src/HOL/Order_Relation.thy

tuned;

(*  Title:      HOL/Order_Relation.thy
    Author:     Tobias Nipkow
    Author:     Andrei Popescu, TU Muenchen
*)

section \<open>Orders as Relations\<close>

theory Order_Relation
imports Wfrec
begin

subsection\<open>Orders on a set\<close>

definition "preorder_on A r \<equiv> refl_on A r \<and> trans r"

definition "partial_order_on A r \<equiv> preorder_on A r \<and> antisym r"

definition "linear_order_on A r \<equiv> partial_order_on A r \<and> total_on A r"

definition "strict_linear_order_on A r \<equiv> trans r \<and> irrefl r \<and> total_on A r"

definition "well_order_on A r \<equiv> linear_order_on A r \<and> wf(r - Id)"

lemmas order_on_defs =
  preorder_on_def partial_order_on_def linear_order_on_def
  strict_linear_order_on_def well_order_on_def


lemma preorder_on_empty[simp]: "preorder_on {} {}"
by(simp add:preorder_on_def trans_def)

lemma partial_order_on_empty[simp]: "partial_order_on {} {}"
by(simp add:partial_order_on_def)

lemma lnear_order_on_empty[simp]: "linear_order_on {} {}"
by(simp add:linear_order_on_def)

lemma well_order_on_empty[simp]: "well_order_on {} {}"
by(simp add:well_order_on_def)


lemma preorder_on_converse[simp]: "preorder_on A (r^-1) = preorder_on A r"
by (simp add:preorder_on_def)

lemma partial_order_on_converse[simp]:
  "partial_order_on A (r^-1) = partial_order_on A r"
by (simp add: partial_order_on_def)

lemma linear_order_on_converse[simp]:
  "linear_order_on A (r^-1) = linear_order_on A r"
by (simp add: linear_order_on_def)


lemma strict_linear_order_on_diff_Id:
  "linear_order_on A r \<Longrightarrow> strict_linear_order_on A (r-Id)"
by(simp add: order_on_defs trans_diff_Id)


subsection\<open>Orders on the field\<close>

abbreviation "Refl r \<equiv> refl_on (Field r) r"

abbreviation "Preorder r \<equiv> preorder_on (Field r) r"

abbreviation "Partial_order r \<equiv> partial_order_on (Field r) r"

abbreviation "Total r \<equiv> total_on (Field r) r"

abbreviation "Linear_order r \<equiv> linear_order_on (Field r) r"

abbreviation "Well_order r \<equiv> well_order_on (Field r) r"


lemma subset_Image_Image_iff:
  "\<lbrakk> Preorder r; A \<subseteq> Field r; B \<subseteq> Field r\<rbrakk> \<Longrightarrow>
   r `` A \<subseteq> r `` B \<longleftrightarrow> (\<forall>a\<in>A.\<exists>b\<in>B. (b,a):r)"
unfolding preorder_on_def refl_on_def Image_def
apply (simp add: subset_eq)
unfolding trans_def by fast

lemma subset_Image1_Image1_iff:
  "\<lbrakk> Preorder r; a : Field r; b : Field r\<rbrakk> \<Longrightarrow> r `` {a} \<subseteq> r `` {b} \<longleftrightarrow> (b,a):r"
by(simp add:subset_Image_Image_iff)

lemma Refl_antisym_eq_Image1_Image1_iff:
  assumes r: "Refl r" and as: "antisym r" and abf: "a \<in> Field r" "b \<in> Field r"
  shows "r `` {a} = r `` {b} \<longleftrightarrow> a = b"
proof
  assume "r `` {a} = r `` {b}"
  hence e: "\<And>x. (a, x) \<in> r \<longleftrightarrow> (b, x) \<in> r" by (simp add: set_eq_iff)
  have "(a, a) \<in> r" "(b, b) \<in> r" using r abf by (simp_all add: refl_on_def)
  hence "(a, b) \<in> r" "(b, a) \<in> r" using e[of a] e[of b] by simp_all
  thus "a = b" using as[unfolded antisym_def] by blast
qed fast

lemma Partial_order_eq_Image1_Image1_iff:
  "\<lbrakk>Partial_order r; a:Field r; b:Field r\<rbrakk> \<Longrightarrow> r `` {a} = r `` {b} \<longleftrightarrow> a=b"
by(auto simp:order_on_defs Refl_antisym_eq_Image1_Image1_iff)

lemma Total_Id_Field:
assumes TOT: "Total r" and NID: "\<not> (r <= Id)"
shows "Field r = Field(r - Id)"
using mono_Field[of "r - Id" r] Diff_subset[of r Id]
proof(auto)
  have "r \<noteq> {}" using NID by fast
  then obtain b and c where "b \<noteq> c \<and> (b,c) \<in> r" using NID by auto
  hence 1: "b \<noteq> c \<and> {b,c} \<le> Field r" by (auto simp: Field_def)

  fix a assume *: "a \<in> Field r"
  obtain d where 2: "d \<in> Field r" and 3: "d \<noteq> a"
  using * 1 by auto
  hence "(a,d) \<in> r \<or> (d,a) \<in> r" using * TOT
  by (simp add: total_on_def)
  thus "a \<in> Field(r - Id)" using 3 unfolding Field_def by blast
qed


subsection\<open>Orders on a type\<close>

abbreviation "strict_linear_order \<equiv> strict_linear_order_on UNIV"

abbreviation "linear_order \<equiv> linear_order_on UNIV"

abbreviation "well_order \<equiv> well_order_on UNIV"


subsection \<open>Order-like relations\<close>

text\<open>In this subsection, we develop basic concepts and results pertaining
to order-like relations, i.e., to reflexive and/or transitive and/or symmetric and/or
total relations. We also further define upper and lower bounds operators.\<close>


subsubsection \<open>Auxiliaries\<close>

lemma refl_on_domain:
"\<lbrakk>refl_on A r; (a,b) : r\<rbrakk> \<Longrightarrow> a \<in> A \<and> b \<in> A"
by(auto simp add: refl_on_def)

corollary well_order_on_domain:
"\<lbrakk>well_order_on A r; (a,b) \<in> r\<rbrakk> \<Longrightarrow> a \<in> A \<and> b \<in> A"
by (auto simp add: refl_on_domain order_on_defs)

lemma well_order_on_Field:
"well_order_on A r \<Longrightarrow> A = Field r"
by(auto simp add: refl_on_def Field_def order_on_defs)

lemma well_order_on_Well_order:
"well_order_on A r \<Longrightarrow> A = Field r \<and> Well_order r"
using well_order_on_Field by auto

lemma Total_subset_Id:
assumes TOT: "Total r" and SUB: "r \<le> Id"
shows "r = {} \<or> (\<exists>a. r = {(a,a)})"
proof-
  {assume "r \<noteq> {}"
   then obtain a b where 1: "(a,b) \<in> r" by fast
   hence "a = b" using SUB by blast
   hence 2: "(a,a) \<in> r" using 1 by simp
   {fix c d assume "(c,d) \<in> r"
    hence "{a,c,d} \<le> Field r" using 1 unfolding Field_def by blast
    hence "((a,c) \<in> r \<or> (c,a) \<in> r \<or> a = c) \<and>
           ((a,d) \<in> r \<or> (d,a) \<in> r \<or> a = d)"
    using TOT unfolding total_on_def by blast
    hence "a = c \<and> a = d" using SUB by blast
   }
   hence "r \<le> {(a,a)}" by auto
   with 2 have "\<exists>a. r = {(a,a)}" by blast
  }
  thus ?thesis by blast
qed

lemma Linear_order_in_diff_Id:
assumes LI: "Linear_order r" and
        IN1: "a \<in> Field r" and IN2: "b \<in> Field r"
shows "((a,b) \<in> r) = ((b,a) \<notin> r - Id)"
using assms unfolding order_on_defs total_on_def antisym_def Id_def refl_on_def by force


subsubsection \<open>The upper and lower bounds operators\<close>

text\<open>Here we define upper (``above") and lower (``below") bounds operators.
We think of @{text "r"} as a {\em non-strict} relation.  The suffix ``S"
at the names of some operators indicates that the bounds are strict -- e.g.,
@{text "underS a"} is the set of all strict lower bounds of @{text "a"} (w.r.t. @{text "r"}).
Capitalization of the first letter in the name reminds that the operator acts on sets, rather
than on individual elements.\<close>

definition under::"'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "under r a \<equiv> {b. (b,a) \<in> r}"

definition underS::"'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "underS r a \<equiv> {b. b \<noteq> a \<and> (b,a) \<in> r}"

definition Under::"'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "Under r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. (b,a) \<in> r}"

definition UnderS::"'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "UnderS r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. b \<noteq> a \<and> (b,a) \<in> r}"

definition above::"'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "above r a \<equiv> {b. (a,b) \<in> r}"

definition aboveS::"'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "aboveS r a \<equiv> {b. b \<noteq> a \<and> (a,b) \<in> r}"

definition Above::"'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "Above r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. (a,b) \<in> r}"

definition AboveS::"'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "AboveS r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. b \<noteq> a \<and> (a,b) \<in> r}"

definition ofilter :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool"
where "ofilter r A \<equiv> (A \<le> Field r) \<and> (\<forall>a \<in> A. under r a \<le> A)"

text\<open>Note:  In the definitions of @{text "Above[S]"} and @{text "Under[S]"},
  we bounded comprehension by @{text "Field r"} in order to properly cover
  the case of @{text "A"} being empty.\<close>

lemma underS_subset_under: "underS r a \<le> under r a"
by(auto simp add: underS_def under_def)

lemma underS_notIn: "a \<notin> underS r a"
by(simp add: underS_def)

lemma Refl_under_in: "\<lbrakk>Refl r; a \<in> Field r\<rbrakk> \<Longrightarrow> a \<in> under r a"
by(simp add: refl_on_def under_def)

lemma AboveS_disjoint: "A Int (AboveS r A) = {}"
by(auto simp add: AboveS_def)

lemma in_AboveS_underS: "a \<in> Field r \<Longrightarrow> a \<in> AboveS r (underS r a)"
by(auto simp add: AboveS_def underS_def)

lemma Refl_under_underS:
  assumes "Refl r" "a \<in> Field r"
  shows "under r a = underS r a \<union> {a}"
unfolding under_def underS_def
using assms refl_on_def[of _ r] by fastforce

lemma underS_empty: "a \<notin> Field r \<Longrightarrow> underS r a = {}"
by (auto simp: Field_def underS_def)

lemma under_Field: "under r a \<le> Field r"
by(unfold under_def Field_def, auto)

lemma underS_Field: "underS r a \<le> Field r"
by(unfold underS_def Field_def, auto)

lemma underS_Field2:
"a \<in> Field r \<Longrightarrow> underS r a < Field r"
using underS_notIn underS_Field by fast

lemma underS_Field3:
"Field r \<noteq> {} \<Longrightarrow> underS r a < Field r"
by(cases "a \<in> Field r", simp add: underS_Field2, auto simp add: underS_empty)

lemma AboveS_Field: "AboveS r A \<le> Field r"
by(unfold AboveS_def Field_def, auto)

lemma under_incr:
  assumes TRANS: "trans r" and REL: "(a,b) \<in> r"
  shows "under r a \<le> under r b"
proof(unfold under_def, auto)
  fix x assume "(x,a) \<in> r"
  with REL TRANS trans_def[of r]
  show "(x,b) \<in> r" by blast
qed

lemma underS_incr:
assumes TRANS: "trans r" and ANTISYM: "antisym r" and
        REL: "(a,b) \<in> r"
shows "underS r a \<le> underS r b"
proof(unfold underS_def, auto)
  assume *: "b \<noteq> a" and **: "(b,a) \<in> r"
  with ANTISYM antisym_def[of r] REL
  show False by blast
next
  fix x assume "x \<noteq> a" "(x,a) \<in> r"
  with REL TRANS trans_def[of r]
  show "(x,b) \<in> r" by blast
qed

lemma underS_incl_iff:
assumes LO: "Linear_order r" and
        INa: "a \<in> Field r" and INb: "b \<in> Field r"
shows "(underS r a \<le> underS r b) = ((a,b) \<in> r)"
proof
  assume "(a,b) \<in> r"
  thus "underS r a \<le> underS r b" using LO
  by (simp add: order_on_defs underS_incr)
next
  assume *: "underS r a \<le> underS r b"
  {assume "a = b"
   hence "(a,b) \<in> r" using assms
   by (simp add: order_on_defs refl_on_def)
  }
  moreover
  {assume "a \<noteq> b \<and> (b,a) \<in> r"
   hence "b \<in> underS r a" unfolding underS_def by blast
   hence "b \<in> underS r b" using * by blast
   hence False by (simp add: underS_notIn)
  }
  ultimately
  show "(a,b) \<in> r" using assms
  order_on_defs[of "Field r" r] total_on_def[of "Field r" r] by blast
qed


subsection \<open>Variations on Well-Founded Relations\<close>

text \<open>
This subsection contains some variations of the results from @{theory Wellfounded}:
\begin{itemize}
\item means for slightly more direct definitions by well-founded recursion;
\item variations of well-founded induction;
\item means for proving a linear order to be a well-order.
\end{itemize}
\<close>


subsubsection \<open>Characterizations of well-foundedness\<close>

text \<open>A transitive relation is well-founded iff it is ``locally'' well-founded,
i.e., iff its restriction to the lower bounds of of any element is well-founded.\<close>

lemma trans_wf_iff:
assumes "trans r"
shows "wf r = (\<forall>a. wf(r Int (r^-1``{a} \<times> r^-1``{a})))"
proof-
  obtain R where R_def: "R = (\<lambda> a. r Int (r^-1``{a} \<times> r^-1``{a}))" by blast
  {assume *: "wf r"
   {fix a
    have "wf(R a)"
    using * R_def wf_subset[of r "R a"] by auto
   }
  }
  (*  *)
  moreover
  {assume *: "\<forall>a. wf(R a)"
   have "wf r"
   proof(unfold wf_def, clarify)
     fix phi a
     assume **: "\<forall>a. (\<forall>b. (b,a) \<in> r \<longrightarrow> phi b) \<longrightarrow> phi a"
     obtain chi where chi_def: "chi = (\<lambda>b. (b,a) \<in> r \<longrightarrow> phi b)" by blast
     with * have "wf (R a)" by auto
     hence "(\<forall>b. (\<forall>c. (c,b) \<in> R a \<longrightarrow> chi c) \<longrightarrow> chi b) \<longrightarrow> (\<forall>b. chi b)"
     unfolding wf_def by blast
     moreover
     have "\<forall>b. (\<forall>c. (c,b) \<in> R a \<longrightarrow> chi c) \<longrightarrow> chi b"
     proof(auto simp add: chi_def R_def)
       fix b
       assume 1: "(b,a) \<in> r" and 2: "\<forall>c. (c, b) \<in> r \<and> (c, a) \<in> r \<longrightarrow> phi c"
       hence "\<forall>c. (c, b) \<in> r \<longrightarrow> phi c"
       using assms trans_def[of r] by blast
       thus "phi b" using ** by blast
     qed
     ultimately have  "\<forall>b. chi b" by (rule mp)
     with ** chi_def show "phi a" by blast
   qed
  }
  ultimately show ?thesis using R_def by blast
qed

text \<open>The next lemma is a variation of @{text "wf_eq_minimal"} from Wellfounded,
allowing one to assume the set included in the field.\<close>

lemma wf_eq_minimal2:
"wf r = (\<forall>A. A <= Field r \<and> A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. \<not> (a',a) \<in> r))"
proof-
  let ?phi = "\<lambda> A. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. \<not> (a',a) \<in> r)"
  have "wf r = (\<forall>A. ?phi A)"
  by (auto simp: ex_in_conv [THEN sym], erule wfE_min, assumption, blast)
     (rule wfI_min, fast)
  (*  *)
  also have "(\<forall>A. ?phi A) = (\<forall>B \<le> Field r. ?phi B)"
  proof
    assume "\<forall>A. ?phi A"
    thus "\<forall>B \<le> Field r. ?phi B" by simp
  next
    assume *: "\<forall>B \<le> Field r. ?phi B"
    show "\<forall>A. ?phi A"
    proof(clarify)
      fix A::"'a set" assume **: "A \<noteq> {}"
      obtain B where B_def: "B = A Int (Field r)" by blast
      show "\<exists>a \<in> A. \<forall>a' \<in> A. (a',a) \<notin> r"
      proof(cases "B = {}")
        assume Case1: "B = {}"
        obtain a where 1: "a \<in> A \<and> a \<notin> Field r"
        using ** Case1 unfolding B_def by blast
        hence "\<forall>a' \<in> A. (a',a) \<notin> r" using 1 unfolding Field_def by blast
        thus ?thesis using 1 by blast
      next
        assume Case2: "B \<noteq> {}" have 1: "B \<le> Field r" unfolding B_def by blast
        obtain a where 2: "a \<in> B \<and> (\<forall>a' \<in> B. (a',a) \<notin> r)"
        using Case2 1 * by blast
        have "\<forall>a' \<in> A. (a',a) \<notin> r"
        proof(clarify)
          fix a' assume "a' \<in> A" and **: "(a',a) \<in> r"
          hence "a' \<in> B" unfolding B_def Field_def by blast
          thus False using 2 ** by blast
        qed
        thus ?thesis using 2 unfolding B_def by blast
      qed
    qed
  qed
  finally show ?thesis by blast
qed


subsubsection \<open>Characterizations of well-foundedness\<close>

text \<open>The next lemma and its corollary enable one to prove that
a linear order is a well-order in a way which is more standard than
via well-foundedness of the strict version of the relation.\<close>

lemma Linear_order_wf_diff_Id:
assumes LI: "Linear_order r"
shows "wf(r - Id) = (\<forall>A \<le> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r))"
proof(cases "r \<le> Id")
  assume Case1: "r \<le> Id"
  hence temp: "r - Id = {}" by blast
  hence "wf(r - Id)" by (simp add: temp)
  moreover
  {fix A assume *: "A \<le> Field r" and **: "A \<noteq> {}"
   obtain a where 1: "r = {} \<or> r = {(a,a)}" using LI
   unfolding order_on_defs using Case1 Total_subset_Id by auto
   hence "A = {a} \<and> r = {(a,a)}" using * ** unfolding Field_def by blast
   hence "\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r" using 1 by blast
  }
  ultimately show ?thesis by blast
next
  assume Case2: "\<not> r \<le> Id"
  hence 1: "Field r = Field(r - Id)" using Total_Id_Field LI
  unfolding order_on_defs by blast
  show ?thesis
  proof
    assume *: "wf(r - Id)"
    show "\<forall>A \<le> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r)"
    proof(clarify)
      fix A assume **: "A \<le> Field r" and ***: "A \<noteq> {}"
      hence "\<exists>a \<in> A. \<forall>a' \<in> A. (a',a) \<notin> r - Id"
      using 1 * unfolding wf_eq_minimal2 by simp
      moreover have "\<forall>a \<in> A. \<forall>a' \<in> A. ((a,a') \<in> r) = ((a',a) \<notin> r - Id)"
      using Linear_order_in_diff_Id[of r] ** LI by blast
      ultimately show "\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r" by blast
    qed
  next
    assume *: "\<forall>A \<le> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r)"
    show "wf(r - Id)"
    proof(unfold wf_eq_minimal2, clarify)
      fix A assume **: "A \<le> Field(r - Id)" and ***: "A \<noteq> {}"
      hence "\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r"
      using 1 * by simp
      moreover have "\<forall>a \<in> A. \<forall>a' \<in> A. ((a,a') \<in> r) = ((a',a) \<notin> r - Id)"
      using Linear_order_in_diff_Id[of r] ** LI mono_Field[of "r - Id" r] by blast
      ultimately show "\<exists>a \<in> A. \<forall>a' \<in> A. (a',a) \<notin> r - Id" by blast
    qed
  qed
qed

corollary Linear_order_Well_order_iff:
assumes "Linear_order r"
shows "Well_order r = (\<forall>A \<le> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r))"
using assms unfolding well_order_on_def using Linear_order_wf_diff_Id[of r] by blast

end