Quelle  EquivClass.thy   Sprache: Isabelle

(*  Title:      ZF/EquivClass.thy
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1994  University of Cambridge
*)

section‹Equivalence Relations›

theory EquivClass imports Trancl Perm begin

definition
  quotient   :: "[i,i]\i"    (infixl ‹'/'/› 90)  (*set of equiv classes*)  where
      "A//r \ {r``{x} . x \ A}"

definition
  congruent  :: "[i,i\i]\o"  where
      "congruent(r,b) \ \y z. \y,z\:r \ b(y)=b(z)"

definition
  congruent2 :: "[i,i,[i,i]\i]\o"  where
      "congruent2(r1,r2,b) \ \y1 z1 y2 z2.
           ⟨y1,z1⟩:r1 ⟶ ⟨y2,z2⟩:r2 ⟶ b(y1,y2) = b(z1,z2)"

abbreviation
  RESPECTS ::"[i\i, i] \ o"  (infixr ‹respects› 80) where
  "f respects r \ congruent(r,f)"

abbreviation
  RESPECTS2 ::"[i\i\i, i] \ o"  (infixr ‹respects2 › 80) where
  "f respects2 r \ congruent2(r,r,f)"
    🍋 ‹Abbreviation for the common case where the relations are identical›


subsection‹Suppes, Theorem 70:
    🍋‹r› is an equiv relation iff 🍋‹converse(r) O r = r››

(** first half: equiv(A,r) \<Longrightarrow> converse(r) O r = r **)

lemma sym_trans_comp_subset:
    "\sym(r); trans(r)\ \ converse(r) O r \ r"
by (unfold trans_def sym_def, blast)

lemma refl_comp_subset:
    "\refl(A,r); r \ A*A\ \ r \ converse(r) O r"
by (unfold refl_def, blast)

lemma equiv_comp_eq:
    "equiv(A,r) \ converse(r) O r = r"
  unfolding equiv_def
apply (blast del: subsetI intro!: sym_trans_comp_subset refl_comp_subset)
done

(*second half*)
lemma comp_equivI:
    "\converse(r) O r = r; domain(r) = A\ \ equiv(A,r)"
  unfolding equiv_def refl_def sym_def trans_def
apply (erule equalityE)
apply (subgoal_tac "\x y. \x,y\ \ r \ \y,x\ \ r", blast+)
done

(** Equivalence classes **)

(*Lemma for the next result*)
lemma equiv_class_subset:
    "\sym(r); trans(r); \a,b\: r\ \ r``{a} \ r``{b}"
by (unfold trans_def sym_def, blast)

lemma equiv_class_eq:
    "\equiv(A,r); \a,b\: r\ \ r``{a} = r``{b}"
  unfolding equiv_def
apply (safe del: subsetI intro!: equalityI equiv_class_subset)
apply (unfold sym_def, blast)
done

lemma equiv_class_self:
    "\equiv(A,r); a \ A\ \ a \ r``{a}"
by (unfold equiv_def refl_def, blast)

(*Lemma for the next result*)
lemma subset_equiv_class:
    "\equiv(A,r); r``{b} \ r``{a}; b \ A\ \ \a,b\: r"
by (unfold equiv_def refl_def, blast)

lemma eq_equiv_class: "\r``{a} = r``{b}; equiv(A,r); b \ A\ \ \a,b\: r"
by (assumption | rule equalityD2 subset_equiv_class)+

(*thus r``{a} = r``{b} as well*)
lemma equiv_class_nondisjoint:
    "\equiv(A,r); x: (r``{a} \ r``{b})\ \ \a,b\: r"
by (unfold equiv_def trans_def sym_def, blast)

lemma equiv_type: "equiv(A,r) \ r \ A*A"
by (unfold equiv_def, blast)

lemma equiv_class_eq_iff:
     "equiv(A,r) \ \x,y\: r \ r``{x} = r``{y} \ x \ A \ y \ A"
by (blast intro: eq_equiv_class equiv_class_eq dest: equiv_type)

lemma eq_equiv_class_iff:
     "\equiv(A,r); x \ A; y \ A\ \ r``{x} = r``{y} \ \x,y\: r"
by (blast intro: eq_equiv_class equiv_class_eq dest: equiv_type)

(*** Quotients ***)

(** Introduction/elimination rules -- needed? **)

lemma quotientI [TC]: "x \ A \ r``{x}: A//r"
  unfolding quotient_def
apply (erule RepFunI)
done

lemma quotientE:
    "\X \ A//r; \x. \X = r``{x}; x \ A\ \ P\ \ P"
by (unfold quotient_def, blast)

lemma Union_quotient:
    "equiv(A,r) \ \(A//r) = A"
by (unfold equiv_def refl_def quotient_def, blast)

lemma quotient_disj:
    "\equiv(A,r); X \ A//r; Y \ A//r\ \ X=Y | (X \ Y \ 0)"
  unfolding quotient_def
apply (safe intro!: equiv_class_eq, assumption)
apply (unfold equiv_def trans_def sym_def, blast)
done

subsection‹Defining Unary Operations upon Equivalence Classes›

(** Could have a locale with the premises equiv(A,r)  and  congruent(r,b)
**)

(*Conversion rule*)
lemma UN_equiv_class:
    "\equiv(A,r); b respects r; a \ A\ \ (\x\r``{a}. b(x)) = b(a)"
apply (subgoal_tac "\x \ r``{a}. b(x) = b(a)")
 apply simp
 apply (blast intro: equiv_class_self)
apply (unfold equiv_def sym_def congruent_def, blast)
done

(*type checking of  @{term"\<Union>x\<in>r``{a}. b(x)"} *)
lemma UN_equiv_class_type:
    "\equiv(A,r); b respects r; X \ A//r; \x. x \ A \ b(x) \ B\
     ==> (∪x∈X. b(x)) ∈ B"
apply (unfold quotient_def, safe)
apply (simp (no_asm_simp) add: UN_equiv_class)
done

(*Sufficient conditions for injectiveness.  Could weaken premises!
  major premise could be an inclusion; bcong could be \<And>y. y \<in> A \<Longrightarrow> b(y):B
*)
lemma UN_equiv_class_inject:
    "\equiv(A,r); b respects r;
        (∪x∈X. b(x))=(∪y∈Y. b(y));  X ∈ A//r;  Y ∈ A//r;
        ∧x y. [x ∈ A; y ∈ A; b(x)=b(y)] ==> ⟨x,y⟩:r]
     ==> X=Y"
apply (unfold quotient_def, safe)
apply (rule equiv_class_eq, assumption)
apply (simp add: UN_equiv_class [of A r b])
done


subsection‹Defining Binary Operations upon Equivalence Classes›

lemma congruent2_implies_congruent:
    "\equiv(A,r1); congruent2(r1,r2,b); a \ A\ \ congruent(r2,b(a))"
by (unfold congruent_def congruent2_def equiv_def refl_def, blast)

lemma congruent2_implies_congruent_UN:
    "\equiv(A1,r1); equiv(A2,r2); congruent2(r1,r2,b); a \ A2\ \
     congruent(r1, λx1. ∪x2 ∈ r2``{a}. b(x1,x2))"
apply (unfold congruent_def, safe)
apply (frule equiv_type [THEN subsetD], assumption)
apply clarify
apply (simp add: UN_equiv_class congruent2_implies_congruent)
apply (unfold congruent2_def equiv_def refl_def, blast)
done

lemma UN_equiv_class2:
    "\equiv(A1,r1); equiv(A2,r2); congruent2(r1,r2,b); a1: A1; a2: A2\
     ==> (∪x1 ∈ r1``{a1}. ∪x2 ∈ r2``{a2}. b(x1,x2)) = b(a1,a2)"
by (simp add: UN_equiv_class congruent2_implies_congruent
              congruent2_implies_congruent_UN)

(*type checking*)
lemma UN_equiv_class_type2:
    "\equiv(A,r); b respects2 r;
        X1: A//r;  X2: A//r;
        ∧x1 x2.  [x1: A; x2: A] ==> b(x1,x2) ∈ B
] ==> (∪x1∈X1. ∪x2∈X2. b(x1,x2)) ∈ B"
apply (unfold quotient_def, safe)
apply (blast intro: UN_equiv_class_type congruent2_implies_congruent_UN
                    congruent2_implies_congruent quotientI)
done


(*Suggested by John Harrison -- the two subproofs may be MUCH simpler
  than the direct proof*)
lemma congruent2I:
    "\equiv(A1,r1); equiv(A2,r2);
        ∧y z w. [w ∈ A2;  ⟨y,z⟩ ∈ r1] ==> b(y,w) = b(z,w);
        ∧y z w. [w ∈ A1;  ⟨y,z⟩ ∈ r2] ==> b(w,y) = b(w,z)
] ==> congruent2(r1,r2,b)"
apply (unfold congruent2_def equiv_def refl_def, safe)
apply (blast intro: trans)
done

lemma congruent2_commuteI:
 assumes equivA: "equiv(A,r)"
     and commute: "\y z. \y \ A; z \ A\ \ b(y,z) = b(z,y)"
     and congt:   "\y z w. \w \ A; \y,z\: r\ \ b(w,y) = b(w,z)"
 shows "b respects2 r"
apply (insert equivA [THEN equiv_type, THEN subsetD])
apply (rule congruent2I [OF equivA equivA])
apply (rule commute [THEN trans])
apply (rule_tac [3] commute [THEN trans, symmetric])
apply (rule_tac [5] sym)
apply (blast intro: congt)+
done

(*Obsolete?*)
lemma congruent_commuteI:
    "\equiv(A,r); Z \ A//r;
        ∧w. [w ∈ A] ==> congruent(r, λz. b(w,z));
        ∧x y. [x ∈ A;  y ∈ A] ==> b(y,x) = b(x,y)
] ==> congruent(r, λw. ∪z∈Z. b(w,z))"
apply (simp (no_asm) add: congruent_def)
apply (safe elim!: quotientE)
apply (frule equiv_type [THEN subsetD], assumption)
apply (simp add: UN_equiv_class [of A r])
apply (simp add: congruent_def)
done

end