src/HOL/Library/Disjoint_Sets.thy

Library: add partition_on

(*  Title:      HOL/Library/Disjoint_Sets.thy
    Author:     Johannes Hölzl, TU München
*)

section \<open>Partitions and Disjoint Sets\<close>

theory Disjoint_Sets
  imports Main
begin

lemma range_subsetD: "range f \<subseteq> B \<Longrightarrow> f i \<in> B"
  by blast

lemma Int_Diff_disjoint: "A \<inter> B \<inter> (A - B) = {}"
  by blast

lemma Int_Diff_Un: "A \<inter> B \<union> (A - B) = A"
  by blast

lemma mono_Un: "mono A \<Longrightarrow> (\<Union>i\<le>n. A i) = A n"
  unfolding mono_def by auto

lemma disjnt_equiv_class: "equiv A r \<Longrightarrow> disjnt (r``{a}) (r``{b}) \<longleftrightarrow> (a, b) \<notin> r"
  by (auto dest: equiv_class_self simp: equiv_class_eq_iff disjnt_def)

subsection \<open>Set of Disjoint Sets\<close>

abbreviation disjoint :: "'a set set \<Rightarrow> bool" where "disjoint \<equiv> pairwise disjnt"

lemma disjoint_def: "disjoint A \<longleftrightarrow> (\<forall>a\<in>A. \<forall>b\<in>A. a \<noteq> b \<longrightarrow> a \<inter> b = {})"
  unfolding pairwise_def disjnt_def by auto

lemma disjointI:
  "(\<And>a b. a \<in> A \<Longrightarrow> b \<in> A \<Longrightarrow> a \<noteq> b \<Longrightarrow> a \<inter> b = {}) \<Longrightarrow> disjoint A"
  unfolding disjoint_def by auto

lemma disjointD:
  "disjoint A \<Longrightarrow> a \<in> A \<Longrightarrow> b \<in> A \<Longrightarrow> a \<noteq> b \<Longrightarrow> a \<inter> b = {}"
  unfolding disjoint_def by auto

lemma disjoint_INT:
  assumes *: "\<And>i. i \<in> I \<Longrightarrow> disjoint (F i)"
  shows "disjoint {\<Inter>i\<in>I. X i | X. \<forall>i\<in>I. X i \<in> F i}"
proof (safe intro!: disjointI del: equalityI)
  fix A B :: "'a \<Rightarrow> 'b set" assume "(\<Inter>i\<in>I. A i) \<noteq> (\<Inter>i\<in>I. B i)"
  then obtain i where "A i \<noteq> B i" "i \<in> I"
    by auto
  moreover assume "\<forall>i\<in>I. A i \<in> F i" "\<forall>i\<in>I. B i \<in> F i"
  ultimately show "(\<Inter>i\<in>I. A i) \<inter> (\<Inter>i\<in>I. B i) = {}"
    using *[OF \<open>i\<in>I\<close>, THEN disjointD, of "A i" "B i"]
    by (auto simp: INT_Int_distrib[symmetric])
qed

subsubsection "Family of Disjoint Sets"

definition disjoint_family_on :: "('i \<Rightarrow> 'a set) \<Rightarrow> 'i set \<Rightarrow> bool" where
  "disjoint_family_on A S \<longleftrightarrow> (\<forall>m\<in>S. \<forall>n\<in>S. m \<noteq> n \<longrightarrow> A m \<inter> A n = {})"

abbreviation "disjoint_family A \<equiv> disjoint_family_on A UNIV"

lemma disjoint_family_onD:
  "disjoint_family_on A I \<Longrightarrow> i \<in> I \<Longrightarrow> j \<in> I \<Longrightarrow> i \<noteq> j \<Longrightarrow> A i \<inter> A j = {}"
  by (auto simp: disjoint_family_on_def)

lemma disjoint_family_subset: "disjoint_family A \<Longrightarrow> (\<And>x. B x \<subseteq> A x) \<Longrightarrow> disjoint_family B"
  by (force simp add: disjoint_family_on_def)

lemma disjoint_family_on_bisimulation:
  assumes "disjoint_family_on f S"
  and "\<And>n m. n \<in> S \<Longrightarrow> m \<in> S \<Longrightarrow> n \<noteq> m \<Longrightarrow> f n \<inter> f m = {} \<Longrightarrow> g n \<inter> g m = {}"
  shows "disjoint_family_on g S"
  using assms unfolding disjoint_family_on_def by auto

lemma disjoint_family_on_mono:
  "A \<subseteq> B \<Longrightarrow> disjoint_family_on f B \<Longrightarrow> disjoint_family_on f A"
  unfolding disjoint_family_on_def by auto

lemma disjoint_family_Suc:
  "(\<And>n. A n \<subseteq> A (Suc n)) \<Longrightarrow> disjoint_family (\<lambda>i. A (Suc i) - A i)"
  using lift_Suc_mono_le[of A]
  by (auto simp add: disjoint_family_on_def)
     (metis insert_absorb insert_subset le_SucE le_antisym not_le_imp_less less_imp_le)

lemma disjoint_family_on_disjoint_image:
  "disjoint_family_on A I \<Longrightarrow> disjoint (A ` I)"
  unfolding disjoint_family_on_def disjoint_def by force

lemma disjoint_family_on_vimageI: "disjoint_family_on F I \<Longrightarrow> disjoint_family_on (\<lambda>i. f -` F i) I"
  by (auto simp: disjoint_family_on_def)

lemma disjoint_image_disjoint_family_on:
  assumes d: "disjoint (A ` I)" and i: "inj_on A I"
  shows "disjoint_family_on A I"
  unfolding disjoint_family_on_def
proof (intro ballI impI)
  fix n m assume nm: "m \<in> I" "n \<in> I" and "n \<noteq> m"
  with i[THEN inj_onD, of n m] show "A n \<inter> A m = {}"
    by (intro disjointD[OF d]) auto
qed

lemma disjoint_UN:
  assumes F: "\<And>i. i \<in> I \<Longrightarrow> disjoint (F i)" and *: "disjoint_family_on (\<lambda>i. \<Union>F i) I"
  shows "disjoint (\<Union>i\<in>I. F i)"
proof (safe intro!: disjointI del: equalityI)
  fix A B i j assume "A \<noteq> B" "A \<in> F i" "i \<in> I" "B \<in> F j" "j \<in> I"
  show "A \<inter> B = {}"
  proof cases
    assume "i = j" with F[of i] \<open>i \<in> I\<close> \<open>A \<in> F i\<close> \<open>B \<in> F j\<close> \<open>A \<noteq> B\<close> show "A \<inter> B = {}"
      by (auto dest: disjointD)
  next
    assume "i \<noteq> j"
    with * \<open>i\<in>I\<close> \<open>j\<in>I\<close> have "(\<Union>F i) \<inter> (\<Union>F j) = {}"
      by (rule disjoint_family_onD)
    with \<open>A\<in>F i\<close> \<open>i\<in>I\<close> \<open>B\<in>F j\<close> \<open>j\<in>I\<close>
    show "A \<inter> B = {}"
      by auto
  qed
qed

lemma disjoint_union: "disjoint C \<Longrightarrow> disjoint B \<Longrightarrow> \<Union>C \<inter> \<Union>B = {} \<Longrightarrow> disjoint (C \<union> B)"
  using disjoint_UN[of "{C, B}" "\<lambda>x. x"] by (auto simp add: disjoint_family_on_def)

subsection \<open>Construct Disjoint Sequences\<close>

definition disjointed :: "(nat \<Rightarrow> 'a set) \<Rightarrow> nat \<Rightarrow> 'a set" where
  "disjointed A n = A n - (\<Union>i\<in>{0..<n}. A i)"

lemma finite_UN_disjointed_eq: "(\<Union>i\<in>{0..<n}. disjointed A i) = (\<Union>i\<in>{0..<n}. A i)"
proof (induct n)
  case 0 show ?case by simp
next
  case (Suc n)
  thus ?case by (simp add: atLeastLessThanSuc disjointed_def)
qed

lemma UN_disjointed_eq: "(\<Union>i. disjointed A i) = (\<Union>i. A i)"
  by (rule UN_finite2_eq [where k=0])
     (simp add: finite_UN_disjointed_eq)

lemma less_disjoint_disjointed: "m < n \<Longrightarrow> disjointed A m \<inter> disjointed A n = {}"
  by (auto simp add: disjointed_def)

lemma disjoint_family_disjointed: "disjoint_family (disjointed A)"
  by (simp add: disjoint_family_on_def)
     (metis neq_iff Int_commute less_disjoint_disjointed)

lemma disjointed_subset: "disjointed A n \<subseteq> A n"
  by (auto simp add: disjointed_def)

lemma disjointed_0[simp]: "disjointed A 0 = A 0"
  by (simp add: disjointed_def)

lemma disjointed_mono: "mono A \<Longrightarrow> disjointed A (Suc n) = A (Suc n) - A n"
  using mono_Un[of A] by (simp add: disjointed_def atLeastLessThanSuc_atLeastAtMost atLeast0AtMost)

subsection \<open>Partitions\<close>

text \<open>
  Partitions @{term P} of a set @{term A}. We explicitly disallow empty sets.
\<close>

definition partition_on :: "'a set \<Rightarrow> 'a set set \<Rightarrow> bool"
where
  "partition_on A P \<longleftrightarrow> \<Union>P = A \<and> disjoint P \<and> {} \<notin> P"

lemma partition_onI:
  "\<Union>P = A \<Longrightarrow> (\<And>p q. p \<in> P \<Longrightarrow> q \<in> P \<Longrightarrow> p \<noteq> q \<Longrightarrow> disjnt p q) \<Longrightarrow> {} \<notin> P \<Longrightarrow> partition_on A P"
  by (auto simp: partition_on_def pairwise_def)

lemma partition_onD1: "partition_on A P \<Longrightarrow> A = \<Union>P"
  by (auto simp: partition_on_def)

lemma partition_onD2: "partition_on A P \<Longrightarrow> disjoint P"
  by (auto simp: partition_on_def)

lemma partition_onD3: "partition_on A P \<Longrightarrow> {} \<notin> P"
  by (auto simp: partition_on_def)

subsection \<open>Constructions of partitions\<close>

lemma partition_on_empty: "partition_on {} P \<longleftrightarrow> P = {}"
  unfolding partition_on_def by fastforce

lemma partition_on_space: "A \<noteq> {} \<Longrightarrow> partition_on A {A}"
  by (auto simp: partition_on_def disjoint_def)

lemma partition_on_singletons: "partition_on A ((\<lambda>x. {x}) ` A)"
  by (auto simp: partition_on_def disjoint_def)

lemma partition_on_transform:
  assumes P: "partition_on A P"
  assumes F_UN: "\<Union>(F ` P) = F (\<Union>P)" and F_disjnt: "\<And>p q. p \<in> P \<Longrightarrow> q \<in> P \<Longrightarrow> disjnt p q \<Longrightarrow> disjnt (F p) (F q)"
  shows "partition_on (F A) (F ` P - {{}})"
proof -
  have "\<Union>(F ` P - {{}}) = F A"
    unfolding P[THEN partition_onD1] F_UN[symmetric] by auto
  with P show ?thesis
    by (auto simp add: partition_on_def pairwise_def intro!: F_disjnt)
qed

lemma partition_on_restrict: "partition_on A P \<Longrightarrow> partition_on (B \<inter> A) (op \<inter> B ` P - {{}})"
  by (intro partition_on_transform) (auto simp: disjnt_def)

lemma partition_on_vimage: "partition_on A P \<Longrightarrow> partition_on (f -` A) (op -` f ` P - {{}})"
  by (intro partition_on_transform) (auto simp: disjnt_def)

lemma partition_on_inj_image:
  assumes P: "partition_on A P" and f: "inj_on f A"
  shows "partition_on (f ` A) (op ` f ` P - {{}})"
proof (rule partition_on_transform[OF P])
  show "p \<in> P \<Longrightarrow> q \<in> P \<Longrightarrow> disjnt p q \<Longrightarrow> disjnt (f ` p) (f ` q)" for p q
    using f[THEN inj_onD] P[THEN partition_onD1] by (auto simp: disjnt_def)
qed auto

subsection \<open>Finiteness of partitions\<close>

lemma finitely_many_partition_on:
  assumes "finite A"
  shows "finite {P. partition_on A P}"
proof (rule finite_subset)
  show "{P. partition_on A P} \<subseteq> Pow (Pow A)"
    unfolding partition_on_def by auto
  show "finite (Pow (Pow A))"
    using assms by simp
qed

lemma finite_elements: "finite A \<Longrightarrow> partition_on A P \<Longrightarrow> finite P"
  using partition_onD1[of A P] by (simp add: finite_UnionD)

subsection \<open>Equivalence of partitions and equivalence classes\<close>

lemma partition_on_quotient:
  assumes r: "equiv A r"
  shows "partition_on A (A // r)"
proof (rule partition_onI)
  from r have "refl_on A r"
    by (auto elim: equivE)
  then show "\<Union>(A // r) = A" "{} \<notin> A // r"
    by (auto simp: refl_on_def quotient_def)

  fix p q assume "p \<in> A // r" "q \<in> A // r" "p \<noteq> q"
  then obtain x y where "x \<in> A" "y \<in> A" "p = r `` {x}" "q = r `` {y}"
    by (auto simp: quotient_def)
  with r equiv_class_eq_iff[OF r, of x y] \<open>p \<noteq> q\<close> show "disjnt p q"
    by (auto simp: disjnt_equiv_class)
qed

lemma equiv_partition_on:
  assumes P: "partition_on A P"
  shows "equiv A {(x, y). \<exists>p \<in> P. x \<in> p \<and> y \<in> p}"
proof (rule equivI)
  have "A = \<Union>P" "disjoint P" "{} \<notin> P"
    using P by (auto simp: partition_on_def)
  then show "refl_on A {(x, y). \<exists>p\<in>P. x \<in> p \<and> y \<in> p}"
    unfolding refl_on_def by auto
  show "trans {(x, y). \<exists>p\<in>P. x \<in> p \<and> y \<in> p}"
    using \<open>disjoint P\<close> by (auto simp: trans_def disjoint_def)
qed (auto simp: sym_def)

lemma partition_on_eq_quotient:
  assumes P: "partition_on A P"
  shows "A // {(x, y). \<exists>p \<in> P. x \<in> p \<and> y \<in> p} = P"
  unfolding quotient_def
proof safe
  fix x assume "x \<in> A"
  then obtain p where "p \<in> P" "x \<in> p" "\<And>q. q \<in> P \<Longrightarrow> x \<in> q \<Longrightarrow> p = q"
    using P by (auto simp: partition_on_def disjoint_def)
  then have "{y. \<exists>p\<in>P. x \<in> p \<and> y \<in> p} = p"
    by (safe intro!: bexI[of _ p]) simp
  then show "{(x, y). \<exists>p\<in>P. x \<in> p \<and> y \<in> p} `` {x} \<in> P"
    by (simp add: \<open>p \<in> P\<close>)
next
  fix p assume "p \<in> P"
  then have "p \<noteq> {}"
    using P by (auto simp: partition_on_def)
  then obtain x where "x \<in> p"
    by auto
  then have "x \<in> A" "\<And>q. q \<in> P \<Longrightarrow> x \<in> q \<Longrightarrow> p = q"
    using P \<open>p \<in> P\<close> by (auto simp: partition_on_def disjoint_def)
  with \<open>p\<in>P\<close> \<open>x \<in> p\<close> have "{y. \<exists>p\<in>P. x \<in> p \<and> y \<in> p} = p"
    by (safe intro!: bexI[of _ p]) simp
  then show "p \<in> (\<Union>x\<in>A. {{(x, y). \<exists>p\<in>P. x \<in> p \<and> y \<in> p} `` {x}})"
    by (auto intro: \<open>x \<in> A\<close>)
qed

lemma partition_on_alt: "partition_on A P \<longleftrightarrow> (\<exists>r. equiv A r \<and> P = A // r)"
  by (auto simp: partition_on_eq_quotient intro!: partition_on_quotient intro: equiv_partition_on)

end