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 ex_lim_seq_geom_m (q : R) : q <= -1 -> ~ ex_lim_seq (fun n => q ^ n).