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

Lemma prop4b : forall p q : Z, 0 <= p -> 0 <= q -> p <= q -> rel_prime p q ->
is_sqr (p * (q * (q * q - p * p))) ->
is_sqr p /\ is_sqr q /\ is_sqr (q * q - p * p).