OFFSET
4,2
COMMENTS
Generating function is K(x,1) in the reference, see page 405.
REFERENCES
Miklos Bona, editor, Handbook of Enumerative Combinatorics, CRC Press, 2015, p.404-405.
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 4..80
FORMULA
a(n) ~ 2^(n*(n-1)/2).
MATHEMATICA
Table[n!*SeriesCoefficient[Log[Sum[2^(k*(k-1)/2)*x^k*Exp[k*x*(x-1-k)/(2*(1+x))]/k!, {k, 0, n}]] - x*(2+x+x^2)/(4*(1+x)) - Log[1+x]/2, {x, 0, n}], {n, 4, 15}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Vaclav Kotesovec, May 14 2015
STATUS
approved