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Search: id:A120908
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| A120908 |
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Sum of the lengths of the drops in all ternary words of length n on {0,1,2}. The drops of a ternary word on {0,1,2} are the subwords 10,20 and 21, their lengths being the differences 1, 2 and 1, respectively. |
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+0
2
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| 0, 4, 24, 108, 432, 1620, 5832, 20412, 69984, 236196, 787320, 2598156, 8503056, 27634932, 89282088, 286978140, 918330048, 2927177028, 9298091736, 29443957164, 92980917360, 292889889684, 920511081864, 2887057484028
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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a(n)=4*A027471(n). a(n)=sum(k*A120907(n,k), k>=0).
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FORMULA
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a(n)=4(n-1)3^(n-2). G.f.=4z^2/(1-3z)^2.
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EXAMPLE
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a(2)=4 because the ternary words 00,01,02,11,12 and 22 have no drops, each of the words 10 and 21 has one drop of length 1 and the word 20 has one drop of length 2.
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MAPLE
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seq(4*(n-1)*3^(n-2), n=1..27);
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CROSSREFS
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Cf. A027471, A120906, A120907.
Sequence in context: A006736 A165752 A166036 this_sequence A145655 A059153 A129032
Adjacent sequences: A120905 A120906 A120907 this_sequence A120909 A120910 A120911
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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), Jul 15 2006
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