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A175430
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a(n) = (n-1)! * (n+1)!.
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5
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2, 6, 48, 720, 17280, 604800, 29030400, 1828915200, 146313216000, 14485008384000, 1738201006080000, 248562743869440000, 41758540970065920000, 8142915489162854400000, 1824013069572479385600000, 465123332740982243328000000, 133955519829402886078464000000
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OFFSET
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1,1
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COMMENTS
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Also the number of automorphisms in the (n+1)-alternating group graph for n > 1. - Eric W. Weisstein, Jul 30 2018
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LINKS
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FORMULA
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a(n) = (Product_{k=1..n} k*A020725(k)) / n;
a(n) = (Product_{k=1..n} k*(k+1)) / n.
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EXAMPLE
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a(5) = (5-1)! * (5+1)! = 4! * 6! = 24 * 720 = 17280.
a(5) = ((1*2) * (2*3) * (3*4) * (4*5) * (5*6)) / 5 = 17280.
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MATHEMATICA
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#[[1]] #[[3]] & /@ Partition[Range[0, 20]!, 3, 1] (* Harvey P. Dale, Jun 20 2017 *)
Table[(n - 1)! (n + 1)!, {n, 20}] (* or *) Table[Gamma[n] Gamma[n + 2], {n, 20}] (* Eric W. Weisstein, Jul 30 2018 *)
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PROG
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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