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A137729
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Number of circular permutations of the multiset {1,1,2,2,...,n,n} (up to rotations).
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11
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1, 1, 2, 16, 318, 11352, 623760, 48648960, 5108105520, 694702028160, 118794043549440, 24946749109094400, 6311527524161798400, 1893458257242791500800, 664603848292138865510400, 269829162406607158901145600
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) = ((2n)!/2^n + n!) / (2n).
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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