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A003091
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[ 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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,4
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REFERENCES
| 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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MAPLE
| A003091:n->floor(2^(n*(n-1)/2)/n!);
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CROSSREFS
| Sequence in context: A009345 A084553 A144164 * A119501 A183277 A202081
Adjacent sequences: A003088 A003089 A003090 * A003092 A003093 A003094
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KEYWORD
| nonn,easy
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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