Theory QuotRing

theory QuotRing
imports RingHom
(*  Title:      HOL/Algebra/QuotRing.thy
    Author:     Stephan Hohe
*)

theory QuotRing
imports RingHom
begin

section {* Quotient Rings *}

subsection {* Multiplication on Cosets *}

definition rcoset_mult :: "[('a, _) ring_scheme, 'a set, 'a set, 'a set] => 'a set"
    ("[mod _:] _ \<Otimes>\<index> _" [81,81,81] 80)
  where "rcoset_mult R I A B = (\<Union>a∈A. \<Union>b∈B. I +>R (a ⊗R b))"


text {* @{const "rcoset_mult"} fulfils the properties required by
  congruences *}
lemma (in ideal) rcoset_mult_add:
    "x ∈ carrier R ==> y ∈ carrier R ==> [mod I:] (I +> x) \<Otimes> (I +> y) = I +> (x ⊗ y)"
  apply rule
  apply (rule, simp add: rcoset_mult_def, clarsimp)
  defer 1
  apply (rule, simp add: rcoset_mult_def)
  defer 1
proof -
  fix z x' y'
  assume carr: "x ∈ carrier R" "y ∈ carrier R"
    and x'rcos: "x' ∈ I +> x"
    and y'rcos: "y' ∈ I +> y"
    and zrcos: "z ∈ I +> x' ⊗ y'"

  from x'rcos have "∃h∈I. x' = h ⊕ x"
    by (simp add: a_r_coset_def r_coset_def)
  then obtain hx where hxI: "hx ∈ I" and x': "x' = hx ⊕ x"
    by fast+

  from y'rcos have "∃h∈I. y' = h ⊕ y"
    by (simp add: a_r_coset_def r_coset_def)
  then obtain hy where hyI: "hy ∈ I" and y': "y' = hy ⊕ y"
    by fast+

  from zrcos have "∃h∈I. z = h ⊕ (x' ⊗ y')"
    by (simp add: a_r_coset_def r_coset_def)
  then obtain hz where hzI: "hz ∈ I" and z: "z = hz ⊕ (x' ⊗ y')"
    by fast+

  note carr = carr hxI[THEN a_Hcarr] hyI[THEN a_Hcarr] hzI[THEN a_Hcarr]

  from z have "z = hz ⊕ (x' ⊗ y')" .
  also from x' y' have "… = hz ⊕ ((hx ⊕ x) ⊗ (hy ⊕ y))" by simp
  also from carr have "… = (hz ⊕ (hx ⊗ (hy ⊕ y)) ⊕ x ⊗ hy) ⊕ x ⊗ y" by algebra
  finally have z2: "z = (hz ⊕ (hx ⊗ (hy ⊕ y)) ⊕ x ⊗ hy) ⊕ x ⊗ y" .

  from hxI hyI hzI carr have "hz ⊕ (hx ⊗ (hy ⊕ y)) ⊕ x ⊗ hy ∈ I"
    by (simp add: I_l_closed I_r_closed)

  with z2 have "∃h∈I. z = h ⊕ x ⊗ y" by fast
  then show "z ∈ I +> x ⊗ y" by (simp add: a_r_coset_def r_coset_def)
next
  fix z
  assume xcarr: "x ∈ carrier R"
    and ycarr: "y ∈ carrier R"
    and zrcos: "z ∈ I +> x ⊗ y"
  from xcarr have xself: "x ∈ I +> x" by (intro a_rcos_self)
  from ycarr have yself: "y ∈ I +> y" by (intro a_rcos_self)
  show "∃a∈I +> x. ∃b∈I +> y. z ∈ I +> a ⊗ b"
    using xself and yself and zrcos by fast
qed


subsection {* Quotient Ring Definition *}

definition FactRing :: "[('a,'b) ring_scheme, 'a set] => ('a set) ring"
    (infixl "Quot" 65)
  where "FactRing R I =
    (|carrier = a_rcosetsR I, mult = rcoset_mult R I,
      one = (I +>R \<one>R), zero = I, add = set_add R|)),"


subsection {* Factorization over General Ideals *}

text {* The quotient is a ring *}
lemma (in ideal) quotient_is_ring: "ring (R Quot I)"
apply (rule ringI)
   --{* abelian group *}
   apply (rule comm_group_abelian_groupI)
   apply (simp add: FactRing_def)
   apply (rule a_factorgroup_is_comm_group[unfolded A_FactGroup_def'])
  --{* mult monoid *}
  apply (rule monoidI)
      apply (simp_all add: FactRing_def A_RCOSETS_def RCOSETS_def
             a_r_coset_def[symmetric])
      --{* mult closed *}
      apply (clarify)
      apply (simp add: rcoset_mult_add, fast)
     --{* mult @{text one_closed} *}
     apply force
    --{* mult assoc *}
    apply clarify
    apply (simp add: rcoset_mult_add m_assoc)
   --{* mult one *}
   apply clarify
   apply (simp add: rcoset_mult_add)
  apply clarify
  apply (simp add: rcoset_mult_add)
 --{* distr *}
 apply clarify
 apply (simp add: rcoset_mult_add a_rcos_sum l_distr)
apply clarify
apply (simp add: rcoset_mult_add a_rcos_sum r_distr)
done


text {* This is a ring homomorphism *}

lemma (in ideal) rcos_ring_hom: "(op +> I) ∈ ring_hom R (R Quot I)"
apply (rule ring_hom_memI)
   apply (simp add: FactRing_def a_rcosetsI[OF a_subset])
  apply (simp add: FactRing_def rcoset_mult_add)
 apply (simp add: FactRing_def a_rcos_sum)
apply (simp add: FactRing_def)
done

lemma (in ideal) rcos_ring_hom_ring: "ring_hom_ring R (R Quot I) (op +> I)"
apply (rule ring_hom_ringI)
     apply (rule is_ring, rule quotient_is_ring)
   apply (simp add: FactRing_def a_rcosetsI[OF a_subset])
  apply (simp add: FactRing_def rcoset_mult_add)
 apply (simp add: FactRing_def a_rcos_sum)
apply (simp add: FactRing_def)
done

text {* The quotient of a cring is also commutative *}
lemma (in ideal) quotient_is_cring:
  assumes "cring R"
  shows "cring (R Quot I)"
proof -
  interpret cring R by fact
  show ?thesis
    apply (intro cring.intro comm_monoid.intro comm_monoid_axioms.intro)
      apply (rule quotient_is_ring)
     apply (rule ring.axioms[OF quotient_is_ring])
    apply (simp add: FactRing_def A_RCOSETS_defs a_r_coset_def[symmetric])
    apply clarify
    apply (simp add: rcoset_mult_add m_comm)
    done
qed

text {* Cosets as a ring homomorphism on crings *}
lemma (in ideal) rcos_ring_hom_cring:
  assumes "cring R"
  shows "ring_hom_cring R (R Quot I) (op +> I)"
proof -
  interpret cring R by fact
  show ?thesis
    apply (rule ring_hom_cringI)
      apply (rule rcos_ring_hom_ring)
     apply (rule is_cring)
    apply (rule quotient_is_cring)
   apply (rule is_cring)
   done
qed


subsection {* Factorization over Prime Ideals *}

text {* The quotient ring generated by a prime ideal is a domain *}
lemma (in primeideal) quotient_is_domain: "domain (R Quot I)"
  apply (rule domain.intro)
   apply (rule quotient_is_cring, rule is_cring)
  apply (rule domain_axioms.intro)
   apply (simp add: FactRing_def) defer 1
    apply (simp add: FactRing_def A_RCOSETS_defs a_r_coset_def[symmetric], clarify)
    apply (simp add: rcoset_mult_add) defer 1
proof (rule ccontr, clarsimp)
  assume "I +> \<one> = I"
  then have "\<one> ∈ I" by (simp only: a_coset_join1 one_closed a_subgroup)
  then have "carrier R ⊆ I" by (subst one_imp_carrier, simp, fast)
  with a_subset have "I = carrier R" by fast
  with I_notcarr show False by fast
next
  fix x y
  assume carr: "x ∈ carrier R" "y ∈ carrier R"
    and a: "I +> x ⊗ y = I"
    and b: "I +> y ≠ I"

  have ynI: "y ∉ I"
  proof (rule ccontr, simp)
    assume "y ∈ I"
    then have "I +> y = I" by (rule a_rcos_const)
    with b show False by simp
  qed

  from carr have "x ⊗ y ∈ I +> x ⊗ y" by (simp add: a_rcos_self)
  then have xyI: "x ⊗ y ∈ I" by (simp add: a)

  from xyI and carr have xI: "x ∈ I ∨ y ∈ I" by (simp add: I_prime)
  with ynI have "x ∈ I" by fast
  then show "I +> x = I" by (rule a_rcos_const)
qed

text {* Generating right cosets of a prime ideal is a homomorphism
        on commutative rings *}
lemma (in primeideal) rcos_ring_hom_cring: "ring_hom_cring R (R Quot I) (op +> I)"
  by (rule rcos_ring_hom_cring) (rule is_cring)


subsection {* Factorization over Maximal Ideals *}

text {* In a commutative ring, the quotient ring over a maximal ideal
        is a field.
        The proof follows ``W. Adkins, S. Weintraub: Algebra --
        An Approach via Module Theory'' *}
lemma (in maximalideal) quotient_is_field:
  assumes "cring R"
  shows "field (R Quot I)"
proof -
  interpret cring R by fact
  show ?thesis
    apply (intro cring.cring_fieldI2)
      apply (rule quotient_is_cring, rule is_cring)
     defer 1
     apply (simp add: FactRing_def A_RCOSETS_defs a_r_coset_def[symmetric], clarsimp)
     apply (simp add: rcoset_mult_add) defer 1
  proof (rule ccontr, simp)
    --{* Quotient is not empty *}
    assume "\<zero>R Quot I = \<one>R Quot I"
    then have II1: "I = I +> \<one>" by (simp add: FactRing_def)
    from a_rcos_self[OF one_closed] have "\<one> ∈ I"
      by (simp add: II1[symmetric])
    then have "I = carrier R" by (rule one_imp_carrier)
    with I_notcarr show False by simp
  next
    --{* Existence of Inverse *}
    fix a
    assume IanI: "I +> a ≠ I" and acarr: "a ∈ carrier R"

    --{* Helper ideal @{text "J"} *}
    def J  "(carrier R #> a) <+> I :: 'a set"
    have idealJ: "ideal J R"
      apply (unfold J_def, rule add_ideals)
       apply (simp only: cgenideal_eq_rcos[symmetric], rule cgenideal_ideal, rule acarr)
      apply (rule is_ideal)
      done

    --{* Showing @{term "J"} not smaller than @{term "I"} *}
    have IinJ: "I ⊆ J"
    proof (rule, simp add: J_def r_coset_def set_add_defs)
      fix x
      assume xI: "x ∈ I"
      have Zcarr: "\<zero> ∈ carrier R" by fast
      from xI[THEN a_Hcarr] acarr
      have "x = \<zero> ⊗ a ⊕ x" by algebra
      with Zcarr and xI show "∃xa∈carrier R. ∃k∈I. x = xa ⊗ a ⊕ k" by fast
    qed

    --{* Showing @{term "J ≠ I"} *}
    have anI: "a ∉ I"
    proof (rule ccontr, simp)
      assume "a ∈ I"
      then have "I +> a = I" by (rule a_rcos_const)
      with IanI show False by simp
    qed

    have aJ: "a ∈ J"
    proof (simp add: J_def r_coset_def set_add_defs)
      from acarr
      have "a = \<one> ⊗ a ⊕ \<zero>" by algebra
      with one_closed and additive_subgroup.zero_closed[OF is_additive_subgroup]
      show "∃x∈carrier R. ∃k∈I. a = x ⊗ a ⊕ k" by fast
    qed

    from aJ and anI have JnI: "J ≠ I" by fast

    --{* Deducing @{term "J = carrier R"} because @{term "I"} is maximal *}
    from idealJ and IinJ have "J = I ∨ J = carrier R"
    proof (rule I_maximal, unfold J_def)
      have "carrier R #> a ⊆ carrier R"
        using subset_refl acarr by (rule r_coset_subset_G)
      then show "carrier R #> a <+> I ⊆ carrier R"
        using a_subset by (rule set_add_closed)
    qed

    with JnI have Jcarr: "J = carrier R" by simp

    --{* Calculating an inverse for @{term "a"} *}
    from one_closed[folded Jcarr]
    have "∃r∈carrier R. ∃i∈I. \<one> = r ⊗ a ⊕ i"
      by (simp add: J_def r_coset_def set_add_defs)
    then obtain r i where rcarr: "r ∈ carrier R"
      and iI: "i ∈ I" and one: "\<one> = r ⊗ a ⊕ i" by fast
    from one and rcarr and acarr and iI[THEN a_Hcarr]
    have rai1: "a ⊗ r = \<ominus>i ⊕ \<one>" by algebra

    --{* Lifting to cosets *}
    from iI have "\<ominus>i ⊕ \<one> ∈ I +> \<one>"
      by (intro a_rcosI, simp, intro a_subset, simp)
    with rai1 have "a ⊗ r ∈ I +> \<one>" by simp
    then have "I +> \<one> = I +> a ⊗ r"
      by (rule a_repr_independence, simp) (rule a_subgroup)

    from rcarr and this[symmetric]
    show "∃r∈carrier R. I +> a ⊗ r = I +> \<one>" by fast
  qed
qed

end