(********************************************)
(**           Riemann's Integral            *)
(********************************************)

Definition Riemann_integrable (f:R -> R) (a b:R) : Type :=
  forall eps:posreal,
    { phi:StepFun a b &
      { psi:StepFun a b |
        (forall t:R,
          Rmin a b <= t <= Rmax a b -> Rabs (f t - phi t) <= psi t) /\
        Rabs (RiemannInt_SF psi) < eps } }.Definition phi_sequence (un:nat -> posreal) (f:R -> R)
  (a b:R) (pr:Riemann_integrable f a b) (n:nat) :=
  projT1 (pr (un n)).

Lemma RiemannInt_exists : forall (f:R -> R) (a b:R) (pr:Riemann_integrable f a b) (un:nat -> posreal), Un_cv un 0 -> { l:R | Un_cv (fun N:nat => RiemannInt_SF (phi_sequence un pr N)) l }.