(* 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] \<Rightarrow> 'a set"
("[mod _:] _ \<Otimes>\<index> _" [81,81,81] 80)
where "rcoset_mult R I A B = (\<Union>a\<in>A. \<Union>b\<in>B. I +>\<^bsub>R\<^esub> (a \<otimes>\<^bsub>R\<^esub> b))"
text {* @{const "rcoset_mult"} fulfils the properties required by
congruences *}
lemma (in ideal) rcoset_mult_add:
"x \<in> carrier R \<Longrightarrow> y \<in> carrier R \<Longrightarrow> [mod I:] (I +> x) \<Otimes> (I +> y) = I +> (x \<otimes> 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 \<in> carrier R" "y \<in> carrier R"
and x'rcos: "x' \<in> I +> x"
and y'rcos: "y' \<in> I +> y"
and zrcos: "z \<in> I +> x' \<otimes> y'"
from x'rcos have "\<exists>h\<in>I. x' = h \<oplus> x"
by (simp add: a_r_coset_def r_coset_def)
then obtain hx where hxI: "hx \<in> I" and x': "x' = hx \<oplus> x"
by fast+
from y'rcos have "\<exists>h\<in>I. y' = h \<oplus> y"
by (simp add: a_r_coset_def r_coset_def)
then obtain hy where hyI: "hy \<in> I" and y': "y' = hy \<oplus> y"
by fast+
from zrcos have "\<exists>h\<in>I. z = h \<oplus> (x' \<otimes> y')"
by (simp add: a_r_coset_def r_coset_def)
then obtain hz where hzI: "hz \<in> I" and z: "z = hz \<oplus> (x' \<otimes> y')"
by fast+
note carr = carr hxI[THEN a_Hcarr] hyI[THEN a_Hcarr] hzI[THEN a_Hcarr]
from z have "z = hz \<oplus> (x' \<otimes> y')" .
also from x' y' have "\<dots> = hz \<oplus> ((hx \<oplus> x) \<otimes> (hy \<oplus> y))" by simp
also from carr have "\<dots> = (hz \<oplus> (hx \<otimes> (hy \<oplus> y)) \<oplus> x \<otimes> hy) \<oplus> x \<otimes> y" by algebra
finally have z2: "z = (hz \<oplus> (hx \<otimes> (hy \<oplus> y)) \<oplus> x \<otimes> hy) \<oplus> x \<otimes> y" .
from hxI hyI hzI carr have "hz \<oplus> (hx \<otimes> (hy \<oplus> y)) \<oplus> x \<otimes> hy \<in> I"
by (simp add: I_l_closed I_r_closed)
with z2 have "\<exists>h\<in>I. z = h \<oplus> x \<otimes> y" by fast
then show "z \<in> I +> x \<otimes> y" by (simp add: a_r_coset_def r_coset_def)
next
fix z
assume xcarr: "x \<in> carrier R"
and ycarr: "y \<in> carrier R"
and zrcos: "z \<in> I +> x \<otimes> y"
from xcarr have xself: "x \<in> I +> x" by (intro a_rcos_self)
from ycarr have yself: "y \<in> I +> y" by (intro a_rcos_self)
show "\<exists>a\<in>I +> x. \<exists>b\<in>I +> y. z \<in> I +> a \<otimes> b"
using xself and yself and zrcos by fast
qed
subsection {* Quotient Ring Definition *}
definition FactRing :: "[('a,'b) ring_scheme, 'a set] \<Rightarrow> ('a set) ring"
(infixl "Quot" 65)
where "FactRing R I =
\<lparr>carrier = a_rcosets\<^bsub>R\<^esub> I, mult = rcoset_mult R I,
one = (I +>\<^bsub>R\<^esub> \<one>\<^bsub>R\<^esub>), zero = I, add = set_add R\<rparr>"
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) \<in> 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> \<in> I" by (simp only: a_coset_join1 one_closed a_subgroup)
then have "carrier R \<subseteq> 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 \<in> carrier R" "y \<in> carrier R"
and a: "I +> x \<otimes> y = I"
and b: "I +> y \<noteq> I"
have ynI: "y \<notin> I"
proof (rule ccontr, simp)
assume "y \<in> I"
then have "I +> y = I" by (rule a_rcos_const)
with b show False by simp
qed
from carr have "x \<otimes> y \<in> I +> x \<otimes> y" by (simp add: a_rcos_self)
then have xyI: "x \<otimes> y \<in> I" by (simp add: a)
from xyI and carr have xI: "x \<in> I \<or> y \<in> I" by (simp add: I_prime)
with ynI have "x \<in> 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>\<^bsub>R Quot I\<^esub> = \<one>\<^bsub>R Quot I\<^esub>"
then have II1: "I = I +> \<one>" by (simp add: FactRing_def)
from a_rcos_self[OF one_closed] have "\<one> \<in> 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 \<noteq> I" and acarr: "a \<in> carrier R"
--{* Helper ideal @{text "J"} *}
def J \<equiv> "(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 \<subseteq> J"
proof (rule, simp add: J_def r_coset_def set_add_defs)
fix x
assume xI: "x \<in> I"
have Zcarr: "\<zero> \<in> carrier R" by fast
from xI[THEN a_Hcarr] acarr
have "x = \<zero> \<otimes> a \<oplus> x" by algebra
with Zcarr and xI show "\<exists>xa\<in>carrier R. \<exists>k\<in>I. x = xa \<otimes> a \<oplus> k" by fast
qed
--{* Showing @{term "J \<noteq> I"} *}
have anI: "a \<notin> I"
proof (rule ccontr, simp)
assume "a \<in> I"
then have "I +> a = I" by (rule a_rcos_const)
with IanI show False by simp
qed
have aJ: "a \<in> J"
proof (simp add: J_def r_coset_def set_add_defs)
from acarr
have "a = \<one> \<otimes> a \<oplus> \<zero>" by algebra
with one_closed and additive_subgroup.zero_closed[OF is_additive_subgroup]
show "\<exists>x\<in>carrier R. \<exists>k\<in>I. a = x \<otimes> a \<oplus> k" by fast
qed
from aJ and anI have JnI: "J \<noteq> I" by fast
--{* Deducing @{term "J = carrier R"} because @{term "I"} is maximal *}
from idealJ and IinJ have "J = I \<or> J = carrier R"
proof (rule I_maximal, unfold J_def)
have "carrier R #> a \<subseteq> carrier R"
using subset_refl acarr by (rule r_coset_subset_G)
then show "carrier R #> a <+> I \<subseteq> 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 "\<exists>r\<in>carrier R. \<exists>i\<in>I. \<one> = r \<otimes> a \<oplus> i"
by (simp add: J_def r_coset_def set_add_defs)
then obtain r i where rcarr: "r \<in> carrier R"
and iI: "i \<in> I" and one: "\<one> = r \<otimes> a \<oplus> i" by fast
from one and rcarr and acarr and iI[THEN a_Hcarr]
have rai1: "a \<otimes> r = \<ominus>i \<oplus> \<one>" by algebra
--{* Lifting to cosets *}
from iI have "\<ominus>i \<oplus> \<one> \<in> I +> \<one>"
by (intro a_rcosI, simp, intro a_subset, simp)
with rai1 have "a \<otimes> r \<in> I +> \<one>" by simp
then have "I +> \<one> = I +> a \<otimes> r"
by (rule a_repr_independence, simp) (rule a_subgroup)
from rcarr and this[symmetric]
show "\<exists>r\<in>carrier R. I +> a \<otimes> r = I +> \<one>" by fast
qed
qed
end