If y = 2l for some l ∈ N, then px = (z+ql)(z−ql). So 2ql = (z+ql)−(z−ql) =
pα(pβ−α − 1) where α < β and α + β = x. Again, because p 6= 2, q, then α = 0
and 2ql = px − 1. Note that if q ≡4 −1, then 2ql ≡4 2. Also, since px ≡4 1
implies that px − 1 ≡4 0, then 2ql 6≡4 px − 1.