OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..40
FORMULA
a(n) = Sum_{d|6} phi(6/d)*(n*d)!/(d!^n*n*6) if n>0 and a(0) = 1.
EXAMPLE
a(0) = 1: the empty necklace.
a(1) = 1: {000000}.
a(2) = 80: {000000111111, ..., 010101010101}.
MAPLE
with(numtheory);
a:= n-> `if`(n=0, 1, add(phi(6/d) *(n*d)!/(d!^n *6*n), d={1, 2, 3, 6})):
seq(a(n), n=0..12);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Feb 24 2012
STATUS
approved