%I A052937
%S A052937 2,3,6,13,30,71,170,409,986,2379,5742,13861,33462,80783,195026,470833,
%T A052937 1136690,2744211,6625110,15994429,38613966,93222359,225058682,
%U A052937 543339721,1311738122,3166815963,7645370046,18457556053,44560482150
%N A052937 A simple regular expression.
%H A052937 INRIA Algorithms Project, <a href="http://algo.inria.fr/bin/encyclopedia?Search=ECSnb&argsearch=928">
               Encyclopedia of Combinatorial Structures 928</a>
%F A052937 G.f.: -(-2+3*x+x^2)/(-1+x)/(-1+2*x+x^2)
%F A052937 Recurrence: {a(2)=6, a(1)=3, a(0)=2, -a(n)-2*a(n+1)+a(n+2)+2}
%F A052937 1+Sum(1/4*(1+_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^2))
%F A052937 a(n) = A000129(n+1)+1, where A000129 are the Pell Numbers. - Graeme McRae 
               (g_m(AT)mcraefamily.com), Aug 03 2006
%p A052937 spec := [S,{S=Union(Sequence(Z),Sequence(Union(Z,Z,Prod(Z,Z))))},unlabeled 
               ]: seq(combstruct[count ](spec, size=n), n=0..20);
%Y A052937 Cf. A001333.
%Y A052937 Sequence in context: A107316 A124682 A079512 this_sequence A005554 A077212 
               A076836
%Y A052937 Adjacent sequences: A052934 A052935 A052936 this_sequence A052938 A052939 
               A052940
%K A052937 easy,nonn
%O A052937 0,1
%A A052937 encyclopedia(AT)pommard.inria.fr, Jan 25 2000
%E A052937 More terms from James A. Sellers (sellersj(AT)math.psu.edu), Jun 05 2000