, (-n^3-8*n^2-17*n-10)*a(n)+(-2-n)*a(n+1)+(n+4)*a(n+2), 
               a(3)=18}
%F A052687 (7/4+1/4*(-1)^(-n)+1/2*n)*n!
%p A052687 spec := [S,{S=Prod(Sequence(Z),Union(Z,Sequence(Prod(Z,Z))))},labeled]: 
               seq(combstruct[count](spec,size=n), n=0..20);
%Y A052687 Sequence in context: A053505 A000138 A028857 this_sequence A162059 A162060 
               A162061
%Y A052687 Adjacent sequences: A052684 A052685 A052686 this_sequence A052688 A052689 
               A052690
%K A052687 easy,nonn
%O A052687 0,2
%A A052687 encyclopedia(AT)pommard.inria.fr, Jan 25 2000