OFFSET
0,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..70
FORMULA
a(n) = Sum_{d|n} phi(n/d)*(6*d)!/(d!^6*6*n) if n>0 and a(0) = 1.
a(n) ~ 6^(6*n-1/2) / (2^(5/2) * Pi^(5/2) * n^(7/2)). - Vaclav Kotesovec, Aug 23 2015
EXAMPLE
a(0) = 1: the empty necklace.
a(1) = 120: {012345, 012354, ..., 054321}.
MAPLE
with(numtheory):
a:= n-> `if`(n=0, 1, add(phi(n/d) *(6*d)!/(d!^6 *6*n), d=divisors(n))):
seq(a(n), n=0..12);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Feb 24 2012
STATUS
approved