If `phi` denotes null set then find `P(phi)`

If phi is a null set, then which one of the following is correct?

If phi denotes reduction potential, then which is true ?

If phi is an empty set, then n(phi)= ________.

If phi is an impossible event, then P(phi)= ............

The set phi is :

If phi(n) denotes the number of positive integers less than n and relatively prime to n and A={n| phi(n)=17}, then find the number of elements in set A.

If A= phi then how many elements are in n[P[P(phi)]] ?

If A = phi , then the number elements in P(A), (i.e., the number of elements in the power set of A) is

If A is the null set, find the number of elements in the power set P(P(A)) .

If A is the null set, find the number of elements in the power set P(P(A)) .