OFFSET
1,4
COMMENTS
The maximal order of an abelian subgroup in S_n is given by A000792.
Sequence becomes non-monotonic at n=20.
FORMULA
E.g.f.: exp(x^3/3!)*(1 + x^2/2 + 7*x^4/4!) + x - 1.
From Benedict W. J. Irwin, May 24 2016: (Start)
If n=1, a(n) = 1.
If n=2,5,8,11,..., a(n) = n!*2^(-(n+1)/3)*3^((2-n)/3)/Gamma((n+1)/3).
If n=3,6,9,12,..., a(n) = n!*6^(-n/3)/Gamma((n+3)/3).
If n=4,7,10,13,..., a(n) = n!*7*2^(-(n+5)/3)*3^((1-n)/3)/Gamma((n-1)/3).
(End)
EXAMPLE
a(4) = 7 because we have three cyclic groups: <(1234)> = <(1432)>, <(1243)> = <(1342)>, <(1324)> = <(1423)> and four groups isomorphhic to C_2 X C_2: <(12),(34)>, <(13),(24)>, <(14),(23)> , <(12)(34),(13)(24)> for a total of 7 distinct subgroups of maximal order 4.
MATHEMATICA
nn = 25; Drop[Range[0, nn]! CoefficientList[Series[Exp[x^3/3!] (1 + x^2/2 + 7 x^4/4!) + x - 1, {x, 0, nn}], x], 1]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Geoffrey Critzer, Dec 06 2015
STATUS
approved