Definition R_fact (x y : Z) :=
1 < x /\ 1 < y /\ exists q : Z, y = q * x /\ 1 < q.

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