OFFSET
0,3
COMMENTS
Previous name was: A simple grammar.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 713
FORMULA
a(n) ~ c * d^n / n^(3/2), where d = 7.969494030514425004826375511986491746399264355846412073489715938424..., c = 0.12982932099206082951153936270704832022771078... . - Vaclav Kotesovec, Feb 24 2015
From Ilya Gutkovskiy, Apr 13 2019: (Start)
G.f. A(x) satisfies: A(x) = x*exp(3*Sum_{k>=1} (-1)^(k+1)*A(x^k)/k).
G.f.: A(x) = Sum_{n>=1} a(n)*x^n = x * Product_{n>=1} (1 + x^n)^(3*a(n)). (End)
EXAMPLE
a(3) = 12:
o o o o o o o o o o o o
| | | | | | | | | / \ / \ / \
1 1 1 2 2 2 3 3 3 1 2 1 3 2 3
| | | | | | | | |
1 2 3 1 2 3 1 2 3 - Alois P. Heinz, Feb 24 2015
MAPLE
spec := [S, {S=Prod(B, B, B, Z), B=PowerSet(S)}, unlabeled]: seq(combstruct[count](spec, size=n), n=0..20);
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
EXTENSIONS
New name from Vaclav Kotesovec, Feb 24 2015
STATUS
approved