intros; set (f := u eps); reg.
apply (d_u eps).
Qed.

(**********)
Lemma D_Rsqr_u_epsilon_1 :
forall (eps : posreal) (t : R),
derive_pt (fun y : R => Rsqr (u eps y)) t (fct_der2 eps t) =
(2 * u eps t * derive_pt (u eps) t (d_u eps t))%R.