src/HOL/Algebra/Ideal.thy

eliminated markup for plain identifiers (frequent but insignificant);
reduced "black" markup (outer string etc. takes care of it already);

(*  Title:      HOL/Algebra/Ideal.thy
    Author:     Stephan Hohe, TU Muenchen
*)

theory Ideal
imports Ring AbelCoset
begin

section {* Ideals *}

subsection {* Definitions *}

subsubsection {* General definition *}

locale ideal = additive_subgroup I R + ring R for I and R (structure) +
  assumes I_l_closed: "\<lbrakk>a \<in> I; x \<in> carrier R\<rbrakk> \<Longrightarrow> x \<otimes> a \<in> I"
    and I_r_closed: "\<lbrakk>a \<in> I; x \<in> carrier R\<rbrakk> \<Longrightarrow> a \<otimes> x \<in> I"

sublocale ideal \<subseteq> abelian_subgroup I R
  apply (intro abelian_subgroupI3 abelian_group.intro)
    apply (rule ideal.axioms, rule ideal_axioms)
   apply (rule abelian_group.axioms, rule ring.axioms, rule ideal.axioms, rule ideal_axioms)
  apply (rule abelian_group.axioms, rule ring.axioms, rule ideal.axioms, rule ideal_axioms)
  done

lemma (in ideal) is_ideal: "ideal I R"
  by (rule ideal_axioms)

lemma idealI:
  fixes R (structure)
  assumes "ring R"
  assumes a_subgroup: "subgroup I \<lparr>carrier = carrier R, mult = add R, one = zero R\<rparr>"
    and I_l_closed: "\<And>a x. \<lbrakk>a \<in> I; x \<in> carrier R\<rbrakk> \<Longrightarrow> x \<otimes> a \<in> I"
    and I_r_closed: "\<And>a x. \<lbrakk>a \<in> I; x \<in> carrier R\<rbrakk> \<Longrightarrow> a \<otimes> x \<in> I"
  shows "ideal I R"
proof -
  interpret ring R by fact
  show ?thesis  apply (intro ideal.intro ideal_axioms.intro additive_subgroupI)
     apply (rule a_subgroup)
    apply (rule is_ring)
   apply (erule (1) I_l_closed)
  apply (erule (1) I_r_closed)
  done
qed


subsubsection (in ring) {* Ideals Generated by a Subset of @{term "carrier R"} *}

definition genideal :: "_ \<Rightarrow> 'a set \<Rightarrow> 'a set"  ("Idl\<index> _" [80] 79)
  where "genideal R S = Inter {I. ideal I R \<and> S \<subseteq> I}"

subsubsection {* Principal Ideals *}

locale principalideal = ideal +
  assumes generate: "\<exists>i \<in> carrier R. I = Idl {i}"

lemma (in principalideal) is_principalideal: "principalideal I R"
  by (rule principalideal_axioms)

lemma principalidealI:
  fixes R (structure)
  assumes "ideal I R"
  assumes generate: "\<exists>i \<in> carrier R. I = Idl {i}"
  shows "principalideal I R"
proof -
  interpret ideal I R by fact
  show ?thesis
    by (intro principalideal.intro principalideal_axioms.intro)
      (rule is_ideal, rule generate)
qed


subsubsection {* Maximal Ideals *}

locale maximalideal = ideal +
  assumes I_notcarr: "carrier R \<noteq> I"
    and I_maximal: "\<lbrakk>ideal J R; I \<subseteq> J; J \<subseteq> carrier R\<rbrakk> \<Longrightarrow> J = I \<or> J = carrier R"

lemma (in maximalideal) is_maximalideal: "maximalideal I R"
  by (rule maximalideal_axioms)

lemma maximalidealI:
  fixes R
  assumes "ideal I R"
  assumes I_notcarr: "carrier R \<noteq> I"
    and I_maximal: "\<And>J. \<lbrakk>ideal J R; I \<subseteq> J; J \<subseteq> carrier R\<rbrakk> \<Longrightarrow> J = I \<or> J = carrier R"
  shows "maximalideal I R"
proof -
  interpret ideal I R by fact
  show ?thesis
    by (intro maximalideal.intro maximalideal_axioms.intro)
      (rule is_ideal, rule I_notcarr, rule I_maximal)
qed


subsubsection {* Prime Ideals *}

locale primeideal = ideal + cring +
  assumes I_notcarr: "carrier R \<noteq> I"
    and I_prime: "\<lbrakk>a \<in> carrier R; b \<in> carrier R; a \<otimes> b \<in> I\<rbrakk> \<Longrightarrow> a \<in> I \<or> b \<in> I"

lemma (in primeideal) is_primeideal: "primeideal I R"
  by (rule primeideal_axioms)

lemma primeidealI:
  fixes R (structure)
  assumes "ideal I R"
  assumes "cring R"
  assumes I_notcarr: "carrier R \<noteq> I"
    and I_prime: "\<And>a b. \<lbrakk>a \<in> carrier R; b \<in> carrier R; a \<otimes> b \<in> I\<rbrakk> \<Longrightarrow> a \<in> I \<or> b \<in> I"
  shows "primeideal I R"
proof -
  interpret ideal I R by fact
  interpret cring R by fact
  show ?thesis
    by (intro primeideal.intro primeideal_axioms.intro)
      (rule is_ideal, rule is_cring, rule I_notcarr, rule I_prime)
qed

lemma primeidealI2:
  fixes R (structure)
  assumes "additive_subgroup I R"
  assumes "cring R"
  assumes I_l_closed: "\<And>a x. \<lbrakk>a \<in> I; x \<in> carrier R\<rbrakk> \<Longrightarrow> x \<otimes> a \<in> I"
    and I_r_closed: "\<And>a x. \<lbrakk>a \<in> I; x \<in> carrier R\<rbrakk> \<Longrightarrow> a \<otimes> x \<in> I"
    and I_notcarr: "carrier R \<noteq> I"
    and I_prime: "\<And>a b. \<lbrakk>a \<in> carrier R; b \<in> carrier R; a \<otimes> b \<in> I\<rbrakk> \<Longrightarrow> a \<in> I \<or> b \<in> I"
  shows "primeideal I R"
proof -
  interpret additive_subgroup I R by fact
  interpret cring R by fact
  show ?thesis apply (intro_locales)
    apply (intro ideal_axioms.intro)
    apply (erule (1) I_l_closed)
    apply (erule (1) I_r_closed)
    apply (intro primeideal_axioms.intro)
    apply (rule I_notcarr)
    apply (erule (2) I_prime)
    done
qed


subsection {* Special Ideals *}

lemma (in ring) zeroideal: "ideal {\<zero>} R"
  apply (intro idealI subgroup.intro)
        apply (rule is_ring)
       apply simp+
    apply (fold a_inv_def, simp)
   apply simp+
  done

lemma (in ring) oneideal: "ideal (carrier R) R"
  by (rule idealI) (auto intro: is_ring add.subgroupI)

lemma (in "domain") zeroprimeideal: "primeideal {\<zero>} R"
  apply (intro primeidealI)
     apply (rule zeroideal)
    apply (rule domain.axioms, rule domain_axioms)
   defer 1
   apply (simp add: integral)
proof (rule ccontr, simp)
  assume "carrier R = {\<zero>}"
  then have "\<one> = \<zero>" by (rule one_zeroI)
  with one_not_zero show False by simp
qed


subsection {* General Ideal Properies *}

lemma (in ideal) one_imp_carrier:
  assumes I_one_closed: "\<one> \<in> I"
  shows "I = carrier R"
  apply (rule)
  apply (rule)
  apply (rule a_Hcarr, simp)
proof
  fix x
  assume xcarr: "x \<in> carrier R"
  with I_one_closed have "x \<otimes> \<one> \<in> I" by (intro I_l_closed)
  with xcarr show "x \<in> I" by simp
qed

lemma (in ideal) Icarr:
  assumes iI: "i \<in> I"
  shows "i \<in> carrier R"
  using iI by (rule a_Hcarr)


subsection {* Intersection of Ideals *}

text {* \paragraph{Intersection of two ideals} The intersection of any
  two ideals is again an ideal in @{term R} *}
lemma (in ring) i_intersect:
  assumes "ideal I R"
  assumes "ideal J R"
  shows "ideal (I \<inter> J) R"
proof -
  interpret ideal I R by fact
  interpret ideal J R by fact
  show ?thesis
    apply (intro idealI subgroup.intro)
          apply (rule is_ring)
         apply (force simp add: a_subset)
        apply (simp add: a_inv_def[symmetric])
       apply simp
      apply (simp add: a_inv_def[symmetric])
     apply (clarsimp, rule)
      apply (fast intro: ideal.I_l_closed ideal.intro assms)+
    apply (clarsimp, rule)
     apply (fast intro: ideal.I_r_closed ideal.intro assms)+
    done
qed

text {* The intersection of any Number of Ideals is again
        an Ideal in @{term R} *}
lemma (in ring) i_Intersect:
  assumes Sideals: "\<And>I. I \<in> S \<Longrightarrow> ideal I R"
    and notempty: "S \<noteq> {}"
  shows "ideal (Inter S) R"
  apply (unfold_locales)
  apply (simp_all add: Inter_eq)
        apply rule unfolding mem_Collect_eq defer 1
        apply rule defer 1
        apply rule defer 1
        apply (fold a_inv_def, rule) defer 1
        apply rule defer 1
        apply rule defer 1
proof -
  fix x y
  assume "\<forall>I\<in>S. x \<in> I"
  then have xI: "\<And>I. I \<in> S \<Longrightarrow> x \<in> I" by simp
  assume "\<forall>I\<in>S. y \<in> I"
  then have yI: "\<And>I. I \<in> S \<Longrightarrow> y \<in> I" by simp

  fix J
  assume JS: "J \<in> S"
  interpret ideal J R by (rule Sideals[OF JS])
  from xI[OF JS] and yI[OF JS] show "x \<oplus> y \<in> J" by (rule a_closed)
next
  fix J
  assume JS: "J \<in> S"
  interpret ideal J R by (rule Sideals[OF JS])
  show "\<zero> \<in> J" by simp
next
  fix x
  assume "\<forall>I\<in>S. x \<in> I"
  then have xI: "\<And>I. I \<in> S \<Longrightarrow> x \<in> I" by simp

  fix J
  assume JS: "J \<in> S"
  interpret ideal J R by (rule Sideals[OF JS])

  from xI[OF JS] show "\<ominus> x \<in> J" by (rule a_inv_closed)
next
  fix x y
  assume "\<forall>I\<in>S. x \<in> I"
  then have xI: "\<And>I. I \<in> S \<Longrightarrow> x \<in> I" by simp
  assume ycarr: "y \<in> carrier R"

  fix J
  assume JS: "J \<in> S"
  interpret ideal J R by (rule Sideals[OF JS])

  from xI[OF JS] and ycarr show "y \<otimes> x \<in> J" by (rule I_l_closed)
next
  fix x y
  assume "\<forall>I\<in>S. x \<in> I"
  then have xI: "\<And>I. I \<in> S \<Longrightarrow> x \<in> I" by simp
  assume ycarr: "y \<in> carrier R"

  fix J
  assume JS: "J \<in> S"
  interpret ideal J R by (rule Sideals[OF JS])

  from xI[OF JS] and ycarr show "x \<otimes> y \<in> J" by (rule I_r_closed)
next
  fix x
  assume "\<forall>I\<in>S. x \<in> I"
  then have xI: "\<And>I. I \<in> S \<Longrightarrow> x \<in> I" by simp

  from notempty have "\<exists>I0. I0 \<in> S" by blast
  then obtain I0 where I0S: "I0 \<in> S" by auto

  interpret ideal I0 R by (rule Sideals[OF I0S])

  from xI[OF I0S] have "x \<in> I0" .
  with a_subset show "x \<in> carrier R" by fast
next

qed


subsection {* Addition of Ideals *}

lemma (in ring) add_ideals:
  assumes idealI: "ideal I R"
      and idealJ: "ideal J R"
  shows "ideal (I <+> J) R"
  apply (rule ideal.intro)
    apply (rule add_additive_subgroups)
     apply (intro ideal.axioms[OF idealI])
    apply (intro ideal.axioms[OF idealJ])
   apply (rule is_ring)
  apply (rule ideal_axioms.intro)
   apply (simp add: set_add_defs, clarsimp) defer 1
   apply (simp add: set_add_defs, clarsimp) defer 1
proof -
  fix x i j
  assume xcarr: "x \<in> carrier R"
    and iI: "i \<in> I"
    and jJ: "j \<in> J"
  from xcarr ideal.Icarr[OF idealI iI] ideal.Icarr[OF idealJ jJ]
  have c: "(i \<oplus> j) \<otimes> x = (i \<otimes> x) \<oplus> (j \<otimes> x)"
    by algebra
  from xcarr and iI have a: "i \<otimes> x \<in> I"
    by (simp add: ideal.I_r_closed[OF idealI])
  from xcarr and jJ have b: "j \<otimes> x \<in> J"
    by (simp add: ideal.I_r_closed[OF idealJ])
  from a b c show "\<exists>ha\<in>I. \<exists>ka\<in>J. (i \<oplus> j) \<otimes> x = ha \<oplus> ka"
    by fast
next
  fix x i j
  assume xcarr: "x \<in> carrier R"
    and iI: "i \<in> I"
    and jJ: "j \<in> J"
  from xcarr ideal.Icarr[OF idealI iI] ideal.Icarr[OF idealJ jJ]
  have c: "x \<otimes> (i \<oplus> j) = (x \<otimes> i) \<oplus> (x \<otimes> j)" by algebra
  from xcarr and iI have a: "x \<otimes> i \<in> I"
    by (simp add: ideal.I_l_closed[OF idealI])
  from xcarr and jJ have b: "x \<otimes> j \<in> J"
    by (simp add: ideal.I_l_closed[OF idealJ])
  from a b c show "\<exists>ha\<in>I. \<exists>ka\<in>J. x \<otimes> (i \<oplus> j) = ha \<oplus> ka"
    by fast
qed


subsection (in ring) {* Ideals generated by a subset of @{term "carrier R"} *}

text {* @{term genideal} generates an ideal *}
lemma (in ring) genideal_ideal:
  assumes Scarr: "S \<subseteq> carrier R"
  shows "ideal (Idl S) R"
unfolding genideal_def
proof (rule i_Intersect, fast, simp)
  from oneideal and Scarr
  show "\<exists>I. ideal I R \<and> S \<le> I" by fast
qed

lemma (in ring) genideal_self:
  assumes "S \<subseteq> carrier R"
  shows "S \<subseteq> Idl S"
  unfolding genideal_def by fast

lemma (in ring) genideal_self':
  assumes carr: "i \<in> carrier R"
  shows "i \<in> Idl {i}"
proof -
  from carr have "{i} \<subseteq> Idl {i}" by (fast intro!: genideal_self)
  then show "i \<in> Idl {i}" by fast
qed

text {* @{term genideal} generates the minimal ideal *}
lemma (in ring) genideal_minimal:
  assumes a: "ideal I R"
    and b: "S \<subseteq> I"
  shows "Idl S \<subseteq> I"
  unfolding genideal_def by rule (elim InterD, simp add: a b)

text {* Generated ideals and subsets *}
lemma (in ring) Idl_subset_ideal:
  assumes Iideal: "ideal I R"
    and Hcarr: "H \<subseteq> carrier R"
  shows "(Idl H \<subseteq> I) = (H \<subseteq> I)"
proof
  assume a: "Idl H \<subseteq> I"
  from Hcarr have "H \<subseteq> Idl H" by (rule genideal_self)
  with a show "H \<subseteq> I" by simp
next
  fix x
  assume HI: "H \<subseteq> I"
  from Iideal and HI have "I \<in> {I. ideal I R \<and> H \<subseteq> I}" by fast
  then show "Idl H \<subseteq> I" unfolding genideal_def by fast
qed

lemma (in ring) subset_Idl_subset:
  assumes Icarr: "I \<subseteq> carrier R"
    and HI: "H \<subseteq> I"
  shows "Idl H \<subseteq> Idl I"
proof -
  from HI and genideal_self[OF Icarr] have HIdlI: "H \<subseteq> Idl I"
    by fast

  from Icarr have Iideal: "ideal (Idl I) R"
    by (rule genideal_ideal)
  from HI and Icarr have "H \<subseteq> carrier R"
    by fast
  with Iideal have "(H \<subseteq> Idl I) = (Idl H \<subseteq> Idl I)"
    by (rule Idl_subset_ideal[symmetric])

  with HIdlI show "Idl H \<subseteq> Idl I" by simp
qed

lemma (in ring) Idl_subset_ideal':
  assumes acarr: "a \<in> carrier R" and bcarr: "b \<in> carrier R"
  shows "(Idl {a} \<subseteq> Idl {b}) = (a \<in> Idl {b})"
  apply (subst Idl_subset_ideal[OF genideal_ideal[of "{b}"], of "{a}"])
    apply (fast intro: bcarr, fast intro: acarr)
  apply fast
  done

lemma (in ring) genideal_zero: "Idl {\<zero>} = {\<zero>}"
  apply rule
   apply (rule genideal_minimal[OF zeroideal], simp)
  apply (simp add: genideal_self')
  done

lemma (in ring) genideal_one: "Idl {\<one>} = carrier R"
proof -
  interpret ideal "Idl {\<one>}" "R" by (rule genideal_ideal) fast
  show "Idl {\<one>} = carrier R"
  apply (rule, rule a_subset)
  apply (simp add: one_imp_carrier genideal_self')
  done
qed


text {* Generation of Principal Ideals in Commutative Rings *}

definition cgenideal :: "_ \<Rightarrow> 'a \<Rightarrow> 'a set"  ("PIdl\<index> _" [80] 79)
  where "cgenideal R a = {x \<otimes>\<^bsub>R\<^esub> a | x. x \<in> carrier R}"

text {* genhideal (?) really generates an ideal *}
lemma (in cring) cgenideal_ideal:
  assumes acarr: "a \<in> carrier R"
  shows "ideal (PIdl a) R"
  apply (unfold cgenideal_def)
  apply (rule idealI[OF is_ring])
     apply (rule subgroup.intro)
        apply simp_all
        apply (blast intro: acarr)
        apply clarsimp defer 1
        defer 1
        apply (fold a_inv_def, clarsimp) defer 1
        apply clarsimp defer 1
        apply clarsimp defer 1
proof -
  fix x y
  assume xcarr: "x \<in> carrier R"
    and ycarr: "y \<in> carrier R"
  note carr = acarr xcarr ycarr

  from carr have "x \<otimes> a \<oplus> y \<otimes> a = (x \<oplus> y) \<otimes> a"
    by (simp add: l_distr)
  with carr show "\<exists>z. x \<otimes> a \<oplus> y \<otimes> a = z \<otimes> a \<and> z \<in> carrier R"
    by fast
next
  from l_null[OF acarr, symmetric] and zero_closed
  show "\<exists>x. \<zero> = x \<otimes> a \<and> x \<in> carrier R" by fast
next
  fix x
  assume xcarr: "x \<in> carrier R"
  note carr = acarr xcarr

  from carr have "\<ominus> (x \<otimes> a) = (\<ominus> x) \<otimes> a"
    by (simp add: l_minus)
  with carr show "\<exists>z. \<ominus> (x \<otimes> a) = z \<otimes> a \<and> z \<in> carrier R"
    by fast
next
  fix x y
  assume xcarr: "x \<in> carrier R"
     and ycarr: "y \<in> carrier R"
  note carr = acarr xcarr ycarr
  
  from carr have "y \<otimes> a \<otimes> x = (y \<otimes> x) \<otimes> a"
    by (simp add: m_assoc) (simp add: m_comm)
  with carr show "\<exists>z. y \<otimes> a \<otimes> x = z \<otimes> a \<and> z \<in> carrier R"
    by fast
next
  fix x y
  assume xcarr: "x \<in> carrier R"
     and ycarr: "y \<in> carrier R"
  note carr = acarr xcarr ycarr

  from carr have "x \<otimes> (y \<otimes> a) = (x \<otimes> y) \<otimes> a"
    by (simp add: m_assoc)
  with carr show "\<exists>z. x \<otimes> (y \<otimes> a) = z \<otimes> a \<and> z \<in> carrier R"
    by fast
qed

lemma (in ring) cgenideal_self:
  assumes icarr: "i \<in> carrier R"
  shows "i \<in> PIdl i"
  unfolding cgenideal_def
proof simp
  from icarr have "i = \<one> \<otimes> i"
    by simp
  with icarr show "\<exists>x. i = x \<otimes> i \<and> x \<in> carrier R"
    by fast
qed

text {* @{const "cgenideal"} is minimal *}

lemma (in ring) cgenideal_minimal:
  assumes "ideal J R"
  assumes aJ: "a \<in> J"
  shows "PIdl a \<subseteq> J"
proof -
  interpret ideal J R by fact
  show ?thesis
    unfolding cgenideal_def
    apply rule
    apply clarify
    using aJ
    apply (erule I_l_closed)
    done
qed

lemma (in cring) cgenideal_eq_genideal:
  assumes icarr: "i \<in> carrier R"
  shows "PIdl i = Idl {i}"
  apply rule
   apply (intro cgenideal_minimal)
    apply (rule genideal_ideal, fast intro: icarr)
   apply (rule genideal_self', fast intro: icarr)
  apply (intro genideal_minimal)
   apply (rule cgenideal_ideal [OF icarr])
  apply (simp, rule cgenideal_self [OF icarr])
  done

lemma (in cring) cgenideal_eq_rcos: "PIdl i = carrier R #> i"
  unfolding cgenideal_def r_coset_def by fast

lemma (in cring) cgenideal_is_principalideal:
  assumes icarr: "i \<in> carrier R"
  shows "principalideal (PIdl i) R"
  apply (rule principalidealI)
  apply (rule cgenideal_ideal [OF icarr])
proof -
  from icarr have "PIdl i = Idl {i}"
    by (rule cgenideal_eq_genideal)
  with icarr show "\<exists>i'\<in>carrier R. PIdl i = Idl {i'}"
    by fast
qed


subsection {* Union of Ideals *}

lemma (in ring) union_genideal:
  assumes idealI: "ideal I R"
    and idealJ: "ideal J R"
  shows "Idl (I \<union> J) = I <+> J"
  apply rule
   apply (rule ring.genideal_minimal)
     apply (rule is_ring)
    apply (rule add_ideals[OF idealI idealJ])
   apply (rule)
   apply (simp add: set_add_defs) apply (elim disjE) defer 1 defer 1
   apply (rule) apply (simp add: set_add_defs genideal_def) apply clarsimp defer 1
proof -
  fix x
  assume xI: "x \<in> I"
  have ZJ: "\<zero> \<in> J"
    by (intro additive_subgroup.zero_closed) (rule ideal.axioms[OF idealJ])
  from ideal.Icarr[OF idealI xI] have "x = x \<oplus> \<zero>"
    by algebra
  with xI and ZJ show "\<exists>h\<in>I. \<exists>k\<in>J. x = h \<oplus> k"
    by fast
next
  fix x
  assume xJ: "x \<in> J"
  have ZI: "\<zero> \<in> I"
    by (intro additive_subgroup.zero_closed, rule ideal.axioms[OF idealI])
  from ideal.Icarr[OF idealJ xJ] have "x = \<zero> \<oplus> x"
    by algebra
  with ZI and xJ show "\<exists>h\<in>I. \<exists>k\<in>J. x = h \<oplus> k"
    by fast
next
  fix i j K
  assume iI: "i \<in> I"
    and jJ: "j \<in> J"
    and idealK: "ideal K R"
    and IK: "I \<subseteq> K"
    and JK: "J \<subseteq> K"
  from iI and IK have iK: "i \<in> K" by fast
  from jJ and JK have jK: "j \<in> K" by fast
  from iK and jK show "i \<oplus> j \<in> K"
    by (intro additive_subgroup.a_closed) (rule ideal.axioms[OF idealK])
qed


subsection {* Properties of Principal Ideals *}

text {* @{text "\<zero>"} generates the zero ideal *}
lemma (in ring) zero_genideal: "Idl {\<zero>} = {\<zero>}"
  apply rule
  apply (simp add: genideal_minimal zeroideal)
  apply (fast intro!: genideal_self)
  done

text {* @{text "\<one>"} generates the unit ideal *}
lemma (in ring) one_genideal: "Idl {\<one>} = carrier R"
proof -
  have "\<one> \<in> Idl {\<one>}"
    by (simp add: genideal_self')
  then show "Idl {\<one>} = carrier R"
    by (intro ideal.one_imp_carrier) (fast intro: genideal_ideal)
qed


text {* The zero ideal is a principal ideal *}
corollary (in ring) zeropideal: "principalideal {\<zero>} R"
  apply (rule principalidealI)
   apply (rule zeroideal)
  apply (blast intro!: zero_genideal[symmetric])
  done

text {* The unit ideal is a principal ideal *}
corollary (in ring) onepideal: "principalideal (carrier R) R"
  apply (rule principalidealI)
   apply (rule oneideal)
  apply (blast intro!: one_genideal[symmetric])
  done


text {* Every principal ideal is a right coset of the carrier *}
lemma (in principalideal) rcos_generate:
  assumes "cring R"
  shows "\<exists>x\<in>I. I = carrier R #> x"
proof -
  interpret cring R by fact
  from generate obtain i where icarr: "i \<in> carrier R" and I1: "I = Idl {i}"
    by fast+

  from icarr and genideal_self[of "{i}"] have "i \<in> Idl {i}"
    by fast
  then have iI: "i \<in> I" by (simp add: I1)

  from I1 icarr have I2: "I = PIdl i"
    by (simp add: cgenideal_eq_genideal)

  have "PIdl i = carrier R #> i"
    unfolding cgenideal_def r_coset_def by fast

  with I2 have "I = carrier R #> i"
    by simp

  with iI show "\<exists>x\<in>I. I = carrier R #> x"
    by fast
qed


subsection {* Prime Ideals *}

lemma (in ideal) primeidealCD:
  assumes "cring R"
  assumes notprime: "\<not> primeideal I R"
  shows "carrier R = I \<or> (\<exists>a b. a \<in> carrier R \<and> b \<in> carrier R \<and> a \<otimes> b \<in> I \<and> a \<notin> I \<and> b \<notin> I)"
proof (rule ccontr, clarsimp)
  interpret cring R by fact
  assume InR: "carrier R \<noteq> I"
    and "\<forall>a. a \<in> carrier R \<longrightarrow> (\<forall>b. a \<otimes> b \<in> I \<longrightarrow> b \<in> carrier R \<longrightarrow> a \<in> I \<or> b \<in> I)"
  then have I_prime: "\<And> a b. \<lbrakk>a \<in> carrier R; b \<in> carrier R; a \<otimes> b \<in> I\<rbrakk> \<Longrightarrow> a \<in> I \<or> b \<in> I"
    by simp
  have "primeideal I R"
    apply (rule primeideal.intro [OF is_ideal is_cring])
    apply (rule primeideal_axioms.intro)
     apply (rule InR)
    apply (erule (2) I_prime)
    done
  with notprime show False by simp
qed

lemma (in ideal) primeidealCE:
  assumes "cring R"
  assumes notprime: "\<not> primeideal I R"
  obtains "carrier R = I"
    | "\<exists>a b. a \<in> carrier R \<and> b \<in> carrier R \<and> a \<otimes> b \<in> I \<and> a \<notin> I \<and> b \<notin> I"
proof -
  interpret R: cring R by fact
  assume "carrier R = I ==> thesis"
    and "\<exists>a b. a \<in> carrier R \<and> b \<in> carrier R \<and> a \<otimes> b \<in> I \<and> a \<notin> I \<and> b \<notin> I \<Longrightarrow> thesis"
  then show thesis using primeidealCD [OF R.is_cring notprime] by blast
qed

text {* If @{text "{\<zero>}"} is a prime ideal of a commutative ring, the ring is a domain *}
lemma (in cring) zeroprimeideal_domainI:
  assumes pi: "primeideal {\<zero>} R"
  shows "domain R"
  apply (rule domain.intro, rule is_cring)
  apply (rule domain_axioms.intro)
proof (rule ccontr, simp)
  interpret primeideal "{\<zero>}" "R" by (rule pi)
  assume "\<one> = \<zero>"
  then have "carrier R = {\<zero>}" by (rule one_zeroD)
  from this[symmetric] and I_notcarr show False
    by simp
next
  interpret primeideal "{\<zero>}" "R" by (rule pi)
  fix a b
  assume ab: "a \<otimes> b = \<zero>" and carr: "a \<in> carrier R" "b \<in> carrier R"
  from ab have abI: "a \<otimes> b \<in> {\<zero>}"
    by fast
  with carr have "a \<in> {\<zero>} \<or> b \<in> {\<zero>}"
    by (rule I_prime)
  then show "a = \<zero> \<or> b = \<zero>" by simp
qed

corollary (in cring) domain_eq_zeroprimeideal: "domain R = primeideal {\<zero>} R"
  apply rule
   apply (erule domain.zeroprimeideal)
  apply (erule zeroprimeideal_domainI)
  done


subsection {* Maximal Ideals *}

lemma (in ideal) helper_I_closed:
  assumes carr: "a \<in> carrier R" "x \<in> carrier R" "y \<in> carrier R"
    and axI: "a \<otimes> x \<in> I"
  shows "a \<otimes> (x \<otimes> y) \<in> I"
proof -
  from axI and carr have "(a \<otimes> x) \<otimes> y \<in> I"
    by (simp add: I_r_closed)
  also from carr have "(a \<otimes> x) \<otimes> y = a \<otimes> (x \<otimes> y)"
    by (simp add: m_assoc)
  finally show "a \<otimes> (x \<otimes> y) \<in> I" .
qed

lemma (in ideal) helper_max_prime:
  assumes "cring R"
  assumes acarr: "a \<in> carrier R"
  shows "ideal {x\<in>carrier R. a \<otimes> x \<in> I} R"
proof -
  interpret cring R by fact
  show ?thesis apply (rule idealI)
    apply (rule cring.axioms[OF is_cring])
    apply (rule subgroup.intro)
    apply (simp, fast)
    apply clarsimp apply (simp add: r_distr acarr)
    apply (simp add: acarr)
    apply (simp add: a_inv_def[symmetric], clarify) defer 1
    apply clarsimp defer 1
    apply (fast intro!: helper_I_closed acarr)
  proof -
    fix x
    assume xcarr: "x \<in> carrier R"
      and ax: "a \<otimes> x \<in> I"
    from ax and acarr xcarr
    have "\<ominus>(a \<otimes> x) \<in> I" by simp
    also from acarr xcarr
    have "\<ominus>(a \<otimes> x) = a \<otimes> (\<ominus>x)" by algebra
    finally show "a \<otimes> (\<ominus>x) \<in> I" .
    from acarr have "a \<otimes> \<zero> = \<zero>" by simp
  next
    fix x y
    assume xcarr: "x \<in> carrier R"
      and ycarr: "y \<in> carrier R"
      and ayI: "a \<otimes> y \<in> I"
    from ayI and acarr xcarr ycarr have "a \<otimes> (y \<otimes> x) \<in> I"
      by (simp add: helper_I_closed)
    moreover
    from xcarr ycarr have "y \<otimes> x = x \<otimes> y"
      by (simp add: m_comm)
    ultimately
    show "a \<otimes> (x \<otimes> y) \<in> I" by simp
  qed
qed

text {* In a cring every maximal ideal is prime *}
lemma (in cring) maximalideal_is_prime:
  assumes "maximalideal I R"
  shows "primeideal I R"
proof -
  interpret maximalideal I R by fact
  show ?thesis apply (rule ccontr)
    apply (rule primeidealCE)
    apply (rule is_cring)
    apply assumption
    apply (simp add: I_notcarr)
  proof -
    assume "\<exists>a b. a \<in> carrier R \<and> b \<in> carrier R \<and> a \<otimes> b \<in> I \<and> a \<notin> I \<and> b \<notin> I"
    then obtain a b where
      acarr: "a \<in> carrier R" and
      bcarr: "b \<in> carrier R" and
      abI: "a \<otimes> b \<in> I" and
      anI: "a \<notin> I" and
      bnI: "b \<notin> I" by fast
    def J \<equiv> "{x\<in>carrier R. a \<otimes> x \<in> I}"
    
    from is_cring and acarr have idealJ: "ideal J R"
      unfolding J_def by (rule helper_max_prime)
    
    have IsubJ: "I \<subseteq> J"
    proof
      fix x
      assume xI: "x \<in> I"
      with acarr have "a \<otimes> x \<in> I"
        by (intro I_l_closed)
      with xI[THEN a_Hcarr] show "x \<in> J"
        unfolding J_def by fast
    qed
    
    from abI and acarr bcarr have "b \<in> J"
      unfolding J_def by fast
    with bnI have JnI: "J \<noteq> I" by fast
    from acarr
    have "a = a \<otimes> \<one>" by algebra
    with anI have "a \<otimes> \<one> \<notin> I" by simp
    with one_closed have "\<one> \<notin> J"
      unfolding J_def by fast
    then have Jncarr: "J \<noteq> carrier R" by fast
    
    interpret ideal J R by (rule idealJ)
    
    have "J = I \<or> J = carrier R"
      apply (intro I_maximal)
      apply (rule idealJ)
      apply (rule IsubJ)
      apply (rule a_subset)
      done
    
    with JnI and Jncarr show False by simp
  qed
qed


subsection {* Derived Theorems *}

--"A non-zero cring that has only the two trivial ideals is a field"
lemma (in cring) trivialideals_fieldI:
  assumes carrnzero: "carrier R \<noteq> {\<zero>}"
    and haveideals: "{I. ideal I R} = {{\<zero>}, carrier R}"
  shows "field R"
  apply (rule cring_fieldI)
  apply (rule, rule, rule)
   apply (erule Units_closed)
  defer 1
    apply rule
  defer 1
proof (rule ccontr, simp)
  assume zUnit: "\<zero> \<in> Units R"
  then have a: "\<zero> \<otimes> inv \<zero> = \<one>" by (rule Units_r_inv)
  from zUnit have "\<zero> \<otimes> inv \<zero> = \<zero>"
    by (intro l_null) (rule Units_inv_closed)
  with a[symmetric] have "\<one> = \<zero>" by simp
  then have "carrier R = {\<zero>}" by (rule one_zeroD)
  with carrnzero show False by simp
next
  fix x
  assume xcarr': "x \<in> carrier R - {\<zero>}"
  then have xcarr: "x \<in> carrier R" by fast
  from xcarr' have xnZ: "x \<noteq> \<zero>" by fast
  from xcarr have xIdl: "ideal (PIdl x) R"
    by (intro cgenideal_ideal) fast

  from xcarr have "x \<in> PIdl x"
    by (intro cgenideal_self) fast
  with xnZ have "PIdl x \<noteq> {\<zero>}" by fast
  with haveideals have "PIdl x = carrier R"
    by (blast intro!: xIdl)
  then have "\<one> \<in> PIdl x" by simp
  then have "\<exists>y. \<one> = y \<otimes> x \<and> y \<in> carrier R"
    unfolding cgenideal_def by blast
  then obtain y where ycarr: " y \<in> carrier R" and ylinv: "\<one> = y \<otimes> x"
    by fast+
  from ylinv and xcarr ycarr have yrinv: "\<one> = x \<otimes> y"
    by (simp add: m_comm)
  from ycarr and ylinv[symmetric] and yrinv[symmetric]
  have "\<exists>y \<in> carrier R. y \<otimes> x = \<one> \<and> x \<otimes> y = \<one>" by fast
  with xcarr show "x \<in> Units R"
    unfolding Units_def by fast
qed

lemma (in field) all_ideals: "{I. ideal I R} = {{\<zero>}, carrier R}"
  apply (rule, rule)
proof -
  fix I
  assume a: "I \<in> {I. ideal I R}"
  then interpret ideal I R by simp

  show "I \<in> {{\<zero>}, carrier R}"
  proof (cases "\<exists>a. a \<in> I - {\<zero>}")
    case True
    then obtain a where aI: "a \<in> I" and anZ: "a \<noteq> \<zero>"
      by fast+
    from aI[THEN a_Hcarr] anZ have aUnit: "a \<in> Units R"
      by (simp add: field_Units)
    then have a: "a \<otimes> inv a = \<one>" by (rule Units_r_inv)
    from aI and aUnit have "a \<otimes> inv a \<in> I"
      by (simp add: I_r_closed del: Units_r_inv)
    then have oneI: "\<one> \<in> I" by (simp add: a[symmetric])

    have "carrier R \<subseteq> I"
    proof
      fix x
      assume xcarr: "x \<in> carrier R"
      with oneI have "\<one> \<otimes> x \<in> I" by (rule I_r_closed)
      with xcarr show "x \<in> I" by simp
    qed
    with a_subset have "I = carrier R" by fast
    then show "I \<in> {{\<zero>}, carrier R}" by fast
  next
    case False
    then have IZ: "\<And>a. a \<in> I \<Longrightarrow> a = \<zero>" by simp

    have a: "I \<subseteq> {\<zero>}"
    proof
      fix x
      assume "x \<in> I"
      then have "x = \<zero>" by (rule IZ)
      then show "x \<in> {\<zero>}" by fast
    qed

    have "\<zero> \<in> I" by simp
    then have "{\<zero>} \<subseteq> I" by fast

    with a have "I = {\<zero>}" by fast
    then show "I \<in> {{\<zero>}, carrier R}" by fast
  qed
qed (simp add: zeroideal oneideal)

--"Jacobson Theorem 2.2"
lemma (in cring) trivialideals_eq_field:
  assumes carrnzero: "carrier R \<noteq> {\<zero>}"
  shows "({I. ideal I R} = {{\<zero>}, carrier R}) = field R"
  by (fast intro!: trivialideals_fieldI[OF carrnzero] field.all_ideals)


text {* Like zeroprimeideal for domains *}
lemma (in field) zeromaximalideal: "maximalideal {\<zero>} R"
  apply (rule maximalidealI)
    apply (rule zeroideal)
proof-
  from one_not_zero have "\<one> \<notin> {\<zero>}" by simp
  with one_closed show "carrier R \<noteq> {\<zero>}" by fast
next
  fix J
  assume Jideal: "ideal J R"
  then have "J \<in> {I. ideal I R}" by fast
  with all_ideals show "J = {\<zero>} \<or> J = carrier R"
    by simp
qed

lemma (in cring) zeromaximalideal_fieldI:
  assumes zeromax: "maximalideal {\<zero>} R"
  shows "field R"
  apply (rule trivialideals_fieldI, rule maximalideal.I_notcarr[OF zeromax])
  apply rule apply clarsimp defer 1
   apply (simp add: zeroideal oneideal)
proof -
  fix J
  assume Jn0: "J \<noteq> {\<zero>}"
    and idealJ: "ideal J R"
  interpret ideal J R by (rule idealJ)
  have "{\<zero>} \<subseteq> J" by (rule ccontr) simp
  from zeromax and idealJ and this and a_subset
  have "J = {\<zero>} \<or> J = carrier R"
    by (rule maximalideal.I_maximal)
  with Jn0 show "J = carrier R"
    by simp
qed

lemma (in cring) zeromaximalideal_eq_field: "maximalideal {\<zero>} R = field R"
  apply rule
   apply (erule zeromaximalideal_fieldI)
  apply (erule field.zeromaximalideal)
  done

end