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A137749
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Number of circular permutations of the multiset {1,1,2,2,...,2n,2n} (up to rotations) with even distances between equal elements.
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7
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1, 30, 13560, 27785520, 162030637440, 2156625389318400, 56857271240920550400, 2686506065987036477184000, 211180868835057744408834048000, 26072812428113877344085395644416000, 4829206317935252350431489703482654720000
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OFFSET
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1,2
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LINKS
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FORMULA
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a(n) = ((2n)!^3 / n!^2 / 2^(2n) + (2n)!) / (4n).
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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