

A126184


Number of hex trees with n edges and having no nonroot nodes of outdegree 2.


2



1, 3, 10, 33, 108, 351, 1134, 3645, 11664, 37179, 118098, 373977, 1180980, 3720087, 11691702, 36669429, 114791256, 358722675, 1119214746, 3486784401, 10847773692, 33705582543, 104603532030, 324270949293, 1004193907488
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OFFSET

0,2


COMMENTS

A hex tree is a rooted tree where each vertex has 0, 1, or 2 children and, when only one child is present, it is either a left child, or a middle child, or a right child (name due to an obvious bijection with certain treelike polyhexes; see the HararyRead reference).


LINKS

Table of n, a(n) for n=0..24.
F. Harary and R. C. Read, The enumeration of treelike polyhexes, Proc. Edinburgh Math. Soc. (2) 17 (1970), 113.


FORMULA

a(n) = A126183(n,0).
a(n) = (n+8)*3^(n2) for n>=1; a(0)=1.
G.f.: (13z+z^2)/(13z)^2.


MAPLE

1, seq(3^(n2)*(n+8), n=1..28);


CROSSREFS

Cf. A126183.
Sequence in context: A120897 A077825 A049219 * A292397 A060557 A018920
Adjacent sequences: A126181 A126182 A126183 * A126185 A126186 A126187


KEYWORD

nonn


AUTHOR

Emeric Deutsch, Dec 19 2006


STATUS

approved



