OFFSET
0,4
COMMENTS
a(n) is the number of involutions with an even number of cycles. - Geoffrey Critzer, Mar 17 2013
MATHEMATICA
nn=25; Range[0, nn]!CoefficientList[Series[Cosh[x+x^2/2], {x, 0, nn}], x] (* Geoffrey Critzer, Mar 17 2013 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Paul Barry, Jun 27 2003
STATUS
approved