If x = 2k for some k ∈ N, then qy = (z + pk)(z − pk). It follows that
2pk = (z + pk) − (z − pk) = qα(qβ−α − 1) where α < β and α + β = y. Since
p 6= 2, q, then α = 0 and 2pk = qy − 1. Note that if p ≡4 1, then p ≡6 −1 and
2pk ≡3 1, 2. Furthermore, since q ≡6 1 and qy − 1 ≡3 0 whenever q ≡4 −1, then
2pk 6≡3 qy + 1.