OFFSET
1,2
COMMENTS
Also (1/8) times the number of n-member subsets of [9n-1] whose elements sum to a multiple of n.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..737
FORMULA
a(n) = 1/(9n) * Sum_{d|n} binomial(9d,d)*(-1)^(n+d)*phi(n/d).
MAPLE
with(numtheory):
a:= n-> add(binomial(9*d, d)*(-1)^(n+d)*
phi(n/d), d in divisors(n))/(9*n):
seq(a(n), n=1..25);
PROG
(PARI) a(n) = 1/(9*n) * sumdiv(n, d, binomial(9*d, d)*(-1)^(n+d)*eulerphi(n/d)); \\ Michel Marcus, Jul 19 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Jul 15 2019
STATUS
approved