%I A052621
%S A052621 2,1,2,6,48,120,720,5040,80640,362880,3628800,39916800,958003200,
%T A052621 6227020800,87178291200,1307674368000,41845579776000,355687428096000,
%U A052621 6402373705728000,121645100408832000,4865804016353280000
%N A052621 A simple regular expression in a labeled universe.
%H A052621 INRIA Algorithms Project, <a href="http://algo.inria.fr/bin/encyclopedia?Search=ECSnb&argsearch=567">
               Encyclopedia of Combinatorial Structures 567</a>
%F A052621 E.g.f.: -(x^3+x^2+x+2)/(-1+x^4)
%F A052621 Recurrence: {a(1)=1, a(3)=6, a(2)=2, a(0)=2, (-n^4-35*n^2-50*n-24-10*n^3)*a(n)+a(n+4)}
%F A052621 Sum(1/4*(_alpha^3+_alpha^2+2*_alpha+1)*_alpha^(-1-n), _alpha=RootOf(-1+_Z^4))*n!
%F A052621 2n! if n is 0 mod 4, n! otherwise.
%p A052621 spec := [S,{S=Union(Sequence(Z), Sequence(Prod(Z,Z,Z,Z)))},labeled]: 
               seq(combstruct[count](spec,size=n), n=0..20);
%Y A052621 Sequence in context: A038078 A000139 A114572 this_sequence A131057 A051852 
               A054495
%Y A052621 Adjacent sequences: A052618 A052619 A052620 this_sequence A052622 A052623 
               A052624
%K A052621 easy,nonn
%O A052621 0,1
%A A052621 encyclopedia(AT)pommard.inria.fr, Jan 25 2000