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Search: id:A090932
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| 1, 1, 1, 3, 6, 30, 90, 630, 2520, 22680, 113400, 1247400, 7484400, 97297200, 681080400, 10216206000, 81729648000, 1389404016000, 12504636144000, 237588086736000, 2375880867360000, 49893498214560000
(list; graph; listen)
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OFFSET
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0,4
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COMMENT
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Number of ordered permutations of the n-th row of Pascal's triangle.
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FORMULA
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a(n) = binomial(n-1, 2) * a(n-2). E.g.f.: (1+x)/(1-1/2*x^2).
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EXAMPLE
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a(5) = 5!/2^2 = 120/4 = 30.
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MAPLE
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a:=n->mul(denom(k/binomial(k, 2)), k=3..n): seq(a(n), n=1..22); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Apr 27 2008
a:=n->mul(numer ((k+1)/(k+3)), k=0..n): seq(a(n), n=-1..20); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Apr 27 2008
a:=n->mul(denom ((k+1)/(k+3)), k=0..n): seq(a(n), n=-3..18); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Apr 27 2008
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PROGRAM
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(PARI) a(n)=n!/2^floor(n/2)
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CROSSREFS
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Cf. A052277, A007019.
The function appears in several expansions: A009775, A046979, A046981, A007415, A007452.
Sequence in context: A136944 A136946 A125521 this_sequence A157534 A133799 A088436
Adjacent sequences: A090929 A090930 A090931 this_sequence A090933 A090934 A090935
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KEYWORD
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nonn
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AUTHOR
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Jon Perry (perry(AT)globalnet.co.uk), Feb 26 2004
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EXTENSIONS
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Edited by Ralf Stephan (ralf(AT)ark.in-berlin.de), Sep 07 2004
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