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A010786
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Floor-factorial numbers: a(n) = Product_{k=1..n} floor(n/k).
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26
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1, 1, 2, 3, 8, 10, 36, 42, 128, 216, 600, 660, 3456, 3744, 9408, 18900, 61440, 65280, 279936, 295488, 1152000, 2116800, 4878720, 5100480, 31850496, 41472000, 93450240, 163762560, 568995840, 589317120, 3265920000, 3374784000, 11324620800, 19269550080, 42188636160
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OFFSET
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0,3
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COMMENTS
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Product floor(n/1)*floor(n/2)*floor(n/3)*...*floor(n/n).
a(n) is the number of functions f:[n]->[n] where f(x) is a multiple of x for all x in [n]. We note that there are floor[n/x] possible choices for each image of x under f. [Dennis P. Walsh, Nov 06 2014]
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LINKS
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FORMULA
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log(a(n)) ~ c * (n - log(2*Pi*n)/2), where c = 0.7885...
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EXAMPLE
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For n=4 the a(4)=8 functions are given by the image sequences <1,2,3,4>, <1,4,3,4>, <2,2,3,4>, <2,4,3,4>, <3,2,3,4>, <3,4,3,4>, <4,2,3,4>, and <4,4,3,4>. [Dennis P. Walsh, Nov 06 2014]
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MAPLE
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a := n -> mul( floor(n/k), k=1..n);
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MATHEMATICA
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Table[Product[Floor[n/k], {k, n}], {n, 40}] (* Harvey P. Dale, May 09 2017 *)
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PROG
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(Haskell)
a010786 n = product $ map (div n) [1..n]
(PARI) vector(50, n, prod(k=1, n, n\k)) \\ Michel Marcus, Nov 10 2014
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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