Proof. Suppose that there are non-negative integers x and z such that 7x+
1 = z2. If x = 0, then z2 = 2 which is impossible. Then x ≥ 1. Thus,
z2 = 7x + 1 ≥ 71 + 1 = 8. Then z ≥ 3. Now, we consider on the equation
z2 − 7x = 1. By Proposition 2.1, we have x = 1. Then z2 = 8. This is a
contradiction. Hence, the equation 7x + 1 = z2 has no non-negative integer
solution.