The nth Mobius number is given by the piecewise function:
/ 0 if n has any repeated factors

/
u(n) =  1 if n=1
\
 k
\ (1) if n is a product of k distinct primes
The first several values of u(n) (n=1,2,3,...) are u(n)=1, 1, 1, 0, 1, 1, ... Mobius numbers are multiplicative, i.e.:
/ u(m)*u(n) if gcd(m,n)=1
u(m*n) = 
\ 0 otherwise
All information gathered from http://mathworld.wolfram.com/MoebiusFunction.html