intro; unfold yP in |- *; reg.
apply pr_nu.
apply yi_derivable.
Qed.

(* Specifications: *)
Lemma fct_der3 :
forall (eps : posreal) (t : R),
derivable_pt (fun y : R => (Rsqr (xP y) + (Rsqr (yP y) + Rsqr eps))%R) t.