# Library Coq.Sets.Image

Require Export Finite_sets.
Require Export Constructive_sets.
Require Export Classical_Type.
Require Export Classical_sets.
Require Export Powerset.
Require Export Powerset_facts.
Require Export Powerset_Classical_facts.
Require Export Gt.
Require Export Lt.
Require Export Le.
Require Export Finite_sets_facts.
Section Image.
Variables U V : Type.
Inductive Im (X:Ensemble U) (f:U -> V) : Ensemble V :=
Im_intro : forall x:U, In _ X x -> forall y:V, y = f x -> In _ (Im X f) y.
Lemma Im_def :
forall (X:Ensemble U) (f:U -> V) (x:U), In _ X x -> In _ (Im X f) (f x).
Lemma Im_add :
forall (X:Ensemble U) (x:U) (f:U -> V),
Im (Add _ X x) f = Add _ (Im X f) (f x).
Lemma image_empty : forall f:U -> V, Im (Empty_set U) f = Empty_set V.
Lemma finite_image :
forall (X:Ensemble U) (f:U -> V), Finite _ X -> Finite _ (Im X f).
Lemma Im_inv :
forall (X:Ensemble U) (f:U -> V) (y:V),
In _ (Im X f) y -> exists x : U , In _ X x /\ f x = y.
Definition injective (f:U -> V) := forall x y:U, f x = f y -> x = y.
Lemma not_injective_elim :
forall f:U -> V,
~ injective f -> exists x : _ , ( exists y : _ , f x = f y /\ x <> y ).
Lemma cardinal_Im_intro :
forall (A:Ensemble U) (f:U -> V) (n:nat),
cardinal _ A n -> exists p : nat , cardinal _ (Im A f) p.
Lemma In_Image_elim :
forall (A:Ensemble U) (f:U -> V),
injective f -> forall x:U, In _ (Im A f) (f x) -> In _ A x.
Lemma injective_preserves_cardinal :
forall (A:Ensemble U) (f:U -> V) (n:nat),
injective f ->
cardinal _ A n -> forall n':nat, cardinal _ (Im A f) n' -> n' = n.
Lemma cardinal_decreases :
forall (A:Ensemble U) (f:U -> V) (n:nat),
cardinal U A n -> forall n':nat, cardinal V (Im A f) n' -> n' <= n.
Theorem Pigeonhole :
forall (A:Ensemble U) (f:U -> V) (n:nat),
cardinal U A n ->
forall n':nat, cardinal V (Im A f) n' -> n' < n -> ~ injective f.
Lemma Pigeonhole_principle :
forall (A:Ensemble U) (f:U -> V) (n:nat),
cardinal _ A n ->
forall n':nat,
cardinal _ (Im A f) n' ->
n' < n -> exists x : _ , ( exists y : _ , f x = f y /\ x <> y ).
End Image.
Hint Resolve Im_def image_empty finite_image: sets v62.