%I A052989
%S A052989 1,0,1,1,3,3,7,10,19,29,52,84,145,239,407,678,1146,1918,3232,5421,9121,
%T A052989 15314,25749,43252,72701,122146,205282,344931,579662,974038,1636836,
%U A052989 2750523,4622090,7766989,13051877,21932553,36855997,61933449,104074334
%N A052989 A simple regular expression.
%H A052989 INRIA Algorithms Project, <a href="http://algo.inria.fr/bin/encyclopedia?Search=ECSnb&argsearch=1063">
               Encyclopedia of Combinatorial Structures 1063</a>
%F A052989 G.f.: -(-1+x)/(1-x-x^2-x^4+x^5)
%F A052989 Recurrence: {a(1)=0, a(0)=1, a(2)=1, a(3)=1, a(4)=3, a(n)-a(n+1)-a(n+3)-a(n+4)+a(n+5)}
%F A052989 Sum(-1/8519*(-389-2111*_alpha+619*_alpha^2-358*_alpha^3+541*_alpha^4)*_alpha^(-1-n), 
               _alpha=RootOf(1-_Z-_Z^2-_Z^4+_Z^5))
%p A052989 spec := [S,{S=Sequence(Prod(Union(Prod(Z,Z),Sequence(Z)),Z,Z))},unlabeled 
               ]: seq(combstruct[count ](spec,size=n), n=0..20);
%Y A052989 Sequence in context: A157933 A013915 A136445 this_sequence A022403 A082550 
               A036056
%Y A052989 Adjacent sequences: A052986 A052987 A052988 this_sequence A052990 A052991 
               A052992
%K A052989 easy,nonn
%O A052989 0,5
%A A052989 encyclopedia(AT)pommard.inria.fr, Jan 25 2000
%E A052989 More terms from James A. Sellers (sellersj(AT)math.psu.edu), Jun 06 2000