Proof. If d is a divisor of mn, then by unique factorization we can represent d uniquely as the product of a divisor of m and a divisor of n (since they have no common factors). That is, every term of σ(mn) (the sum of all divisors of mn) occurs exactly once in the sum σ(m)σ(n) (the product of all divisors of m and n). The converse is also true: every such product is a divisor of mn, so the sums must be the same. This suffices to establish the proposition. It might be easier also just to appeal to the very special way in which σ is defined as a divisor sum (for the details consult12]). Hence σ is completely determined when its value is known for every prime-power argument. (Such functions are, of course, the multiplicative functions.) This yields
the very useful: