Definition ex_lim_seq_cauchy (u : nat -> R) :=
  forall eps : posreal, exists N : nat, forall n m,
    (N <= n)%nat -> (N <= m)%nat -> Rabs (u n - u m) < eps.

Lemma is_lim_seq_opp (u : nat -> R) (l : Rbar) : is_lim_seq u l <-> is_lim_seq (fun n => -u n) (Rbar_opp l).