OFFSET
0,3
COMMENTS
Number of permutations of 2*n elements with distinct cycle lengths and without odd cycles. - Vladeta Jovovic, Aug 17 2004
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..225
FORMULA
a(n) ~ 2*exp(-gamma/2) * (2*n)! / (Pi*sqrt(n)), where gamma is the Euler-Mascheroni constant A001620. - Vaclav Kotesovec, Jul 23 2019
MAPLE
b:= proc(n, i) option remember; `if`((i/2)*(i/2+1)<n, 0,
`if`(n=0, 1, b(n, i-2)+`if`(i>n, 0, (i-1)!*
b(n-i, i-2)*binomial(n, i))))
end:
a:= n-> b(2*n$2):
seq(a(n), n=0..17); # Alois P. Heinz, Nov 01 2017
MATHEMATICA
nmax = 20; Table[(CoefficientList[Series[Product[1 + x^(2*k)/(2*k), {k, 1, 2*nmax}], {x, 0, 2*nmax}], x]*Range[0, 2*nmax]!)[[2*n + 1]], {n, 0, nmax}] (* Vaclav Kotesovec, Jul 23 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Yuval Dekel (dekelyuval(AT)hotmail.com), Nov 06 2003
EXTENSIONS
More terms from Christian G. Bower, Jan 06 2006
STATUS
approved