Module mathcomp.classical.cardinality

From HB Require Import structures.
From mathcomp Require Import boot order finmap ssralg ssrnum ssrint rat.
From mathcomp Require Import boolp classical_sets functions.

# Cardinality This file provides an account of cardinality properties of classical sets. This includes standard results of set theory such as the Pigeon Hole principle, the Cantor-Bernstein Theorem, or lemmas about the cardinal of nat, nat * nat, and rat. Since universe polymorphism is not yet available in our framework, we develop a relational theory of cardinals: there is no type for cardinals only relations A #<= B and A #= B to compare the cardinals of two sets (on two possibly different types). ``` A #<= B == the cardinal of A is smaller or equal to the one of B A #>= B := B #<= A A #= B == the cardinal of A is equal to the cardinal of B A #!= B := ~~ (A #= B) finite_set A == the set A is finite := exists n, A #= `I_n <-> exists X : {fset T}, A = [set` X] <-> ~ ([set: nat] #<= A) infinite_set A := ~ finite_set A cofinite_set A := finite_set (~` A) countable A <-> A is countable := A #<= [set: nat] fset_set A == the finite set corresponding if A : set T is finite, set0 otherwise (T : choiceType) A.`1 := [fset x.1 | x in A] A.`2 := [fset x.2 | x in A] {fimfun aT >-> T} == type of functions with a finite image ```

Unset SsrOldRewriteGoalsOrder.
Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.

Reserved Notation "A '#<=' B" (at level 79, format "A '#<=' B").
Reserved Notation "A '#>=' B" (at level 79, format "A '#>=' B").
Reserved Notation "A '#=' B" (at level 79, format "A '#=' B").
Reserved Notation "A '#!=' B" (at level 79, format "A '#!=' B").

Import Order.Theory GRing.Theory.

Local Open Scope classical_set_scope.
Local Open Scope ring_scope.
Local Open Scope function_scope.

Declare Scope card_scope.
Delimit Scope card_scope with card.
Local Open Scope card_scope.

Definition ( : set T) ( : set U) :=
  `[< $|{injfun [set: A] >-> [set: B]}| >].
Notation := (card_le A B) : card_scope.
Notation := (card_le B A) (only parsing) : card_scope.

Definition ( : set T) ( : set U) :=
  `[< $|{bij [set: A] >-> [set: B]}| >].
Notation := (card_eq A B) : card_scope.
Notation := (~~ (card_eq A B)) : card_scope.

Definition {} ( : set T) := exists , A #= `I_n.
Notation A := (~ finite_set A).
Notation A := (finite_set (~` A)).

Lemma { } { : set T} :
   $|{inj A >-> U}| <-> exists : T -> U, set_inj A f.

Proof.

by split=> [[f]|[_ /Pinj[f _]]]; first by exists f. Qed.

Lemma { } { : set T} { : set U} :
$|{surj A >-> B}| <-> exists , set_surj A B f.

Proof.

by split=> [[f]|[_ /Psurj[f _]]]; first by exists f. Qed.

Lemma { } { : set T} { : set U} :
$|{bij A >-> B}| <-> exists , set_bij A B f.

Proof.

by split=> [[f]|[_ /Pbij[f _]]]; first by exists f. Qed.

Lemma { } { : set T} { : set U} :
$|{surjfun A >-> B}| <-> exists , B = f @` A.

Proof.

split=> [[f]|[f ->]]; last by squash [fun f in A].
by exists f; apply/seteqP; split=> //; apply: surj.
Qed.

Lemma { } { : set T} { : set U}:
$|{injfun A >-> B}| <-> exists2 : T -> U, set_fun A B f & set_inj A f.

Proof.

by split=> [[f]|[_ /Pfun[? ->] /funPinj[f]]]; [exists f | squash f]. Qed.

Lemma { } { : set T} { : set U} :
reflect $|{injfun [set: A] >-> [set: B]}| (A #<= B).

Proof.

exact: asboolP. Qed.

Lemma { } { : set T} { : set U} : {injfun A >-> B} -> (A #<= B).

Proof.

by move=> f; apply/card_leP; squash (sigLR f). Qed.

Lemma {} { : pointedType} { : set T} { : set U} :
reflect $|{injfun A >-> B}| (A #<= B).

Proof.

by apply: (iffP card_leP) => -[f]; [squash (valLR point f) | squash (sigLR f)].
Qed.

Lemma {} { : pointedType} { : set T} :
reflect $|{inj A >-> U}| (A #<= [set: U]).

Proof.

by apply: (iffP pcard_leP) => -[f]; [squash f | squash ('totalfun_A f)].
Qed.

Lemma {} { : pointedType} { : set T} :
reflect (exists : T -> U, {in A &, injective f}) (A #<= [set: U]).

Proof.

by apply: (iffP pcard_leTP); rewrite injPex. Qed.

Lemma { : pointedType} { : set T} { : set U} :
reflect $|{splitinjfun A >-> B}| (A #<= B).

Proof.

by apply: (iffP pcard_leP) => -[f]; squash (split f). Qed.

Lemma ( : set U) : @set0 T #<= S.

Proof.

by apply/card_leP; squash set0fun. Qed.

#[global] Hint Resolve card_ge0 : core.

Lemma ( : set T) : reflect (A = set0) (A #<= @set0 U).

Proof.

apply: (iffP idP) => [/card_leP[f]|->//].
by rewrite -subset0 => a /mem_set aA; have [x /set_mem] := f (SigSub aA).
Qed.

Lemma ( : set T) : (A #<= @set0 U) = (A == set0).

Proof.

exact/card_le0P/eqP. Qed.

Lemma { } { : set T} { : set U} :
reflect $|{bij [set: A] >-> [set: B]}| (A #= B).

Proof.

exact: asboolP. Qed.

Lemma { } { : set T} { : set U} : {bij A >-> B} -> A #= B.

Proof.

by move=> f; apply/card_eqP; squash (sigLR f). Qed.

Lemma {} { : pointedType} { : set T} { : set U} :
reflect $| {bij A >-> B} | (A #= B).

Proof.

by apply: (iffP card_eqP) => -[f]; [squash (valLR point f) | squash (sigLR f)].
Qed.

Lemma { } { : set T} { : set U} :
reflect (exists : A -> B, bijective f) (A #= B).

Proof.

by apply: (iffP card_eqP) => [[f]|[_ /PbijTT[f _]]]; [exists f|squash f].
Qed.

Lemma { } { : set T} { : set U} :
reflect $|{splitbij [set: A] >-> [set: B]}| (A #= B).

Proof.

by apply: (iffP card_bijP) => [[_ /PbijTT[f _]]//|[f]]; exists f. Qed.

Lemma {} { : pointedType} { : set T} { : set U} :
reflect (exists , set_bij A B f) (A #= B).

Proof.

by apply: (iffP pcard_eqP) => [[f]|[_ /Pbij[f _]]]; [exists f|squash f].
Qed.

Lemma { : pointedType} { : set T} { : set U} :
reflect $| {splitbij A >-> B} | (A #= B).

Proof.

by apply: (iffP pcard_eqP) => -[f]; [squash (split f)|squash f]. Qed.

Lemma ( : set T) : A #= A.

Proof.

by apply/card_eqP; squash idfun. Qed.

#[global] Hint Resolve card_eqxx : core.

Lemma : @set0 T #= @set0 U.

Proof.

apply/card_eqP/squash; apply: @bijection_of_bijective set0fun _.
by exists set0fun => -[x x0]; have := set_mem x0.
Qed.

#[global] Hint Resolve card_eq00 : core.

Section empty1.
Implicit Types (T : emptyType).
Lemma : all_equal_to (set0 : set T).

Proof.

by move=> X; apply/setF_eq0/no. Qed.

Lemma ( : set T) ( : set U) : A #<= B.

Proof.

by rewrite empty_eq0. Qed.

Lemma ( : set T) ( : set U) : (B #<= A) = (B == set0).

Proof.

by rewrite empty_eq0; apply/idP/eqP=> [/card_le0P|->//]. Qed.

Definition := (empty_eq0, card_le_emptyl, card_le_emptyr, eq_opE).
End empty1.

Theorem ( : set T) ( : set U) :
A #<= B -> B #<= A -> A #= B.

Proof.

elim/Ppointed: T => T in A *; first by rewrite !emptyE_subdef => _ ->.
elim/Ppointed: U => U in B *; first by rewrite !emptyE_subdef => ->.
suff {A B} card_eq ( : set U) : B `<=` A -> A #<= B -> A #= B.
  move=> /ppcard_leP[f] /ppcard_leP[g].
  have /(_ _)/ppcard_eqP[|h] := card_eq _ _ (fun_image_sub f).
    by apply/pcard_leP; squash ([fun f in A] \o g).
  by apply/pcard_eqP; squash ((split h)^-1 \o [fun f in A]).
move=> BA /ppcard_leP[u]; have uAB := 'funS_u.
pose C_ := fix C := if n is n.+1 then u @` C n else A `\` B.
pose C := \bigcup_ C_ n; have CA : C `<=` A.
  by move=> + [] => /[swap]; elim=> [|i IH] y _ []// x /IH/uAB/BA + <-; apply.
have uC: {homo u : / x \in C}.
  by move=> x; rewrite !inE => -[i _ Cix]; exists i.+1 => //; exists x.
apply/card_set_bijP; exists (fun => if x \in C then u x else x); split.
- move=> x Ax; case: ifPn; first by move=> _; apply: uAB.
  by move/negP; apply: contra_notP => NBx; rewrite inE; exists 0%N.
- move=> x y xA yA; have := 'inj_u xA yA.
  have [xC|] := boolP (x \in C); have [yC|] := boolP (y \in C) => // + _.
    by move=> /[swap]<-; rewrite uC// xC.
  by move=> /[swap]->; rewrite uC// yC.
- move=> y /[dup] By /BA Ay/=.
  case: (boolP (y \in C)); last by exists y; rewrite // ifN.
  rewrite inE => -[[|i]/= _ []// x Cix <-]; have Cx : C x by exists i.
  by exists x; [exact: CA|rewrite ifT// inE].
Qed.

Lemma ( : set T) ( : set U) : A #= B -> B #= A.

Proof.

by move=> /card_eqVP[f]; apply/card_eqP; squash f^-1. Qed.

Lemma ( : set T) ( : set U) :
(A #= B) = (A #<= B) && (B #<= A).

Proof.

apply/idP/andP => [/card_eqVP[f]|[]]; last exact: Cantor_Bernstein.
by split; apply/card_leP; [squash f|squash f^-1].
Qed.

Lemma ( : set T) ( : set U) :
(A #= B) <-> (A #<= B) /\ (B #<= A).

Proof.

by rewrite card_eq_le (rwP andP). Qed.

Lemma ( : set T) : A #<= A.

Proof.

by apply/card_leP; squash idfun. Qed.

#[global] Hint Resolve card_lexx : core.

Lemma ( : set T) : S #<= [set: T].

Proof.

by apply/card_leP; squash (to_setT \o inclT _ \o val). Qed.

Lemma ( : set T) : A `<=` B -> A #<= B.

Proof.

by move=> AB; apply/card_leP; squash (inclT _ \o subfun AB). Qed.

Lemma { } ( : T -> U) ( : set T) : f @` A #<= A.

Proof.

elim/Ppointed: T => T in A f *; first by rewrite !emptyE_subdef image_set0.
by apply/pcard_leP; squash (pinv A f).
Qed.

Lemma { } {} { : T -> U} : {in A &, injective f} -> f @` A #= A.

Proof.

by move=> /inj_bij/pcard_eq/card_esym. Qed.

Arguments inj_card_eq {T U A f}.

Lemma {} { : set T} : some @` A #= A.

Proof.

exact: inj_card_eq. Qed.

Lemma { } { : set T} ( : {inj A >-> U}) : f @` A #= A.

Proof.

exact: inj_card_eq. Qed.

Lemma { } ( : set T) ( : {inj A >-> U}) : X `<=` A -> f @` X #= X.

Proof.

by move=> XA; rewrite (card_image [inj of f \o incl XA]). Qed.

Lemma ( : Type) ( : set U) ( : set T) ( : set V) :
A #<= B -> B #<= C -> A #<= C.

Proof.

by move=> /card_leP[f]/card_leP[g]; apply/card_leP; squash (g \o f). Qed.

Lemma ( : set T) ( : set U) : (A #= B) = (B #= A).

Proof.

by rewrite !card_eq_le andbC. Qed.

Lemma ( : set T) ( : set U) ( : set V) :
A #= B -> B #= C -> A #= C.

Proof.

by move=> /card_eqP[f]/card_eqP[g]; apply/card_eqP; squash (g \o f). Qed.

Lemma ( : set T) ( : set T') [ : set T''] :
A #= B -> (A #<= C) = (B #<= C).

Proof.

by move=> /card_eqPle[*]; apply/idP/idP; apply: card_le_trans. Qed.

Lemma ( : set T) ( : set T') [ : set T''] :
A #= B -> (C #<= A) = (C #<= B).

Proof.

by move=> /card_eqPle[*]; apply/idP/idP => /card_le_trans; apply. Qed.

Lemma ( : set T) ( : set T') [ : set T''] :
A #= B -> (A #= C) = (B #= C).

Proof.

by move=> e; rewrite !card_eq_le (card_le_eql e) (card_le_eqr e). Qed.

Lemma ( : set T) ( : set T') [ : set T''] :
A #= B -> (C #= A) = (C #= B).

Proof.

by move=> e; rewrite !card_eq_le (card_le_eql e) (card_le_eqr e). Qed.

Lemma { } { : set T} ( : {inj A >-> U}) ( : set V) :
X `<=` A -> (f @` X #<= Y) = (X #<= Y).

Proof.

by move=> XA; rewrite (card_le_eql (card_imsub _ _)). Qed.

Lemma { } { : set T} ( : {inj A >-> U}) ( : set V) :
X `<=` A -> (Y #<= f @` X) = (Y #<= X).

Proof.

by move=> XA; rewrite (card_le_eqr (card_imsub _ _)). Qed.

Lemma { } ( : set T) ( : {inj A >-> U}) :
X `<=` A -> Y `<=` A ->
(f @` X #<= f @` Y) = (X #<= Y).

Proof.

by move=> *; rewrite card_ge_image// card_le_image. Qed.

Lemma { } { : set T} ( : {inj A >-> U}) ( : set V) :
X `<=` A -> (f @` X #= Y) = (X #= Y).

Proof.

by move=> XA; rewrite (card_eql (card_imsub _ _)). Qed.

Lemma { } { : set T} ( : {inj A >-> U}) ( : set V) :
X `<=` A -> (Y #= f @` X) = (Y #= X).

Proof.

by move=> XA; rewrite (card_eqr (card_imsub _ _)). Qed.

Lemma { } ( : set T) ( : {inj A >-> U}) :
X `<=` A -> Y `<=` A ->
(f @` X #= f @` Y) = (X #= Y).

Proof.

by move=> *; rewrite card_eq_image// card_eq_imager. Qed.

Lemma { } { : set T} { : set T'} :
(some @` A #<= B) = (A #<= B).

Proof.

by rewrite (card_le_eql card_some). Qed.

Lemma { } { : set T} { : set T'} :
(A #<= some @` B) = (A #<= B).

Proof.

by rewrite (card_le_eqr card_some). Qed.

Lemma { } { : set T} { : set T'} :
(some @` A #<= some @` B) = (A #<= B).

Proof.

by rewrite card_ge_some card_le_some. Qed.

Lemma { } { : set T} { : set T'} :
(some @` A #= B) = (A #= B).

Proof.

by rewrite (card_eql card_some). Qed.

Lemma { } { : set T} { : set T'} :
(A #= some @` B) = (A #= B).

Proof.

by rewrite (card_eqr card_some). Qed.

Lemma { } { : set T} { : set T'} :
(some @` A #= some @` B) = (A #= B).

Proof.

by rewrite card_eq_somel card_eq_somer. Qed.

Lemma { } { : set T} : (A #= @set0 U) = (A == set0).

Proof.

by rewrite card_eq_le card_le0 card_ge0 andbT. Qed.

Lemma {} { : T} : [set x] #= `I_1.

Proof.

apply/pcard_eqP; suff /Pbij[f]: set_bij [set x] `I_1 (fun=> 0%N) by squash f.
by split=> [//|y z /[!in_setE]-> ->//|[]//]; exists x.
Qed.

Lemma { } ( : T) ( : U) : [set x] #= [set y].

Proof.

by rewrite (card_eql card_set1) (card_eqr card_set1). Qed.

Lemma ( : emptyType) ( : set T) ( : set U) :
(B #= A) = (B == set0).

Proof.

by rewrite empty_eq0; exact: card_eq0. Qed.

Lemma ( : emptyType) ( : set T) ( : set U) :
(A #= B) = (B == set0).

Proof.

by rewrite card_eq_sym card_eq_emptyr. Qed.

Definition := (emptyE_subdef, card_eq_emptyr, card_eq_emptyl).

Lemma ( : set T) : [set: A] #= A.

Proof.

by apply/card_esym/card_eqP; squash to_setT. Qed.

#[global] Hint Resolve card_setT : core.

Lemma ( : set T) : A #= [set: A].

Proof.

exact/card_esym/card_setT. Qed.

#[global] Hint Resolve card_setT : core.

Lemma { } { : set T} { : set U} : {surj B >-> A} -> A #<= B.

Proof.

by move=> g; rewrite (card_le_trans (subset_card_le 'surj_g)) ?card_image_le.
Qed.

Arguments surj_card_ge {T U A B} g.

Lemma { : pointedType} {} { : set T} { : set U} :
reflect (exists , set_surj B A g) (A #<= B).

Proof.

apply: (iffP idP) => [|[_ /Psurj[g _]]]; last exact: surj_card_ge.
elim/Ppointed: U => U in B *; first by rewrite ?emptyE => ->; exists any.
by move=> /pcard_leP[f]; exists (pinv A f); apply: subl_surj surj.
Qed.

Lemma { : pointedType} {} { : set T} { : set U} :
reflect $|{surj B >-> A}| (A #<= B).

Proof.

by apply: (iffP pcard_surjP); rewrite surjPex. Qed.

Lemma { } { : set T} { : set U} :
reflect $|{surj B >-> some @` A}| (A #<= B).

Proof.

by elim/Pchoice: T => T in A *; rewrite -card_ge_some; apply: pcard_geP.
Qed.

Lemma { } { : set T} { : set U} :
reflect (A = set0 \/ $|{surjfun B >-> A}|) (A #<= B).

Proof.

apply: (iffP idP); last by move=> [->//|[f]]; apply: surj_card_ge; exact: f.
elim/Ppointed: T => T in A *; first by rewrite !emptyE; left.
elim/Ppointed: U => U in B *; first by rewrite !emptyE => ->; right; squash any.
move=> /pcard_geP[f]; case: (eqVneq A set0); first by left.
move=> /set0P[x Ax]; right; apply/surjfunPex.
exists (fun => if f y \in A then f y else x).
apply/seteqP; split.
by move=> x' /[dup] /= /'surj_f [y By <-] Afy; exists y; rewrite ?ifT// inE.
by apply/image_subP => y By; case: ifPn; rewrite (inE, notin_setE).
Qed.

Lemma : (`I_n #<= `I_m) = (n <= m)%N.

Proof.

apply/idP/idP=> [/card_leP[f]|?];
last by apply/subset_card_le => k /leq_trans; apply.
by have /leq_card := in2TT 'inj_(IIord \o f \o IIord^-1); rewrite !card_ord.
Qed.

Lemma { } { : set T} { : set U} :
reflect $|{bij A >-> some @` B}| (A #= B).

Proof.

elim/Pchoice: U => U in B *.
by rewrite -(card_eqr card_some); exact: (iffP pcard_eqP).
Qed.

Lemma { } { : set T} { : set U} :
reflect $|{splitbij some @` A >-> some @` B}| (A #= B).

Proof.

elim/Pchoice: U => U in B *; elim/Pchoice: T => T in A *.
rewrite -(card_eql card_some) -(card_eqr card_some).
exact: (iffP ppcard_eqP).
Qed.

Lemma { } : reflect (n = m) (`I_n #= `I_m).

Proof.

by rewrite card_eq_le !card_le_II -eqn_leq; apply: eqP. Qed.

Lemma {} { : set T} ( : set A) : set_val @` X `<=` A.

Proof.

by move=> x [/= a Xa <-]; apply: set_valP. Qed.

Arguments sub_setP {T A}.
Arguments image_subset {aT rT} f [A B].

Lemma ( : set T) ( : set U) :
reflect (exists2 , C #= A & C `<=` B) (A #<= B).

Proof.

apply: (iffP idP) => [/card_leP[f]|[C CA CB]]; last first.
by rewrite -(card_le_eql CA); apply/card_leP; squash (inclT _ \o subfun CB).
exists (set_val @` range f); last exact: (subset_trans (sub_setP _)).
by rewrite ?(card_eql (inj_card_eq _))//; apply: in2W; apply: in2TT; apply: inj.
Qed.

#[deprecated(since="mathcomp-analysis 1.15.0", note="To be removed, use other lemmas instead.")]
Lemma ( : nat -> nat) : {in `I_m &, injective f} ->
f @` `I_m `<=` `I_n -> (m <= n)%N.

Proof.

move=> /Pinj[{}f->] /subset_card_le.
by rewrite (card_le_eql (inj_card_eq _))// card_le_II.
Qed.

Definition ( : set T) := A #<= @setT nat.

Lemma ( : set T) ( : set U) :
A #= B -> countable A = countable B.

Proof.

by move=> /card_le_eql leA; rewrite /countable leA. Qed.

Lemma ( : countType) ( : set T) : countable A.

Proof.

by apply/card_leP; squash (to_setT \o choice.pickle). Qed.

#[global] Hint Resolve countableP : core.

Lemma : countable (@set0 T)

Proof.

exact: card_ge0. Qed.

#[global] Hint Resolve countable0 : core.

Lemma ( : set T) :
reflect (exists : T -> nat, {in A &, injective f}) (countable A).

Proof.

exact: pcard_injP. Qed.

Lemma ( : set T) :
reflect (exists : set nat, (A #= B)%card) (countable A).

Proof.

apply: (iffP idP); last by move=> [B] /eq_countable ->.
move=> /pcard_leP[f]; exists (f @` A).
by apply/pcard_eqP; squash [fun f in A].
Qed.

Lemma ( : set T) ( : set U) : A #<= B ->
countable B -> countable A.

Proof.

exact: card_le_trans. Qed.

Lemma ( : set T) : finite_set A <-> exists , A #= `I_n.

Proof.

by []. Qed.

Lemma : finite_set `I_n

Proof.

by apply/finite_setP; exists n. Qed.

#[global] Hint Resolve finite_II : core.

Lemma {} : `I_n #= [set: 'I_n].

Proof.

by apply/card_esym/pcard_eqP/bijPex; exists val; split. Qed.

Lemma { : choiceType} { : set T} :
finite_set A <-> exists : {fset T}, A = [set` X].

Proof.

rewrite finite_setP; split=> [[n]|[X {A}->]]; last first.
exists #|{: X}|; rewrite (card_eqr card_II).
by apply/card_eqP; squash (to_setT \o enum_rank \o val_finset).
rewrite (card_eqr card_II) => /card_esym/card_eqVP[f]; pose g := f \o to_setT.
exists [fset val (g i) | in 'I_n]%fset.
apply/seteqP; split=> [x /mem_set Ax|_ /imfsetP[i _ ->]]; last exact: set_valP.
by apply/imfsetP; exists (g^-1 (SigSub Ax)); rewrite ?[g _]invK//= inE.
Qed.

Lemma { : choiceType} ( : {fset T}) { : set T} :
A `<=` [set` X] -> finite_set A.

Proof.

move=> AX; apply/finite_fsetP; exists [fset in X | x \in A]%fset.
apply/seteqP; split=> x; rewrite /= ?inE; last by move=> /andP[_ /set_mem].
by move=> Ax; rewrite mem_set ?andbT//; apply: AX.
Qed.

Arguments finite_subfset {T} X {A}.

Lemma : finite_set (set0 : set T).

Proof.

by apply/finite_setP; exists 0%N; rewrite II0. Qed.

#[global] Hint Resolve finite_set0 : core.

Lemma : cofinite_set [set: T].

Proof.

by rewrite setCT. Qed.

#[global] Hint Resolve cofinite_setT : core.

Lemma {} ( : set T) : infinite_set A -> A !=set0.

Proof.

by rewrite -set0P; apply: contra_not_neq => ->. Qed.

Lemma { : eqType} :
finite_set A <-> exists : seq T, A = [set` s].

Proof.

elim/eqPchoice: T => T in A *; rewrite finite_fsetP.
split=> [[X ->]|[s ->]]; first by exists X.
by exists [fset x | in s]%fset; apply/seteqP; split=> x /=; rewrite inE.
Qed.

Lemma { : eqType} ( : seq T) : finite_set [set` s].

Proof.

by apply/finite_seqP; exists s. Qed.

#[global] Hint Resolve finite_seq : core.

Lemma { : choiceType} ( : {fset T}) : finite_set [set` X].

Proof.

by apply/finite_fsetP; exists X. Qed.

#[global] Hint Resolve finite_fset : core.

Lemma { : finType} { : predType T} ( : pT) :
finite_set [set` P].

Proof.

rewrite finite_seqP; exists (enum P).
by apply/seteqP; split=> x/=; rewrite mem_enum.
Qed.

#[global]
Hint Extern 0 (finite_set [set` _]) => solve [apply: finite_finpred] : core.

Lemma { : finType} { : set T} : finite_set X.

Proof.

by have -> : X = [set` mem X] by apply/seteqP; split=> x /=; rewrite ?inE.
Qed.

#[global] Hint Resolve finite_finset : core.

Lemma ( : set T) : finite_set A -> countable A.

Proof.

by move=> /finite_setP[n /eq_countable->]. Qed.

Lemma ( : set T) : infinite_set A <-> [set: nat] #<= A.

Proof.

elim/Ppointed: T => T in A *.
  by rewrite !emptyE; split=> // /(congr1 (@^~ 0%N))/=; rewrite propeqE => -[].
split=> [Ainfinite| + /finite_setP[n eqAI]]; last first.
  rewrite (card_le_eqr eqAI) => le_nat_n.
  suff: `I_n.+1 #<= `I_n by rewrite card_le_II ltnn.
  exact: card_le_trans (subset_card_le _) le_nat_n.
have /all_sig2[f Af fX] : forall : {fset T}, { | x \in A & x \notin X}.
  move=> X; apply/sig2W; apply: contra_notP Ainfinite => nAX; apply/finite_fsetP.
  exists [fset in X | x \in A]%fset; rewrite eqEsubset; split; last first.
    by move=> x/=; rewrite !inE => /andP[_]; rewrite inE.
  move=> x Ax /=; rewrite !inE/=; apply/andP; split; rewrite ?inE//.
  by apply: contra_notT nAX => xNX; exists x; rewrite ?inE.
do [under [forall : {fset _}, _]eq_forall do rewrite inE] in Af *.
suff [g gE] : exists : nat -> T,
    forall , g n = f [fset g k | in iota 0 n]%fset.
  have /Pinj[h hE] : {in setT &, injective g}.
    move=> i j _ _; apply: contra_eq; wlog lt_ij : i j / (i < j)%N => [hwlog|_].
    by case: ltngtP => // ij _; [|rewrite eq_sym];
       apply: hwlog=> //; rewrite lt_eqF//.
    rewrite [g j]gE; set X := (X in f X); have := fX X.
    by apply: contraNneq => <-; apply/imfsetP; exists i => //=; rewrite mem_iota.
  have/injPfun[i _] : {homo h : / setT x >-> A x} by move=> i; rewrite -hE gE.
  by apply/pcard_leP; squash i.
pose g := fix g := if n isn't n'.+1 then f fset0
                       else f [fset g n' i | in iota 0 k]%fset.
exists (fun => g n n) => n.
suff {n} gn n k : (k <= n)%N -> g n k = f [fset g k k | in iota 0 k]%fset.
  by rewrite gn//; congr f; apply/fsetP => k.
have [m] := ubnP n; elim: m n k => //= m IHm [|n] k /=.
  rewrite leqn0 => _ /eqP->/=.
  congr f; apply/fsetP => x; rewrite !inE; symmetry.
  by apply/imfsetP => /= -[].
rewrite ltnS => ltmn lekSn /=; congr f; apply/fsetP => i.
by apply/imfsetP/imfsetP => /= -[j]; rewrite mem_iota/= => jk ->;
   exists j; rewrite ?mem_iota//= ?add0n ?IHm//;
   by [rewrite (leq_trans jk)// (leq_trans lekSn)|rewrite -ltnS (leq_trans jk)].
Qed.

Lemma ( : set T) : finite_set A <-> ~ ([set: nat] #<= A).

Proof.

by rewrite -infiniteP notK. Qed.

Lemma ( : set T) ( : set U) :
A #<= B -> finite_set B -> finite_set A.

Proof.

by move=> ?; rewrite !finite_setPn; apply: contra_not => /card_le_trans; apply.
Qed.

Lemma ( : set T) : A `<=` B ->
finite_set B -> finite_set A.

Proof.

by move=> ?; apply/card_le_finite/subset_card_le. Qed.

Lemma ( : set T) : A `<=` B ->
cofinite_set A -> cofinite_set B.

Proof.

by move=> /subsetC/sub_finite_set. Qed.

Lemma ( : set T) : A `<=` B ->
infinite_set A -> infinite_set B.

Proof.

by move=> AB; apply/contra_not/sub_finite_set. Qed.

Lemma ( : set T) : finite_set A <-> exists , A #<= `I_n.

Proof.

split=> [[n /card_eqPle[]]|[n leAn]]; first by exists n.
by apply: card_le_finite leAn _; exists n.
Qed.

Lemma { } ( : set U) ( : T -> U) :
{in f @^-1` B &, injective f} -> f @^-1` B #<= B.

Proof.

move=> /Pinj[g eqg]; rewrite -(card_le_eql (card_image g)) -eqg.
by apply: subset_card_le; apply: image_preimage_subset.
Qed.

Corollary { } ( : set U) ( : T -> U) :
{in f @^-1` B &, injective f} -> finite_set B -> finite_set (f @^-1` B).

Proof.

by move=> /card_ge_preimage fB; apply: card_le_finite. Qed.

Lemma ( : set T) ( : set U) :
A #= B -> finite_set A = finite_set B.

Proof.

move=> eqAB; apply/propeqP.
by split=> -[n Xn]; exists n; move: Xn; rewrite (card_eql eqAB).
Qed.

Lemma ( : set T) : A `\` B #<= A.

Proof.

by apply: subset_card_le; rewrite setDE; apply: subIset; left. Qed.

Lemma ( : T -> T') : finite_set A -> finite_set (f @` A).

Proof.

exact/card_le_finite/card_image_le. Qed.

Lemma ( : set T) ( : set T') :
finite_set (A `*` B) -> finite_set A \/ finite_set B.

Proof.

have [->|/set0P[a Aa]] := eqVneq A set0; first by left.
have /sub_finite_set : [set a] `*` B `<=` A `*` B by move=> x/= [] -> ?; split.
move => /[apply]/(finite_image snd); rewrite (_ : _ @` _ = B); last by right.
by apply/seteqP; split=> [b [[? ?] [? ?] <-//]|b ?]/=; exists (a, b).
Qed.

Lemma {} { : set T} :
infinite_set A -> infinite_set B -> infinite_set (A `*` B).

Proof.

by move=> iA iB; have /not_orP := conj iA iB; exact/contra_not/finite_setX_or.
Qed.

Lemma ( : T) : finite_set [set x].

Proof.

elim/Pchoice: T => T in x *.
by apply/finite_fsetP; exists (fset1 x); rewrite set_fset1.
Qed.

#[global] Hint Resolve finite_set1 : core.

Lemma ( : set T) : finite_set A -> finite_set (A `\` B).

Proof.

exact/card_le_finite/card_le_setD. Qed.

Lemma ( : set T) : cofinite_set A -> cofinite_set (A `|` B).

Proof.

by rewrite setCU -setDE; apply: finite_setD. Qed.

Lemma ( : set T) : cofinite_set B -> cofinite_set (A `|` B).

Proof.

by rewrite setUC; apply: cofinite_setUl. Qed.

Lemma ( : set T) :
cofinite_set A \/ cofinite_set B -> cofinite_set (A `|` B).

Proof.

by move=> [/cofinite_setUl|/cofinite_setUr]. Qed.

Lemma ( : set T) :
finite_set (A `|` B) = (finite_set A /\ finite_set B).

Proof.

pose fP := @finite_fsetP {classic T}; rewrite propeqE; split.
by move=> finAUB; split; apply: sub_finite_set finAUB.
by case=> /fP[X->]/fP[Y->]; apply/fP; exists (X `|` Y)%fset; rewrite set_fsetU.
Qed.

Lemma ( : set T) :
cofinite_set (A `&` B) = (cofinite_set A /\ cofinite_set B).

Proof.

by rewrite setCI finite_setU. Qed.

Lemma ( : T) : finite_set [set x; y].

Proof.

by rewrite !finite_setU; split; apply: finite_set1. Qed.

#[global] Hint Resolve finite_set2 : core.

Lemma ( : T) : finite_set [set x; y; z].

Proof.

by rewrite !finite_setU; do !split; apply: finite_set1. Qed.

#[global] Hint Resolve finite_set3 : core.

Lemma ( : T) : finite_set [set x; y; z; t].

Proof.

by rewrite !finite_setU; do !split; apply: finite_set1. Qed.

#[global] Hint Resolve finite_set4 : core.

Lemma ( : T) : finite_set [set x; y; z; t; u].

Proof.

by rewrite !finite_setU; do !split; apply: finite_set1. Qed.

#[global] Hint Resolve finite_set5 : core.

Lemma ( : T) : finite_set [set x; y; z; t; u; v].

Proof.

by rewrite !finite_setU; do !split; apply: finite_set1. Qed.

#[global] Hint Resolve finite_set6 : core.

Lemma ( : T) : finite_set [set x; y; z; t; u; v; w].

Proof.

by rewrite !finite_setU; do !split; apply: finite_set1. Qed.

#[global] Hint Resolve finite_set7 : core.

Lemma ( : set T) :
(finite_set A \/ finite_set B) -> finite_set (A `&` B).

Proof.

by case; apply: contraPP; rewrite !infiniteP => /card_le_trans; apply;
apply: subset_card_le.
Qed.

Lemma ( : set T) : finite_set A -> finite_set (A `&` B).

Proof.

by move=> ?; apply: finite_setI; left. Qed.

Lemma ( : set T) : finite_set B -> finite_set (A `&` B).

Proof.

by move=> ?; apply: finite_setI; right. Qed.

Lemma ( : set T) ( : set T') :
finite_set A -> finite_set B -> finite_set (A `*` B).

Proof.

elim/Pchoice: T => T in A *; elim/Pchoice: T' => T' in B *.
move=> /finite_fsetP[{}A ->] /finite_fsetP[{}B ->].
apply/finite_fsetP; exists (A `*` B)%fset; apply/predeqP => x.
by split; rewrite /= inE => /andP.
Qed.

Lemma [ : Type] [ : set aT] [ : set bT]
( : aT -> bT -> rT) :
finite_set A -> finite_set B -> finite_set [set f x y | in A & in B].

Proof.

by move=> fA fB; rewrite image2E; exact/finite_image/finite_setX. Qed.

Lemma [ : Type] [ : set xT]
    ( : aT -> bT -> rT) ( : xT -> aT) ( : xT -> bT) :
    finite_set (fa @` X) -> finite_set (fb @` X) ->
  finite_set [set g (fa x) (fb x) | in X].

Proof.

move=> /(finite_image2 g) /[apply]; apply: sub_finite_set; rewrite image2E.
by move=> r/= [x Xx <-]; exists (fa x, fb x) => //; split; exists x.
Qed.

Definition ( : choiceType) ( : set T) :=
  if pselect (finite_set A) is left Afin
  then projT1 (cid (finite_fsetP.1 Afin)) else fset0.

Lemma ( : choiceType) ( : set T) : finite_set A ->
  [set` fset_set A] = A.

Proof.

by rewrite /fset_set; case: pselect => // Afin _; case: cid. Qed.

Lemma ( : choiceType) ( : set T) : finite_set A ->
fset_set A =i A.

Proof.

by move=> fA x; rewrite -[A in RHS]fset_setK//; apply/idP/idP; rewrite ?inE.
Qed.

Lemma ( : choiceType) ( : set T) :
finite_set A -> finite_set B -> A `<=` B = (fset_set A `<=` fset_set B)%fset.

Proof.

move=> finA finB; apply/propext; split=> [AB|/fsubsetP AB t].
by apply/fsubsetP => t; rewrite in_fset_set// in_fset_set// 2!inE => /AB.
by have := AB t; rewrite !in_fset_set// !inE.
Qed.

Lemma ( : choiceType) ( : set T) : finite_set A ->
fset_set A = fset0 -> A = set0.

Proof.

move=> finA; rewrite /fset_set; case: pselect => // {}finA.
by case: cid => _/= -> ->; rewrite set_fset0.
Qed.

Lemma { : choiceType} : fset_set (set0 : set T) = fset0.

Proof.

by apply/fsetP=> x; rewrite in_fset_set ?inE//; apply/negP; rewrite inE.
Qed.

Lemma { : choiceType} ( : T) : fset_set [set x] = [fset x]%fset.

Proof.

apply/fsetP=> y; rewrite in_fset_set ?inE//.
by apply/idP/idP; rewrite inE => /eqP.
Qed.

Lemma { : choiceType} ( : set T) :
finite_set A -> finite_set B ->
fset_set (A `|` B) = (fset_set A `|` fset_set B)%fset.

Proof.

move=> fA fB; apply/fsetP=> x.
rewrite ?(inE, in_fset_set)//; first by rewrite finite_setU.
by apply/idP/orP; rewrite ?inE.
Qed.

Lemma { : choiceType} ( : set T) :
finite_set A -> finite_set B ->
fset_set (A `&` B) = (fset_set A `&` fset_set B)%fset.

Proof.

move=> fA fB; apply/fsetP=> x.
rewrite ?(inE, in_fset_set)//; first by apply: finite_setI; left.
by apply/idP/andP; rewrite ?inE.
Qed.

Lemma { : choiceType} ( : T) ( : set T) :
finite_set A -> fset_set (x |` A) = (x |` fset_set A)%fset.

Proof.

by move=> fA; rewrite fset_setU// fset_set1. Qed.

Lemma { : choiceType} ( : set T) :
finite_set A -> finite_set B ->
fset_set (A `\` B) = (fset_set A `\` fset_set B)%fset.

Proof.

move=> fA fB; apply/fsetP=> x.
rewrite ?(inE, in_fset_set)//; first exact: finite_setD.
by apply/idP/andP; rewrite ?inE => -[]; rewrite ?notin_setE.
Qed.

Lemma { : choiceType} ( : T) ( : set T) :
finite_set A -> fset_set (A `\ x) = (fset_set A `\ x)%fset.

Proof.

by move=> fA; rewrite fset_setD// fset_set1. Qed.

Lemma { : choiceType} ( : set T1) ( : set T2) :
finite_set A -> finite_set B ->
fset_set (A `*` B) = (fset_set A `*` fset_set B)%fset.

Proof.

move=> Afin Bfin; have ABfin : finite_set (A `*` B) by exact: finite_setX.
apply/fsetP => i; apply/idP/idP; rewrite !(inE, in_fset_set)//=.
by move=> [/mem_set-> /mem_set->].
by move=> /andP[]; rewrite !inE.
Qed.

Definition ( : choiceType) ( : {fset (T1 * T2)}) : {fset T1} :=
  [fset x.1 | in A]%fset.
Definition ( : choiceType) ( : {fset (T1 * T2)}) : {fset T2} :=
  [fset x.2 | in A]%fset.
Notation := (fst_fset A) : fset_scope.
Notation := (snd_fset A) : fset_scope.

Lemma ( : choiceType) ( : set (T1 * T2)) :
  finite_set A -> finite_set A.`1.

Proof.

move=> /finite_fsetP[B A_B]; apply/finite_fsetP; exists (B.`1)%fset.
by apply/seteqP; split=> [x/= [y]|_/= /imfsetP[[x1 x2]/= +] ->]; rewrite A_B;
[move=> xyB; apply/imfsetP; exists (x, y)|move=> ?; exists x2].
Qed.

Lemma ( : choiceType) ( : set (T1 * T2)) :
finite_set A -> finite_set A.`2.

Proof.

move=> /finite_fsetP[B A_B]; apply/finite_fsetP; exists (B.`2)%fset.
apply/seteqP; split=> [y/= [x]|_/= /imfsetP[[x1 x2]/= +] ->]; rewrite A_B;
by [move=> xyB; apply/imfsetP; exists (x, y)|move=> ?; exists x1].
Qed.

Lemma { } ( : set I) ( : I -> set T) :
finite_set D -> (forall , D i -> finite_set (F i)) ->
finite_set (\bigcup_( in D) F i).

Proof.

elim/Pchoice: I => I in D F *.
elim/Ppointed: T => T in F *; first by rewrite emptyE.
move=> Dfin Ffin; pose G ( : fset_set D) := fset_set (F (val i)).
suff: (\bigcup_( in D) F i #<= [set: { & G i}])%card.
by move=> /card_le_finite; apply; apply: finite_finset.
apply/pcard_geP/surjPex.
exists (fun ( : { : fset_set D & G i}) => val (projT2 k)).
move=> y [i Di Fky]/=.
have Dk : i \in fset_set D by rewrite in_fset_set// inE.
pose k : fset_set D := [` Dk]%fset.
have Gy : y \in G k by rewrite in_fset_set ?inE//; apply: Ffin.
by exists (Tagged G [` Gy]%fset).
Qed.

Lemma {} ( : set T) : infinite_set [set: T] ->
cofinite_set A -> infinite_set A.

Proof.

by move=> + ACfin Afin; apply; rewrite -(setvU A) finite_setU. Qed.

Lemma ( : choiceType) ( : nat -> set T) :
  (forall , finite_set (F n)) -> trivIset [set: nat] F ->
  (\sum_( < n) #|` fset_set (F i)| =
   #|` fset_set (\big[setU/set0]_( < n) F k)|)%N.

Proof.

move=> finF tF; elim: n => [|n ih]; first by rewrite !big_ord0 fset_set0.
rewrite big_ord_recr//= ih big_ord_recr/= fset_setU//.
by rewrite -bigcup_mkord; exact: bigcup_finite.
rewrite cardfsU [X in (_ - X)%N](_ : _ = O) ?subn0// ?EFinD ?natrD//.
apply/eqP; rewrite cardfs_eq0 -fset_setI//.
by rewrite -bigcup_mkord; exact: bigcup_finite.
rewrite (@trivIset_bigsetUI _ xpredT)// ?fset_set0//.
by rewrite [X in trivIset X F](_ : _ = [set: nat])//; exact/seteqP.
Qed.

Lemma ( : choiceType) ( : set T) ( : T -> set T') :
finite_set A -> (forall , A x -> finite_set (B x)) -> finite_set (A `*`` B).

Proof.

move=> Afin Bfin; rewrite -bigcupX1l.
by apply: bigcup_finite => // i Ai; exact/finite_setX/Bfin.
Qed.

Lemma ( : choiceType) ( : T' -> set T) ( : set T') :
(forall , B x -> finite_set (A x)) -> finite_set B -> finite_set (A ``*` B).

Proof.

move=> Afin Bfin; rewrite -bigcupX1r.
by apply: bigcup_finite => // i Ai; apply/finite_setX => //; exact: Afin.
Qed.

Lemma : fset_set `I_n = [fset val i | in 'I_n]%fset.

Proof.

apply/fsetP => i; rewrite /= ?inE in_fset_set//.
apply/idP/imfsetP; rewrite ?inE/=.
by move=> lt_in; exists (Ordinal lt_in).
by move=> [j _ ->].
Qed.

Lemma ( : choiceType) ( : {fset T}) : fset_set [set` A] = A.

Proof.

apply/fsetP => x; rewrite in_fset_set//=.
by apply/idP/idP; rewrite ?inE.
Qed.

Lemma { : choiceType} ( : T -> U) ( : set T) :
finite_set A -> fset_set (f @` A) = (f @` fset_set A)%fset.

Proof.

move=> Afset; apply/fsetP=> i.
rewrite !in_fset_set; first exact: finite_image.
apply/idP/imfsetP; rewrite !inE/=.
by move=> [x Ax <-]; exists x; rewrite ?in_fset_set ?inE.
by move=> [x + ->]; rewrite in_fset_set// inE; exists x.
Qed.

Lemma { : choiceType} ( : set T) :
finite_set A -> finite_set B -> fset_set A = fset_set B -> A = B.

Proof.

by move=> Afin Bfin /(congr1 pred_set); rewrite !fset_setK. Qed.

Lemma ( : choiceType) ( : set I) ( : I -> set T) :
finite_set A -> \big[setU/set0]_( <- fset_set A) F i =\bigcup_( in A) F i.

Proof.

move=> finA; rewrite -bigcup_fset /fset_set; case: pselect => [{}finA|//].
apply/seteqP; split=> [x [i /=]|x [i Ai Fix]].
by case: cid => /= B -> iB Fix; exists i.
by exists i => //; case: cid => // B AB /=; move: Ai; rewrite AB.
Qed.

Lemma ( : choiceType) ( : set I) ( : I -> set T) :
finite_set A -> \bigcup_( in A) F i = \big[setU/set0]_( <- fset_set A) F i.

Proof.

by move=> /bigsetU_fset_set->. Qed.

#[deprecated(note="Use -bigsetU_fset_set instead")]
Notation := __deprecated__bigcup_fset_set (only parsing).

Lemma ( : choiceType) ( : set I) ( : I -> set T)
( : pred I) : finite_set A ->
\big[setU/set0]_( <- fset_set A | P i) F i = \bigcup_( in A `&` P) F i.

Proof.

by move=> *; rewrite bigcup_mkcondr big_mkcond -bigsetU_fset_set ?mem_setE.
Qed.

Lemma ( : choiceType) ( : set I) ( : I -> set T)
( : pred I) : finite_set A ->
\bigcup_( in A `&` P) F i = \big[setU/set0]_( <- fset_set A | P i) F i.

Proof.

by move=> /bigsetU_fset_set_cond->. Qed.

#[deprecated(note="Use -bigsetU_fset_set_cond instead")]
Notation := __deprecated__bigcup_fset_set_cond (only parsing).

Lemma ( : choiceType) ( : set I) ( : I -> set T) :
finite_set A -> \big[setI/setT]_( <- fset_set A) F i =\bigcap_( in A) F i.

Proof.

by move=> *; apply: setC_inj; rewrite setC_bigcap setC_bigsetI bigsetU_fset_set.
Qed.

Lemma ( : choiceType) ( : set I) ( : I -> set T) :
finite_set A -> \bigcap_( in A) F i = \big[setI/setT]_( <- fset_set A) F i.

Proof.

by move=> /bigsetI_fset_set->. Qed.

#[deprecated(note="Use -bigsetI_fset_set instead")]
Notation := __deprecated__bigcap_fset_set (only parsing).

Lemma ( : choiceType) ( : set I) ( : I -> set T)
( : pred I) : finite_set A ->
\big[setI/setT]_( <- fset_set A | P i) F i = \bigcap_( in A `&` P) F i.

Proof.

by move=> *; rewrite bigcap_mkcondr big_mkcond -bigsetI_fset_set ?mem_setE.
Qed.

Lemma ( : set T) ( : set U) ( : {bij X >-> Y}) :
X `<=` A -> Y `<=` B -> A `\` X #= B `\` Y ->
exists : {bij A >-> B}, {in X, g =1 f}.

Proof.

elim/Ppointed: U => U in Y B f *.
rewrite !emptyE in f * => XA _; rewrite setD_eq0 => AX.
by suff /seteqP->// : A `<=>` X by exists f.
move=> XA YB /pcard_eqP[g].
rewrite -(joinIB X A) -(joinIB Y B) !meetEset.
have /disj_set2P AX : (A `&` X) `&` (A `\` X) = set0 by apply: meetIB.
have /disj_set2P BY : (B `&` Y) `&` (B `\` Y) = set0 by apply: meetIB.
rewrite !(setIidr XA) !(setIidr YB) in AX BY *.
by exists [bij of glue AX BY f g] => x /= xX; rewrite glue1.
Qed.

Lemma { : choiceType} { : {fset T}} {} :
reflect (#|` A| = n) ([set` A] #= `I_n).

Proof.

elim/choicePpointed: T => T in A *.
  rewrite -{1}[A]set_fsetK !emptyE fset_set0 cardfs0.
  by apply: (iffP eqP) => [/IIn_eq0->//|<-]; rewrite II0.
rewrite (card_eqr card_II) card_eq_sym.
apply: (iffP pcard_eqP) => [[f]|]; last first.
  rewrite cardfE => eqAn.
  by squash (set_val \o finset_val \o enum_val \o cast_ord (esym eqAn)).
suff -> : A = [fset f i | in 'I_n]%fset by rewrite card_imfset ?size_enum_ord.
apply/fsetP => x; apply/idP/imfsetP => /= [xA|[i _ ->]].
  by have [i _ <-] := 'surj_f xA; exists i.
by have /(_ i I) := 'funS_f.
Qed.

Lemma { : choiceType} ( : set T) :
A #= `I_n -> #|`fset_set A| = n.

Proof.

move=> An; apply/card_eq_fsetP; rewrite fset_setK//.
by apply/finite_setP; exists n.
Qed.

Lemma { : choiceType} ( : set T) :
A #<= `I_n -> (#|`fset_set A| <= n)%N.

Proof.

move=> An; have /finite_setP[m Am] : finite_set A
by apply/finite_set_leP; exists n.
by rewrite (card_fset_set Am) -card_le_II -(card_le_eql Am).
Qed.

Lemma { : choiceType} ( : set T) :
finite_set A -> A #>= `I_n -> (#|`fset_set A| >= n)%N.

Proof.

move=> /finite_setP[m Am]; rewrite (card_fset_set Am).
by rewrite (card_le_eqr Am) card_le_II.
Qed.

Lemma {} ( : set T) :
infinite_set A -> finite_set B -> infinite_set (A `\` B).

Proof.

move=> + finB finAB; apply.
have : finite_set ((A `&` ~` B) `|` B) by rewrite finite_setU.
by rewrite setUIl setUCl setIT finite_setU => -[].
Qed.

Lemma {} ( : set T) :
infinite_set A -> cofinite_set B -> infinite_set (A `&` B).

Proof.

by move=> /infinite_setD/[apply]; rewrite setDE setCK. Qed.

Lemma {} ( : set T) :
cofinite_set A -> infinite_set B -> infinite_set (A `&` B).

Proof.

by rewrite setIC => *; apply: infinite_setIl. Qed.

Lemma { : choiceType} ( : set T) :
infinite_set A ->
exists2 : {fset T}, [set` B] `<=` A & (#|` B| >= n)%N.

Proof.

elim/choicePpointed: T => T in A *; first by rewrite emptyE.
move=> /infiniteP/ppcard_leP[f]; exists (fset_set [set f i | in `I_n]).
rewrite fset_setK//; first exact: finite_image.
by apply: subset_trans (fun_image_sub f); apply: image_subset.
rewrite fset_set_image// card_imfset//= fset_set_II/=.
by rewrite card_imfset//= ?size_enum_ord//; apply: val_inj.
Qed.

Lemma { : choiceType} ( : set T) :
infinite_set A <->
forall , exists2 : {fset T}, [set` B] `<=` A & (#|` B| >= n)%N.

Proof.

split; first by move=> ? ?; apply: infinite_set_fset.
elim/choicePpointed: T => T in A *.
move=> /(_ 1%N)[B _]; rewrite cardfs_gt0 => /fset0Pn[x xB].
by have: [set` B] x by []; rewrite emptyE.
move=> Bge /finite_setP[n An]; have [B BA] := Bge n.+1.
apply/negP; rewrite -leqNgt -(card_fset_set An) fsubset_leq_card//.
apply/fsubsetP => x /BA; rewrite in_fset_set ?inE//.
by apply/finite_setP; exists n.
Qed.

Lemma { : choiceType} ( : set T) ( : set T') :
finite_set A -> finite_set B ->
reflect (#|`fset_set A| = #|`fset_set B|) (A #= B).

Proof.

move=> /finite_setP/cid[n An] /finite_setP/cid[m Bm].
rewrite (card_fset_set An) (card_fset_set Bm).
by rewrite (card_eql An) (card_eqr Bm); apply: card_eq_II.
Qed.

Lemma { } : `I_n `\` `I_k #= `I_(n - k)%N.

Proof.

apply/fcard_eq => //; first exact: finite_setD.
rewrite fset_setD//= cardfsD/= -fset_setI// setI_II.
rewrite !fset_set_II !card_imfset// /= !size_enum_ord.
by case: leqP; rewrite // subnn => /eqP->.
Qed.

Lemma ( : set T) : A != set0 ->
A #= `I_n -> S `<=` A ->
exists ( : {bij `I_n >-> A}) , (k <= n)%N /\ `I_n `&` (f @^-1` S) = `I_k.

Proof.

elim/Ppointed: T => T in A S *; first by rewrite !emptyE eqxx.
move=> AN0 An SA; have [k kn Sk] : exists2 , (k <= n)%N & S #= `I_k.
  have /finite_setP[k Sk]: finite_set S by apply: sub_finite_set SA _; exists n.
  exists k => //; rewrite -card_le_II.
  by rewrite -(card_le_eqr An) -(card_le_eql Sk); apply: subset_card_le.
have /card_esym/ppcard_eqP[f] := Sk.
have eqAS : A `\` S #= `I_n `\` `I_k.
  have An' := An; have Sk' := Sk.
  do [have /finite_fsetP[{An'}A ->] : finite_set A by exists n] in An AN0 SA *.
  do [have /finite_fsetP[{Sk'}S ->] : finite_set S by exists k] in Sk f SA *.
  have [/card_eq_fsetP {}An /card_eq_fsetP {}Sk] := (An, Sk).
  rewrite -set_fsetD (card_eqr card_IID); apply/card_eq_fsetP.
  by rewrite cardfsD (fsetIidPr _) ?An ?Sk //; apply/fsubsetP.
case: (super_bij [bij of f^-1] SA _ eqAS) => [x /= /leq_trans->// | g].
have [{}g ->] := pPbij 'bij_g => /= gE.
exists [bij of g^-1], k; split=> //=; rewrite -inv_sub_image //= invV.
by under eq_imagel do rewrite /= gE ?inE//; rewrite image_eq.
Qed.

#[deprecated(note="use countable0 instead")]
Notation := countable0 (only parsing).

Lemma ( : T) : countable [set x].

Proof.

exact: finite_set_countable. Qed.

#[global] Hint Resolve countable1 : core.

Lemma ( : choiceType) ( : {fset T}) : countable [set` X].

Proof.

exact: finite_set_countable. Qed.

#[global] Hint Resolve countable_fset : core.

Lemma ( : finType) ( : predType T) ( : pT) : countable [set` P].

Proof.

exact: finite_set_countable. Qed.

#[global] Hint Extern 0 (is_true (countable [set` _])) => solve [apply: countable_finpred] : core.

Lemma ( : set T):
countable A -> ~ finite_set A -> A #= [set: nat].

Proof.

by move=> Acnt /infiniteP leNA; rewrite card_eq_le leNA andbT. Qed.

Lemma : ~ finite_set [set: nat].

Proof.

exact/infiniteP/card_lexx. Qed.

Lemma : infinite_set [set: nat * nat].

Proof.

by rewrite -setXTT; apply: infinite_setX; exact: infinite_nat. Qed.

Lemma : [set: nat * nat] #= [set: nat].

Proof.

exact/eq_card_nat/infinite_prod_nat/countableP. Qed.

Lemma ( : nat -> nat) : injective f -> forall ( : nat), exists , (n < f m)%N.

Proof.

move=> fI n; suff [m /negP] : ~` (f @^-1` `I_n.+1) !=set0 by rewrite -ltnNge; exists m.
apply: infinite_setN0; apply: cofinite_set_infinite; first exact: infinite_nat.
by rewrite setCK; apply: finite_preimage; first by move=> ? ? ? ?; apply: fI.
Qed.

..Build rat 0.

Lemma : infinite_set [set: rat].

Proof.

apply/infiniteP/pcard_leTP/injPex; exists (GRing.natmul 1) => // m n _ _.
exact/Num.Theory.mulrIn/oner_neq0.
Qed.

Lemma : [set: rat] #= [set: nat].

Proof.

exact/eq_card_nat/infinite_rat/countableP. Qed.

Lemma : infinite_set [set: rat * rat].

Proof.

by rewrite -setXTT; apply: infinite_setX; exact: infinite_rat. Qed.

Lemma : ([set: rat * rat] #= [set: nat])%card.

Proof.

exact/eq_card_nat/infinite_prod_rat/countableP. Qed.

Lemma { : choiceType} : countable [set: T] ->
{ : countType | T = T' :> Type}.

Proof.

move=> /pcard_leP/unsquash f.
pose TcM := PCanIsCountable (in1TT 'funoK_f).
pose TC : countType := HB.pack T TcM.
by exists TC.
Qed.

Lemma { : eqType} : countable [set: T] ->
{ : countType | T = T' :> Type}.

Proof.

by elim/eqPchoice: T => T /choicePcountable. Qed.

Lemma { : Type} : countable [set: T] ->
{ : countType | T = T' :> Type}.

Proof.

by elim/Pchoice: T => T /choicePcountable. Qed.

Lemma { } ( : set I) ( : I -> set T) :
countable D -> (forall , D i -> countable (F i)) ->
countable (\bigcup_( in D) F i).

Proof.

elim/Ppointed: T => T in F *; first by rewrite emptyE.
rewrite -(eq_countable (card_setT _)) => cD cF; rewrite bigcup_set_type.
set G := (fun : D => F (val i)).
have {cF}cG i : countable (G i) by apply: cF; apply: set_valP.
move: (D : Type) cD G cG => {F I}_ /Pcountable[{}D ->] G cG.
suff: (\bigcup_ G i #<= [set: { & G i}])%card.
  have cGT i : countable [set: G i] by rewrite (eq_countable (card_setT _)).
  have /all_sig[H GE] := fun => Pcountable (cGT i).
  by move=> /sub_countable->//; rewrite (eq_fun GE).
apply/pcard_geP/surjPex; exists (fun ( : { & G i}) => val (projT2 k)).
by move=> x [i _] Gix/=; exists (Tagged G (SigSub (mem_set Gix))).
Qed.

Lemma ( : set T) ( : T -> set T') :
countable A -> (forall , A i -> countable (B i)) -> countable (A `*`` B).

Proof.

elim/Ppointed: T => T in A B *; first by rewrite emptyE -bigcupX1l bigcup_set0.
elim/Ppointed: T' => T' in B *.
by rewrite -bigcupX1l bigcup0// => i; rewrite emptyE setX0.
move=> Ac Bc; rewrite -bigcupX1l bigcup_countable// => i Ai.
have /ppcard_leP[f] := Bc i Ai; apply/pcard_geP/surjPex.
exists (fun => (i, f^-1%FUN k)) => -[_ j]/= [-> dj].
by exists (f j) => //=; rewrite funK ?inE.
Qed.

Lemma ( : set T1) ( : set T2) :
countable D1 -> countable D2 -> countable (D1 `*` D2).

Proof.

by move=> D1c D2c; exact: countableXR (fun _ _ => D2c). Qed.

Lemma ( : T' -> set T) ( : set T') :
countable B -> (forall , B i -> countable (A i)) -> countable (A ``*` B).

Proof.

move=> Bc Ac; rewrite -bigcupX1r; apply: bigcup_countable => // i Bi.
by apply: countableX => //; exact: Ac.
Qed.

Lemma ( : set T) ( : T -> set T') :
infinite_set A -> (forall , B i !=set0) -> infinite_set (A `*`` B).

Proof.

move=> /infiniteP/pcard_geP[f] /(_ _)/cid-/all_sig[b Bb].
apply/infiniteP/pcard_geP/surjPex; exists (fun => f x.1).
by move=> i iT; have [a Aa fa] := 'oinvP_f iT; exists (a, b a).
Qed.

Lemma ( : set T) ( : T -> set T') :
(A #= [set: nat] -> (forall , countable (B i) /\ B i !=set0) ->
A `*`` B #= [set: nat])%card.

Proof.

rewrite !card_eq_le => /andP[Acnt /infiniteP Ainfty] /all_and2[Bcnt Bn0].
by rewrite [(_ #<= _)%card]countableXR//=; exact/infiniteP/infiniteXRl.
Qed.

Lemma { : Type} ( : set T) :
reflect (exists2 , A x & A `\ x #= `I_n) (A #= `I_n.+1).

Proof.

elim/Ppointed: T A => T A.
  rewrite !emptyE; apply: (iffP eqP) => [|[]//].
  by move=> /(congr1 (@^~ 0%N))/=; rewrite -falseE ltnS leq0n => /is_true_inj.
apply: (iffP idP) => [|[x Ax]].
  move=> /ppcard_eqP[f]; exists (f^-1 n); first by apply: funS => /=.
  by apply/card_esym/card_set_bijP; exists f^-1; apply: bij_II_D1.
move=> /pcard_eqP[f]; have [//|||g _] := @super_bij _ _ _ A _ `I_n.+1 f.
- by move=> k /=; apply: leq_trans.
- by rewrite setDD (card_eqr card_IID) subSnn// setIidr ?card_set1// => ? ->.
- by apply/pcard_eqP; squash g.
Qed.

Lemma { : Type} ( : set T) :
countable D -> countable [set | A `<=` D /\ A #= `I_n].

Proof.

move=> Dcnt; elim: n => [|n].
rewrite [X in countable X]( _ : _ = [set set0])// eqEsubset II0.
by split=> A /=; [rewrite card_eq0; case=> _ /eqP | move->; split].
move=> /(countableX Dcnt); apply: sub_countable.
apply: card_le_trans (card_image_le (fun => u.1 |` u.2) _).
apply: subset_card_le => B [BD] /eq_cardSP [x Bx BDx].
exists (x, B `\ x) => /=; last by apply: setDUK => ? ->.
by do !split=> //; [exact: BD | apply: subset_trans _ BD; apply: subDsetl].
Qed.

Lemma { : Type} ( : set T) :
countable D -> countable [set | A `<=` D /\ finite_set A ].

Proof.

move=> Dcnt; suff -> : [set | A `<=` D /\ finite_set A ] =
\bigcup_ [set | A `<=` D /\ A #= `I_n ].
by apply: bigcup_countable => // ? _; exact: countable_n_subset.
rewrite eqEsubset; split=> [A [AD /finite_setP[n An]]|A]; first by exists n.
by move=> [n _ [AD An]]; split=> //; apply/finite_setP; exists n.
Qed.

Lemma { : pointedType} ( : set T) :
[set | A `<=` D /\ finite_set A ] #= [set : {fset T} | {subset A <= D}] .

Proof.

apply/card_set_bijP; exists (@fset_set T); split.
- by move=> A [AD fsetA] /= x; rewrite in_fset_set // ?inE; exact: AD.
- by move=> ? ? /set_mem [_ +] /set_mem [_ +]; exact: fset_set_inj.
- move=> B /= BD; exists [set` B]; rewrite ?set_fsetK //.
by split; [by move => x /= /BD /set_mem | exact: finite_fset].
Qed.

Lemma { : pointedType} ( : set T) :
countable D -> countable [set : {fset T} | {subset A <= D}].

Proof.

rewrite -(eq_countable (eq_card_fset_subset _)) => ?.
exact: countable_finite_subset.
Qed.

. ( -> rT) := {
  fimfunP : finite_set (range f)
}.
. of @FiniteImage aT rT f}.

Arguments fimfunP {aT rT} _.
#[global] Hint Extern 0 (finite_set _) => solve [apply: fimfunP] : core.

Reserved Notation "{ 'fimfun' aT >-> T }"
  (at level 0, format "{ 'fimfun' aT >-> T }").
Reserved Notation "[ 'fimfun' 'of' f ]"
  (at level 0, format "[ 'fimfun' 'of' f ]").
Notation := (@FImFun.type aT T) : form_scope.
Notation := [the {fimfun _ >-> _} of f] : form_scope.

Lemma { } ( : {fimfun aT >-> rT}) ( : set aT) :
  finite_set (f @` D).

Proof.

by apply: (@sub_finite_set _ _ (range f)) => // y [x]; exists x. Qed.

#[global] Hint Resolve fimfun_inP : core.

Lemma ( : Type) ( : choiceType) ( : set T1)
    ( : {fimfun T1 >-> T2}) ( : T2 -> T3) :
  fset_set [set (g \o f) x | in D] =
  [fset g x | in fset_set [set f x | in D]]%fset.

Proof.

by rewrite -(image_comp f g) fset_set_image. Qed.

Section fimfun_pred.
Context { : Type}.
Definition : {pred aT -> rT} := mem [set | finite_set (range f)].
Definition : pred_key fimfun.

Proof.

exact. Qed.

Canonical := KeyedPred fimfun_key.
End fimfun_pred.

Section fimfun.
Context { : Type}.
Notation := {fimfun aT >-> rT}.
Notation := (@fimfun aT rT).
Section Sub.
Context ( : aT -> rT) ( : f \in fimfun).
Definition := @FiniteImage.Build aT rT f (set_mem fP).
..
Definition := [fimfun of f].
End Sub.

Lemma ( : T -> Type) :
(forall ( : f \in fimfun), K (fimfun_Sub Pf)) -> forall : T, K u.

Proof.

move=> Ksub [f [[Pf]]]/=.
by suff -> : Pf = (set_mem (@mem_set _ [set | _] f Pf)) by apply: Ksub.
Qed.

Lemma ( : f \in fimfun) : fimfun_Sub Pf = f :> (_ -> _).

Proof.

by []. Qed.

..Build _ _ T fimfun_rect fimfun_valP.
End fimfun.

Lemma ( : {fimfun aT >-> rT}) :
f = g <-> f =1 g.

Proof.

by split=> [->//|fg]; apply/val_inj/funext. Qed.

. () :=
  [Equality of {fimfun aT >-> rT} by <:].

. () :=
  [Choice of {fimfun aT >-> rT} by <:].

Lemma { : Type} ( : rT) :
  finite_set (range (cst x : aT -> _)).

Proof.

elim/Ppointed: aT => aT; rewrite ?emptyE ?image_set0//.
suff -> : cst x @` [set: aT] = [set x] by apply: finite_set1.
by apply/predeqP => y; split=> [[t' _ <-]//|->//] /=; exists point.
Qed.