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Search: id:A126184
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| A126184 |
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Number of hex trees with n edges and having no nonroot nodes of outdegree 2. 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). |
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+0
2
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| 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; listen)
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OFFSET
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0,2
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COMMENT
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a(n)=A126183(n,0).
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REFERENCES
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F. Harary and R. C. Read, The enumeration of tree-like polyhexes, Proc. Edinburgh Math. Soc. (2) 17 (1970), 1-13.
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FORMULA
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a(n)=(n+8)*3^(n-2) for n>=1; a(0)=1. G.f.=(1-3z+z^2)/(1-3z)^2.
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MAPLE
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1, seq(3^(n-2)*(n+8), n=1..28);
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CROSSREFS
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Cf. A126183.
Sequence in context: A120897 A077825 A049219 this_sequence A060557 A018920 A006190
Adjacent sequences: A126181 A126182 A126183 this_sequence A126185 A126186 A126187
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KEYWORD
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nonn
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AUTHOR
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Emeric Deutsch (deutsch(AT)duke.poly.edu), Dec 19 2006
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