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A193775
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Number of signed permutations invariant under the reverse complement and avoiding (-2, -1), (-1, 2), and (1, -2).
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0
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OFFSET
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0,2
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LINKS
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Table of n, a(n) for n=0..9.
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FORMULA
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a(n) = (\lfloor n/2\rfloor)! 2^{\lfloor n/2\rfloor} + \sum_{k=0}^{\lfloor n/2\lceil} k!
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EXAMPLE
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For n = 3, the permutations are (1, 2, 3), (3, 2, 1), (-1, -2, -3), and (-3, 2, -1).
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CROSSREFS
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Sequence in context: A005456 A100773 A250112 * A131120 A115195 A095384
Adjacent sequences: A193772 A193773 A193774 * A193776 A193777 A193778
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KEYWORD
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nonn
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AUTHOR
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Andy Hardt, Aug 04 2011
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STATUS
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approved
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