Proof. Let p and q be twin primes. If p ≡4 1, −1, then q ≡4 −1, 1, respectively. First, suppose that p ≡4 1 and q ≡4 −1. Then, x, y must be of different
parity and z is even. So we consider two possibilities: (i) x is even and y is
odd; and, (ii) x is odd and y is even.