A052824 A simple grammar: cycles of pairs of cycles.

0, 0, 2, 6, 34, 220, 1808, 17388, 194724, 2478096, 35418192, 561533280, 9785418432, 185921027136, 3825633439392, 84756646285920, 2011657535668128, 50924796197369088, 1369659967551038976, 39003791158314816768, 1172394903935534452992, 37094744191300029964800

Offset

0,3

Links

Table of n, a(n) for n=0..21.

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 789

Formula

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

E.g.f.: -log(1-(log(1-x))^2). - Vaclav Kotesovec, Oct 01 2013

a(n) ~ (n-1)! * (exp(1)/(exp(1)-1))^n. - Vaclav Kotesovec, Oct 01 2013

Maple

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

Mathematica

CoefficientList[Series[-Log[1-(Log[1-x])^2], {x, 0, 20}], x]* Range[0, 20]! (* Vaclav Kotesovec, Oct 01 2013 *)

Crossrefs

Cf. A185393.

Sequence in context: A346189 A018953 A009199 * A371768 A019029 A019032

Adjacent sequences: A052821 A052822 A052823 * A052825 A052826 A052827

Keyword

easy,nonn

Author

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

Extensions

More terms from Alois P. Heinz, Mar 16 2016

Status

approved