Given a multiplicative function $f$, is there any general method of getting a upper bound of $\sum^{'}_{n
For example, when the sum is restricted to square-free numbers, we have $\sum^{''}_{n
Given a multiplicative function $f$, is there any general method of getting a upper bound of $\sum^{'}_{n
For example, when the sum is restricted to square-free numbers, we have $\sum^{''}_{n