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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 (list; graph; refs; listen; history; text; internal format)



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 tree-like polyhexes; see the Harary-Read reference).


Table of n, a(n) for n=0..24.

F. Harary and R. C. Read, The enumeration of tree-like polyhexes, Proc. Edinburgh Math. Soc. (2) 17 (1970), 1-13.


a(n) = A126183(n,0).

a(n) = (n+8)*3^(n-2) for n>=1; a(0)=1.

G.f.: (1-3z+z^2)/(1-3z)^2.


1, seq(3^(n-2)*(n+8), n=1..28);


Cf. A126183.

Sequence in context: A120897 A077825 A049219 * A292397 A060557 A018920

Adjacent sequences:  A126181 A126182 A126183 * A126185 A126186 A126187




Emeric Deutsch, Dec 19 2006



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Last modified December 7 22:29 EST 2019. Contains 329850 sequences. (Running on oeis4.)