Definition ps_atan_exists_01 (x : R) (Hx:0 <= x <= 1) :
   {l : R | Un_cv (fun N : nat => sum_f_R0 (tg_alt (Ratan_seq x)) N) l}.

Lemma Ratan_seq_opp : forall x n, Ratan_seq (-x) n = -Ratan_seq x n.