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A055555
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a(n) = n!*(n!+1)/2.
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2
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1, 1, 3, 21, 300, 7260, 259560, 12703320, 812871360, 65841128640, 6584096534400, 796675481078400, 114721266640780800, 19387894024929830400, 3800027228319587865600, 855006126362753549184000, 218881568348707987666944000, 63256773252773762936322048000
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OFFSET
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0,3
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COMMENTS
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a(n) is the number of unordered pairs (not necessarily distinct) of elements in S_n (the symmetric group on n letters). That is, a(n) = binomial(n!,2) + n!. - Geoffrey Critzer, Jan 09 2016
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LINKS
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FORMULA
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a(n) + (-n^2-n-3)*a(n-1) + (n-1)*(n^2+2*n-1)*a(n-2) - 2*(n-1)*(n-2)^2*a(n-3) = 0. - R. J. Mathar, Mar 21 2013
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MATHEMATICA
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PROG
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(Magma) [Factorial(n)*(Factorial(n)+1)/2: n in [0..20]]; // Vincenzo Librandi, Jan 10 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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