Theory HH

(*  Title:      ZF/AC/HH.thy
    Author:     Krzysztof Grabczewski

Some properties of the recursive definition of HH used in the proofs of
  AC17 ⟹ AC1
  AC1 ⟹ WO2
  AC15 ⟹ WO6
*)

theory HH
imports AC_Equiv Hartog
begin

definition
  HH :: "[i, i, i] ⇒ i"  where
    "HH(f,x,a) ≡ transrec(a, λb r. let z = x - (⋃c ∈ b. r`c)
                                    in  if f`z ∈ Pow(z)-{0} then f`z else {x})"

subsection‹Lemmas useful in each of the three proofs›

lemma HH_def_satisfies_eq:
     "HH(f,x,a) = (let z = x - (⋃b ∈ a. HH(f,x,b))   
                   in  if f`z ∈ Pow(z)-{0} then f`z else {x})"
by (rule HH_def [THEN def_transrec, THEN trans], simp)

lemma HH_values: "HH(f,x,a) ∈ Pow(x)-{0} | HH(f,x,a)={x}"
apply (rule HH_def_satisfies_eq [THEN ssubst])
apply (simp add: Let_def Diff_subset [THEN PowI], fast)
done

lemma subset_imp_Diff_eq:
     "B ⊆ A ⟹ X-(⋃a ∈ A. P(a)) = X-(⋃a ∈ A-B. P(a))-(⋃b ∈ B. P(b))"
by fast

lemma Ord_DiffE: "⟦c ∈ a-b; b<a⟧ ⟹ c=b | b<c ∧ c<a"
apply (erule ltE)
apply (drule Ord_linear [of _ c])
apply (fast elim: Ord_in_Ord)
apply (fast intro!: ltI intro: Ord_in_Ord)
done

lemma Diff_UN_eq_self: "(⋀y. y∈A ⟹ P(y) = {x}) ⟹ x - (⋃y ∈ A. P(y)) = x" 
by (simp, fast elim!: mem_irrefl)

lemma HH_eq: "x - (⋃b ∈ a. HH(f,x,b)) = x - (⋃b ∈ a1. HH(f,x,b))   
              ⟹ HH(f,x,a) = HH(f,x,a1)"
apply (subst HH_def_satisfies_eq [of _ _ a1]) 
apply (rule HH_def_satisfies_eq [THEN trans], simp) 
done

lemma HH_is_x_gt_too: "⟦HH(f,x,b)={x}; b<a⟧ ⟹ HH(f,x,a)={x}"
apply (rule_tac P = "b<a" in impE)
prefer 2 apply assumption+
apply (erule lt_Ord2 [THEN trans_induct])
apply (rule impI)
apply (rule HH_eq [THEN trans])
prefer 2 apply assumption+
apply (rule leI [THEN le_imp_subset, THEN subset_imp_Diff_eq, THEN ssubst], 
       assumption)
apply (rule_tac t = "λz. z-X" for X in subst_context)
apply (rule Diff_UN_eq_self)
apply (drule Ord_DiffE, assumption) 
apply (fast elim: ltE, auto) 
done

lemma HH_subset_x_lt_too:
     "⟦HH(f,x,a) ∈ Pow(x)-{0}; b<a⟧ ⟹ HH(f,x,b) ∈ Pow(x)-{0}"
apply (rule HH_values [THEN disjE], assumption)
apply (drule HH_is_x_gt_too, assumption)
apply (drule subst, assumption)
apply (fast elim!: mem_irrefl)
done

lemma HH_subset_x_imp_subset_Diff_UN:
    "HH(f,x,a) ∈ Pow(x)-{0} ⟹ HH(f,x,a) ∈ Pow(x - (⋃b ∈ a. HH(f,x,b)))-{0}"
apply (drule HH_def_satisfies_eq [THEN subst])
apply (rule HH_def_satisfies_eq [THEN ssubst])
apply (simp add: Let_def Diff_subset [THEN PowI])
apply (drule split_if [THEN iffD1])
apply (fast elim!: mem_irrefl)
done

lemma HH_eq_arg_lt:
     "⟦HH(f,x,v)=HH(f,x,w); HH(f,x,v) ∈ Pow(x)-{0}; v ∈ w⟧ ⟹ P"
apply (frule_tac P = "λy. y ∈ Pow (x) -{0}" in subst, assumption)
apply (drule_tac a = w in HH_subset_x_imp_subset_Diff_UN)
apply (drule subst_elem, assumption)
apply (fast intro!: singleton_iff [THEN iffD2] equals0I)
done

lemma HH_eq_imp_arg_eq:
  "⟦HH(f,x,v)=HH(f,x,w); HH(f,x,w) ∈ Pow(x)-{0}; Ord(v); Ord(w)⟧ ⟹ v=w"
apply (rule_tac j = w in Ord_linear_lt)
apply (simp_all (no_asm_simp))
 apply (drule subst_elem, assumption) 
 apply (blast dest: ltD HH_eq_arg_lt)
apply (blast dest: HH_eq_arg_lt [OF sym] ltD) 
done

lemma HH_subset_x_imp_lepoll: 
     "⟦HH(f, x, i) ∈ Pow(x)-{0}; Ord(i)⟧ ⟹ i ≲ Pow(x)-{0}"
  unfolding lepoll_def inj_def
apply (rule_tac x = "λj ∈ i. HH (f, x, j) " in exI)
apply (simp (no_asm_simp))
apply (fast del: DiffE
            elim!: HH_eq_imp_arg_eq Ord_in_Ord HH_subset_x_lt_too 
            intro!: lam_type ballI ltI intro: bexI)
done

lemma HH_Hartog_is_x: "HH(f, x, Hartog(Pow(x)-{0})) = {x}"
apply (rule HH_values [THEN disjE])
prefer 2 apply assumption 
apply (fast del: DiffE
            intro!: Ord_Hartog 
            dest!: HH_subset_x_imp_lepoll 
            elim!: Hartog_lepoll_selfE)
done

lemma HH_Least_eq_x: "HH(f, x, μ i. HH(f, x, i) = {x}) = {x}"
by (fast intro!: Ord_Hartog HH_Hartog_is_x LeastI)

lemma less_Least_subset_x:
     "a ∈ (μ i. HH(f,x,i)={x}) ⟹ HH(f,x,a) ∈ Pow(x)-{0}"
apply (rule HH_values [THEN disjE], assumption)
apply (rule less_LeastE)
apply (erule_tac [2] ltI [OF _ Ord_Least], assumption)
done

subsection‹Lemmas used in the proofs of @{term "AC1 ⟹ WO2"} and @{term "AC17 ⟹ AC1"}›

lemma lam_Least_HH_inj_Pow: 
        "(λa ∈ (μ i. HH(f,x,i)={x}). HH(f,x,a))   
         ∈ inj(μ i. HH(f,x,i)={x}, Pow(x)-{0})"
apply (unfold inj_def, simp)
apply (fast intro!: lam_type dest: less_Least_subset_x 
            elim!: HH_eq_imp_arg_eq Ord_Least [THEN Ord_in_Ord])
done

lemma lam_Least_HH_inj:
     "∀a ∈ (μ i. HH(f,x,i)={x}). ∃z ∈ x. HH(f,x,a) = {z}   
      ⟹ (λa ∈ (μ i. HH(f,x,i)={x}). HH(f,x,a))   
          ∈ inj(μ i. HH(f,x,i)={x}, {{y}. y ∈ x})"
by (rule lam_Least_HH_inj_Pow [THEN inj_strengthen_type], simp)

lemma lam_surj_sing: 
        "⟦x - (⋃a ∈ A. F(a)) = 0;  ∀a ∈ A. ∃z ∈ x. F(a) = {z}⟧   
         ⟹ (λa ∈ A. F(a)) ∈ surj(A, {{y}. y ∈ x})"
apply (simp add: surj_def lam_type Diff_eq_0_iff)
apply (blast elim: equalityE) 
done

lemma not_emptyI2: "y ∈ Pow(x)-{0} ⟹ x ≠ 0"
by auto

lemma f_subset_imp_HH_subset:
     "f`(x - (⋃j ∈ i. HH(f,x,j))) ∈ Pow(x - (⋃j ∈ i. HH(f,x,j)))-{0}   
      ⟹ HH(f, x, i) ∈ Pow(x) - {0}"
apply (rule HH_def_satisfies_eq [THEN ssubst])
apply (simp add: Let_def Diff_subset [THEN PowI] not_emptyI2 [THEN if_P], fast)
done

lemma f_subsets_imp_UN_HH_eq_x:
     "∀z ∈ Pow(x)-{0}. f`z ∈ Pow(z)-{0}
      ⟹ x - (⋃j ∈ (μ i. HH(f,x,i)={x}). HH(f,x,j)) = 0"
apply (case_tac "P ∈ {0}" for P, fast)
apply (drule Diff_subset [THEN PowI, THEN DiffI])
apply (drule bspec, assumption) 
apply (drule f_subset_imp_HH_subset) 
apply (blast dest!: subst_elem [OF _ HH_Least_eq_x [symmetric]] 
             elim!: mem_irrefl)
done

lemma HH_values2: "HH(f,x,i) = f`(x - (⋃j ∈ i. HH(f,x,j))) | HH(f,x,i)={x}"
apply (rule HH_def_satisfies_eq [THEN ssubst])
apply (simp add: Let_def Diff_subset [THEN PowI])
done

lemma HH_subset_imp_eq:
     "HH(f,x,i): Pow(x)-{0} ⟹ HH(f,x,i)=f`(x - (⋃j ∈ i. HH(f,x,j)))"
apply (rule HH_values2 [THEN disjE], assumption)
apply (fast elim!: equalityE mem_irrefl dest!: singleton_subsetD)
done

lemma f_sing_imp_HH_sing:
     "⟦f ∈ (Pow(x)-{0}) -> {{z}. z ∈ x};   
         a ∈ (μ i. HH(f,x,i)={x})⟧ ⟹ ∃z ∈ x. HH(f,x,a) = {z}"
apply (drule less_Least_subset_x)
apply (frule HH_subset_imp_eq)
apply (drule apply_type)
apply (rule Diff_subset [THEN PowI, THEN DiffI])
apply (fast dest!: HH_subset_x_imp_subset_Diff_UN [THEN not_emptyI2], force) 
done

lemma f_sing_lam_bij: 
     "⟦x - (⋃j ∈ (μ i. HH(f,x,i)={x}). HH(f,x,j)) = 0;   
         f ∈ (Pow(x)-{0}) -> {{z}. z ∈ x}⟧   
      ⟹ (λa ∈ (μ i. HH(f,x,i)={x}). HH(f,x,a))   
          ∈ bij(μ i. HH(f,x,i)={x}, {{y}. y ∈ x})"
  unfolding bij_def
apply (fast intro!: lam_Least_HH_inj lam_surj_sing f_sing_imp_HH_sing)
done

lemma lam_singI:
     "f ∈ (∏X ∈ Pow(x)-{0}. F(X))   
      ⟹ (λX ∈ Pow(x)-{0}. {f`X}) ∈ (∏X ∈ Pow(x)-{0}. {{z}. z ∈ F(X)})"
by (fast del: DiffI DiffE
            intro!: lam_type singleton_eq_iff [THEN iffD2] dest: apply_type)

(*FIXME: both uses have the form ...[THEN bij_converse_bij], so 
  simplification is needed!*)
lemmas bij_Least_HH_x =  
    comp_bij [OF f_sing_lam_bij [OF _ lam_singI] 
              lam_sing_bij [THEN bij_converse_bij]]


subsection‹The proof of @{term "AC1 ⟹ WO2"}›

(*Establishing the existence of a bijection, namely
converse
 (converse(λx∈x. {x}) O
  Lambda
   (μ i. HH(λX∈Pow(x) - {0}. {f ` X}, x, i) = {x},
    HH(λX∈Pow(x) - {0}. {f ` X}, x)))
Perhaps it could be simplified. *)

lemma bijection:
     "f ∈ (∏X ∈ Pow(x) - {0}. X) 
      ⟹ ∃g. g ∈ bij(x, μ i. HH(λX ∈ Pow(x)-{0}. {f`X}, x, i) = {x})"
apply (rule exI) 
apply (rule bij_Least_HH_x [THEN bij_converse_bij])
apply (rule f_subsets_imp_UN_HH_eq_x)
apply (intro ballI apply_type) 
apply (fast intro: lam_type apply_type del: DiffE, assumption) 
apply (fast intro: Pi_weaken_type)
done

lemma AC1_WO2: "AC1 ⟹ WO2"
  unfolding AC1_def WO2_def eqpoll_def
  apply (intro allI) 
  apply (drule_tac x = "Pow(A) - {0}" in spec) 
  apply (blast dest: bijection)
  done

end