Automorphisms of a finite cyclic group

Let Zn denote the cyclic group of order n. Suppose mN is relatively prime to n. Define the function μm:ZnZn by m[a]n=[ma]n.

  1. Prove that the map μm is a well-defined automorphism of Zn.
  2. Prove that any automorphism of Zn has the form μm for some m.