%I A052574
%S A052574 1,1,4,30,312,4080,64080,1174320,24595200,579519360,15172012800,
%T A052574 436929292800,13726748851200,467182235520000,17123385600921600,
%U A052574 672444082582272000,28167703419727872000,1253648083943743488000
%N A052574 A simple regular expression in a labeled universe.
%H A052574 INRIA Algorithms Project, <a href="http://algo.inria.fr/bin/encyclopedia?Search=ECSnb&argsearch=517">
               Encyclopedia of Combinatorial Structures 517</a>
%F A052574 E.g.f.: -(-1+2*x)/(1-3*x+x^2)
%F A052574 Recurrence: {a(1)=1, a(0)=1, (n^2+3*n+2)*a(n)+(-6-3*n)*a(n+1)+a(n+2)}
%F A052574 Sum(1/5*(-1+4*_alpha)*_alpha^(-1-n), _alpha=RootOf(_Z^2-3*_Z+1))*n!
%p A052574 spec := [S,{S=Sequence(Prod(Z,Sequence(Prod(Z,Sequence(Z)))))},labeled]: 
               seq(combstruct[count](spec,size=n), n=0..20);
%Y A052574 Sequence in context: A052452 A088794 A145348 this_sequence A158834 A139086 
               A128329
%Y A052574 Adjacent sequences: A052571 A052572 A052573 this_sequence A052575 A052576 
               A052577
%K A052574 easy,nonn
%O A052574 0,3
%A A052574 encyclopedia(AT)pommard.inria.fr, Jan 25 2000