OFFSET
0,4
REFERENCES
Alexander Postnikov. Permutohedra, associahedra, and beyond. in: Conference in Honor of Richard Stanley's Sixtieth Birthday, June 2004. International Mathematics Research Notices, 6:1026-1106, 2009.
LINKS
Matthew Wilson, Bruhat order on fixed-point-free involutions in the symmetric group, Electron. J. Combin., 21(2) (2014), #P2.20.
EXAMPLE
1, 2, 6, 64/3, 250/3, 1728/5, 67228/45, 2097152/315, 1062882/35, 80000000/567, 9431790764/14175, 6115295232/1925, 7168641576148/467775, ...
MAPLE
PROG
(PARI) vector(30, n, n--; denominator(2^n*(n+1)^(n-1)/n!)) \\ Michel Marcus, Jul 18 2015
CROSSREFS
KEYWORD
nonn,frac
AUTHOR
N. J. A. Sloane, May 13 2014
STATUS
approved