A003091 a(n) = floor( 2^(n*(n-1)/2) / n! ). (Formerly M1870)

1, 1, 1, 2, 8, 45, 416, 6657, 189372, 9695869, 902597327, 154043277297, 48535481831642, 28400190511772276, 31020581422991798557, 63530150754287203445810, 244912468225468597942626507, 1783398168624923337196441201196, 24605638395579573858211783276124626

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

G. C. Greubel, Table of n, a(n) for n = 1..85

Formula

a(n) = floor( 2^binomial(n,2) / n! ).

Maple

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

Mathematica

Table[Floor[2^(n*(n-1)/2)/n!], {n, 30}] (* G. C. Greubel, Nov 02 2022 *)

Prog

(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

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