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A114338
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Number of divisors of n!! (double factorial = A006882(n)).
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2
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1, 1, 2, 2, 4, 4, 10, 8, 16, 16, 36, 32, 66, 64, 144, 120, 192, 240, 340, 480, 570, 864, 1200, 1728, 1656, 2880, 3456, 4320, 5616, 8640, 9072, 17280, 10752, 28800, 22176, 46080, 30240, 92160, 62208, 152064, 84240, 304128, 128000, 608256, 201600
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OFFSET
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0,3
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COMMENTS
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LINKS
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FORMULA
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EXAMPLE
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a(5) = 4 since 5!! = 15 and the divisors are 1, 3, 5 and 15.
a(6) = 10 because 6!! = A006882(6) = 48 has precisely ten distinct divisors: 1, 2, 3, 4, 6, 8, 12, 16, 24, 48. - Michel Lagneau, Dec 07 2016
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MAPLE
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f := proc(n)
numtheory[tau](doublefactorial(n)) ;
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MATHEMATICA
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DivisorSigma[0, Range[50]!! ]
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PROG
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(PARI) df(n) = if( n<0, 0, my(E); E = exp(x^2 / 2 + x * O(x^n)); n! * polcoeff( 1 + E * x * (1 + intformal(1 / E)), n)); \\ A006882
vector(100, n, n--; numdiv(df(n))) \\ Altug Alkan, Dec 07 2015
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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