intros y; case y; simpl in |- *; auto with *.
intros x' y Hlt; apply lt_trans with (S x' * y).
rewrite (mult_comm (S x') y); apply mult_lt; auto.
apply mult_lt; omega.
Qed.

Theorem root_monotonic : forall x y : nat, x * x < y * y -> x < y.
exact (monotonic_inverse (fun x : nat => x * x) square_monotonic).