The Diophantine equation 1+91y = z2 has no non-negative
integer solution where y and z are non-negative integers.
Proof. Suppose that there are non-negative integers y and z such that 1 +
91y = z2. If y = 0, then z2 = 2 which is impossible. Then y ≥ 1. Thus,
z2 = 1 + 91y ≥ 1 + 911 = 92. Then z ≥ 10. Now, we consider on the equation
z2 − 91y = 1. By Proposition 2.1, we have y = 1. Then z2 = 92. This is a contradiction. Hence, the equation 1 + 91y = z2 has no non-negative integer
solution.