Thread: equivalence relation

Let R be the relation on N such that aRb when a/b is an integer power of 2. Show the least element of an equivalance class is odd.

Ok so I get its always a prime but how do I prove it.

Let n be the smallest representative of [n] (a smallest element exists by well-ordering). If n is even, then n=2m for some m<n, and then mRn, contradicting that n was the smallest representative of the class.