OFFSET
1,4
LINKS
FORMULA
(1/n) Sum mu(d) M(n/d, 0, 1); d|n, d=1(3) + (1/n) Sum mu(d) M(n/d, 0, 2); d|n, d=2(3) where M(n, t, s) = Sum n!/(i!j!k!); i+j+k=n, j=t(3), k=s(3).
EXAMPLE
a(4) = 3 = |{ 0001, 1222 }|
CROSSREFS
KEYWORD
nonn
AUTHOR
Frank Ruskey, Jan 17 2000
STATUS
approved