(** Definition of ps_atan's derivative *)
Definition Datan_seq := fun (x : R) (n : nat) => x ^ (2*n).

Lemma Datan_eq_DatanSeq_interv : forall x, -1 < x < 1 ->
forall (Pratan:derivable_pt ps_atan x) (Prmymeta:derivable_pt atan x),
derive_pt ps_atan x Pratan = derive_pt atan x Prmymeta.