Lemma 2.2. The Diophantine equation 19x +1= z2 has no non-negative
integer solution.
Proof: Suppose that there are non-negative integers x and z such that
19x +1= z2 . If x = 0 , that z2 = 2 which is impossible. If x =1, then z2 = 20
which is impossible. If x >1, by Proposition 2.1, the Diophantine equation
z2 −19x =1 has no non-negative integer solution.