Definition R_p4 (x y : Z) :=
  0 <= x /\ 1 < y /\ exists d : Z, y = d * d * x /\ 1 < d.

Lemma relp_pq2 : forall p q : Z, (rel_prime p q) -> (distinct_parity p q) -> (rel_prime (2 * p * q) (p * p + q * q)).