OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..300
EXAMPLE
a(3)=20 because there are 27 functions f:{1,2,3}->{1,2,3} but 7 of these have at least two cycles of equal length: (1,2,3);(1,2,1);(1,2,2);(1,1,3);(1,3,3);(2,2,3)(3,2,3) where the functions are represented by their values.
MAPLE
with(combinat):
b:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,
add((i-1)!^j*multinomial(n, n-i*j, i$j)/j!*
b(n-i*j, i-1), j=0..min(1, n/i))))
end:
a:= n-> add(binomial(n-1, j-1)*n^(n-j)*b(j$2), j=0..n):
seq(a(n), n=0..25); # Alois P. Heinz, Aug 10 2014
MATHEMATICA
nn = 20; p = Product[1 + t^n/n, {n, 1, nn}]; t = Sum[n^(n - 1) x^n/n!, {n, 1, nn}]; Range[0, nn]! CoefficientList[Series[p, {x, 0, nn}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Geoffrey Critzer, May 27 2012
EXTENSIONS
Terms a(8)-a(19) recomputed by Alois P. Heinz, Aug 10 2014
STATUS
approved