MA 407 HW 1
1. Let n and a be positive integers, and d = gcd(n, a). Show that the equation
   ax mod n = 1 has a solution if and only if d = 1.
2. Prove the uniqueness portion of the Fundamental Theorem of Arithmetic
   (for a precise statement, see Theorem 0.3. The existence potion is done
   by induction in Example 13 of chapter 0).
3. Show that gcd(a, bc) = 1 if and only if gcd(a, b) = 1 and gcd(a, c) = 1.
4. Use induction (instead of the modular arithmetic trick we did in class)
   to prove that the sum of the cubes any three consecutive positive integers
   is divisible by 9.
5. Prove that every set with n elements has exactly 2n subsets.