Proof. Suppose that there are non-negative integers y and z such that 1 +
13y = z2. If y = 0, then z2 = 2 which is impossible. Then y ≥ 1. Thus,
z2 = 1 + 13y ≥ 1 + 131 = 14. Then z ≥ 4. Now, we consider on the equation
z2 − 13y = 1. By Proposition 2.1, we have y = 1. Then z2 = 14. This is a
contradiction.