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A199495
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Number of permutations of [n] starting and ending with an odd number.
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2
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0, 1, 0, 2, 4, 36, 144, 1440, 8640, 100800, 806400, 10886400, 108864000, 1676505600, 20118067200, 348713164800, 4881984307200, 94152554496000, 1506440871936000, 32011868528640000, 576213633515520000, 13380961044971520000, 267619220899430400000
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OFFSET
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0,4
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LINKS
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FORMULA
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a(n) = 2*(n-2)! * [x^(n-2)] x/((1+x)^2*(1-x)^3) for n>=2, else a(n) = n.
a(n) = 2*A152887(n-2) for n>=2, else a(n) = n. (End)
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MAPLE
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a:= n-> `if`(n<2, n, ceil(n/2)*(ceil(n/2)-1)*(n-2)!):
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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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