intros; reg.
apply yP_derivable.
apply xP_derivable.
Qed.

(**********)
Lemma D_Rsqr_u_epsilon_2 :
forall (eps : posreal) (t : R),
(derive_pt (fun y : R => Rsqr (u eps y)) t (fct_der2 eps t) <= 2 * u0 t * vi)%R.