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A003091
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a(n) = floor( 2^(n*(n-1)/2) / n! ).
(Formerly M1870)
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1
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1, 1, 1, 2, 8, 45, 416, 6657, 189372, 9695869, 902597327, 154043277297, 48535481831642, 28400190511772276, 31020581422991798557, 63530150754287203445810, 244912468225468597942626507, 1783398168624923337196441201196, 24605638395579573858211783276124626
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OFFSET
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1,4
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REFERENCES
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F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 246.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = floor( 2^binomial(n,2) / n! ).
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MAPLE
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A003091:n->floor(2^(n*(n-1)/2)/n!);
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MATHEMATICA
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Table[Floor[2^(n*(n-1)/2)/n!], {n, 30}] (* G. C. Greubel, Nov 02 2022 *)
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PROG
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(Magma) [Floor(2^Binomial(n, 2)/Factorial(n)): n in [1..30]]; // G. C. Greubel, Nov 02 2022
(SageMath) [(2^binomial(n, 2)//factorial(n)) for n in range(1, 30)] # G. C. Greubel, Nov 02 2022
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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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