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A126804
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a(n) = (2n)! / (n-1)!.
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13
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2, 24, 360, 6720, 151200, 3991680, 121080960, 4151347200, 158789030400, 6704425728000, 309744468633600, 15543540607795200, 841941782922240000, 48962152914554880000, 3042648073975910400000, 201220459292273541120000, 14110584707870682071040000
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OFFSET
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1,1
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COMMENTS
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Old name was "Multiplying n X n integers above n".
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LINKS
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FORMULA
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a(n) = (2n)! / (n-1)!.
a(n) = (2*n*(2*n-1)/(n-1))*a(n-1).
E.g.f.: 2*x/(1-4*x)^(3/2). (End)
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EXAMPLE
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a(5) = 151200 because five digits above 5: (6, 7, 8, 9, 10), multiplied by five equals 5*(6*7*8*9*10) = 151200.
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MAPLE
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a:=n->sum((count(Permutation(2*n+2), size=n+1)), j=0..n): seq(a(n), n=0..15); # Zerinvary Lajos, May 03 2007
with(combstruct):with(combinat) :bin := {B=Union(Z, Prod(B, B))}: seq (count([B, bin, labeled], size=n)*(n-1), n=2..17); # Zerinvary Lajos, Dec 05 2007
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MATHEMATICA
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PROG
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(Magma) [Factorial(2*n)/Factorial(n-1) : n in [1..20]]; // Wesley Ivan Hurt, Dec 14 2015
(PARI) x='x+O('x^99); Vec(serlaplace(2*x/(1-4*x)^(3/2))) \\ Altug Alkan, Mar 11 2018
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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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Jonathan R. Love (japanada11(AT)yahoo.ca), Feb 22 2007
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EXTENSIONS
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STATUS
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approved
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