%I #17 Apr 18 2017 07:04:09

%S 0,0,2,6,34,220,1808,17388,194724,2478096,35418192,561533280,

%T 9785418432,185921027136,3825633439392,84756646285920,

%U 2011657535668128,50924796197369088,1369659967551038976,39003791158314816768,1172394903935534452992,37094744191300029964800

%N A simple grammar: cycles of pairs of cycles.

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=789">Encyclopedia of Combinatorial Structures 789</a>

%F E.g.f.: log(-1/(-1+log(-1/(-1+x))^2)).

%F E.g.f.: -log(1-(log(1-x))^2). - _Vaclav Kotesovec_, Oct 01 2013

%F a(n) ~ (n-1)! * (exp(1)/(exp(1)-1))^n. - _Vaclav Kotesovec_, Oct 01 2013

%p spec := [S,{B=Cycle(Z),C=Prod(B,B),S=Cycle(C)},labeled]: seq(combstruct[count](spec,size=n), n=0..20);

%t CoefficientList[Series[-Log[1-(Log[1-x])^2], {x, 0, 20}], x]* Range[0, 20]! (* _Vaclav Kotesovec_, Oct 01 2013 *)

%Y Cf. A185393.

%K easy,nonn

%O 0,3

%A encyclopedia(AT)pommard.inria.fr, Jan 25 2000

%E More terms from _Alois P. Heinz_, Mar 16 2016