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A338375
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Number of digits in (2n)! / (2^n * n!).
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1
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1, 1, 2, 3, 3, 5, 6, 7, 8, 9, 11, 12, 13, 15, 16, 18, 19, 21, 22, 24, 26, 27, 29, 31, 32, 34, 36, 37, 39, 41, 43, 45, 46, 48, 50, 52, 54, 56, 58, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99, 101, 104, 106, 108, 110
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OFFSET
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1,3
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COMMENTS
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a(n) is the number of digits in double factorial of odd numbers (see A001147).
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LINKS
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FORMULA
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a(n) = floor(log((2n)!/((2^n)*n!))/log(10))+1.
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EXAMPLE
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For n = 7, (2*n-1)!! = 13!! = 135135 and the number of digits is 6.
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MAPLE
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seq(1 + ilog10(doublefactorial(2*n-1)), n=1..100); # Robert Israel, Jan 15 2024
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MATHEMATICA
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a[n_] := IntegerLength[(2 n - 1)!!]; Array[a, 65] (* Amiram Eldar, Oct 23 2020 *)
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PROG
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(Sage) floor((log(factorial(2 * n) / ((2 ** n) * factorial(n))) / log(10))) + 1
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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