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A193776
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Number of signed permutations of length n invariant under the reverse complement and avoiding (-2, -1), (-2, 1), (2, -1).
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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(2k) = 2^k k! + \sum_{j=0}^{k-1}(k-j-1)! a(2j)
a(2k+1) = (2^k + 1)k! + \sum_{j=0}^{k-1}(k-j-1)! a(2j+1)
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EXAMPLE
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For n = 2, the permutations are (1, 2), (2, 1), (-1, -2), (-2, -1).
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CROSSREFS
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Sequence in context: A002139 A140489 A331101 * A051915 A064688 A089891
Adjacent sequences: A193773 A193774 A193775 * A193777 A193778 A193779
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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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