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

Lemma relp_pq1 : forall p q : Z, p >= 0 -> p <= q -> (rel_prime p q) ->
(distinct_parity p q) -> (rel_prime (q * q - p * p) (p * p + q * q)).