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A003091 [ 2^{n(n-1)/2} / n! ].
(Formerly M1870)
1
1, 1, 1, 2, 8, 45, 416, 6657, 189372, 9695869, 902597327, 154043277297, 48535481831642, 28400190511772276, 31020581422991798557, 63530150754287203445810, 244912468225468597942626507, 1783398168624923337196441201196, 24605638395579573858211783276124626 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

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).

LINKS

Table of n, a(n) for n=1..19.

MAPLE

A003091:n->floor(2^(n*(n-1)/2)/n!);

CROSSREFS

Sequence in context: A009345 A084553 A144164 * A119501 A183277 A269006

Adjacent sequences:  A003088 A003089 A003090 * A003092 A003093 A003094

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified November 14 17:08 EST 2018. Contains 317210 sequences. (Running on oeis4.)