Definition image_dir (f:R -> R) (D:R -> Prop) (x:R) : Prop :=
  exists y : R, x = f y /\ D y.

(* Specifications: *)
Lemma continuity_compact : forall (f:R -> R) (X:R -> Prop), (forall x:R, continuity_pt f x) -> compact X -> compact (image_dir f X).
Lemma prolongement_C0 : forall (f:R -> R) (a b:R), a <= b -> (forall c:R, a <= c <= b -> continuity_pt f c) -> exists g : R -> R, continuity g /\ (forall c:R, a <= c <= b -> g c = f c).
Lemma continuity_ab_maj : forall (f:R -> R) (a b:R), a <= b -> (forall c:R, a <= c <= b -> continuity_pt f c) -> exists Mx : R, (forall c:R, a <= c <= b -> f c <= f Mx) /\ a <= Mx <= b.
Lemma continuity_ab_min : forall (f:R -> R) (a b:R), a <= b -> (forall c:R, a <= c <= b -> continuity_pt f c) -> exists mx : R, (forall c:R, a <= c <= b -> f mx <= f c) /\ a <= mx <= b.