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Search: id:A089354
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| A089354 |
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Number of generalized {(1,2),(1,-1)}-Dyck paths of length 3n with no peaks at level 2. |
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+0
1
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| 1, 0, 1, 4, 19, 96, 508, 2780, 15607, 89392, 520337, 3069232, 18305876, 110214144, 668950744, 4088824140, 25146253311, 155491812384, 966142729939, 6029139839684, 37771401328459, 237467581184384, 1497754198565104
(list; graph; listen)
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OFFSET
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0,4
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FORMULA
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a(n)=(2/n)sum((-2)^i*(i+1)binomial(3n+1, n-2-i), i=0..n-2), n>=1. G.f.=g/(1+zg^2), where g=1+zg^3, g(0)=1. Also g=2sin(arcsin(3sqrt(3z)/2)/3)/sqrt(3z).
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EXAMPLE
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a(3)=4 because we have UUDUDDDDD, UUUDDDDDD, UUDDUDDDD, and UUDDDUDDD, where
U=(1,2) and D=(1,-1).
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CROSSREFS
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Adjacent sequences: A089351 A089352 A089353 this_sequence A089355 A089356 A089357
Sequence in context: A020060 A122394 A047781 this_sequence A083315 A025573 A006194
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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 26 2003
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