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A129815
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Number of reverse alternating fixed-point-free permutations on n letters.
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3
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0, 0, 1, 2, 6, 22, 102, 506, 2952, 18502, 131112, 991226, 8271792, 73176262, 703077552, 7121578106, 77437418112, 883521487942, 10726837356672, 136104948161786
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OFFSET
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1,4
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COMMENTS
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From Emeric Deutsch, Aug 06 2009: (Start)
Reverse alternating permutations are called also up-down permutations.
a(n) is also the number of reverse alternating permutations having exactly 1 fixed point (see the Stanley reference). Example: a(4)=2 because we have 1423 and 2314.
(End)
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LINKS
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Table of n, a(n) for n=1..20.
R. P. Stanley, Alternating permutations and symmetric functions
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FORMULA
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a(2n-1) = A129817(2n-1). [Emeric Deutsch, Aug 06 2009]
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EXAMPLE
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a(4)=2 because we have 3412 and 2413. [Emeric Deutsch, Aug 06 2009]
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CROSSREFS
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Cf. A000111, A000166, A007779.
Sequence in context: A002772 A000140 A079263 * A103941 A064643 A218531
Adjacent sequences: A129812 A129813 A129814 * A129816 A129817 A129818
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KEYWORD
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more,nonn
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AUTHOR
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Vladeta Jovovic, May 20 2007
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STATUS
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approved
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