The Diophantine equation 1+19y = z2 has no non-negative
integer solution.
Proof. Suppose that there are non-negative integers y and z such that 1 +
19y = z2. If y = 0, then z2 = 2 which is impossible. Then y ≥ 1. Thus,
z2 = 1 + 19y ≥ 1 + 191 = 20. Then z ≥ 5. Now, we consider on the equation
z2 − 19y = 1. By Proposition 2.1, we have y = 1. Then z2 = 20. This is a
contradiction. Hence, the equation 1 + 19y = z2 has no non-negative integer
solution.