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_le_le (u v w : nat -> R) (l : Rbar) : (forall n, u n <= v n <= w n) -> is_lim_seq u l -> is_lim_seq w l -> is_lim_seq v l.