src/HOL/Algebra/Embedded_Algebras.thy

clarified signature;

(*  Title:      HOL/Algebra/Embedded_Algebras.thy
    Author:     Paulo Emílio de Vilhena
*)

theory Embedded_Algebras
  imports Subrings Generated_Groups
begin

section \<open>Definitions\<close>

locale embedded_algebra =
  K?: subfield K R + R?: ring R for K :: "'a set" and R :: "('a, 'b) ring_scheme" (structure)

definition (in ring) line_extension :: "'a set \<Rightarrow> 'a \<Rightarrow> 'a set \<Rightarrow> 'a set"
  where "line_extension K a E = (K #> a) <+>\<^bsub>R\<^esub> E"

fun (in ring) Span :: "'a set \<Rightarrow> 'a list \<Rightarrow> 'a set"
  where "Span K Us = foldr (line_extension K) Us { \<zero> }"

fun (in ring) combine :: "'a list \<Rightarrow> 'a list \<Rightarrow> 'a"
  where
    "combine (k # Ks) (u # Us) = (k \<otimes> u) \<oplus> (combine Ks Us)"
  | "combine Ks Us = \<zero>"

inductive (in ring) independent :: "'a set \<Rightarrow> 'a list \<Rightarrow> bool"
  where
    li_Nil [simp, intro]: "independent K []"
  | li_Cons: "\<lbrakk> u \<in> carrier R; u \<notin> Span K Us; independent K Us \<rbrakk> \<Longrightarrow> independent K (u # Us)"

inductive (in ring) dimension :: "nat \<Rightarrow> 'a set \<Rightarrow> 'a set \<Rightarrow> bool"
  where
    zero_dim [simp, intro]: "dimension 0 K { \<zero> }"
   | Suc_dim: "\<lbrakk> v \<in> carrier R; v \<notin> E; dimension n K E \<rbrakk> \<Longrightarrow> dimension (Suc n) K (line_extension K v E)"


subsubsection \<open>Syntactic Definitions\<close>

abbreviation (in ring) dependent ::  "'a set \<Rightarrow> 'a list \<Rightarrow> bool"
  where "dependent K U \<equiv> \<not> independent K U"

definition over :: "('a \<Rightarrow> 'b) \<Rightarrow> 'a \<Rightarrow> 'b" (infixl "over" 65)
  where "f over a = f a"



context ring
begin


subsection \<open>Basic Properties - First Part\<close>

lemma line_extension_consistent:
  assumes "subring K R" shows "ring.line_extension (R \<lparr> carrier := K \<rparr>) = line_extension"
  unfolding ring.line_extension_def[OF subring_is_ring[OF assms]] line_extension_def
  by (simp add: set_add_def set_mult_def)

lemma Span_consistent:
  assumes "subring K R" shows "ring.Span (R \<lparr> carrier := K \<rparr>) = Span"
  unfolding ring.Span.simps[OF subring_is_ring[OF assms]] Span.simps
            line_extension_consistent[OF assms] by simp

lemma combine_in_carrier [simp, intro]:
  "\<lbrakk> set Ks \<subseteq> carrier R; set Us \<subseteq> carrier R \<rbrakk> \<Longrightarrow> combine Ks Us \<in> carrier R"
  by (induct Ks Us rule: combine.induct) (auto)

lemma combine_r_distr:
  "\<lbrakk> set Ks \<subseteq> carrier R; set Us \<subseteq> carrier R \<rbrakk> \<Longrightarrow>
     k \<in> carrier R \<Longrightarrow> k \<otimes> (combine Ks Us) = combine (map ((\<otimes>) k) Ks) Us"
  by (induct Ks Us rule: combine.induct) (auto simp add: m_assoc r_distr)

lemma combine_l_distr:
  "\<lbrakk> set Ks \<subseteq> carrier R; set Us \<subseteq> carrier R \<rbrakk> \<Longrightarrow>
     u \<in> carrier R \<Longrightarrow> (combine Ks Us) \<otimes> u = combine Ks (map (\<lambda>u'. u' \<otimes> u) Us)"
  by (induct Ks Us rule: combine.induct) (auto simp add: m_assoc l_distr)

lemma combine_eq_foldr:
  "combine Ks Us = foldr (\<lambda>(k, u). \<lambda>l. (k \<otimes> u) \<oplus> l) (zip Ks Us) \<zero>"
  by (induct Ks Us rule: combine.induct) (auto)

lemma combine_replicate:
  "set Us \<subseteq> carrier R \<Longrightarrow> combine (replicate (length Us) \<zero>) Us = \<zero>"
  by (induct Us) (auto)

lemma combine_take:
  "combine (take (length Us) Ks) Us = combine Ks Us"
  by (induct Us arbitrary: Ks)
     (auto, metis combine.simps(1) list.exhaust take.simps(1) take_Suc_Cons)

lemma combine_append_zero:
  "set Us \<subseteq> carrier R \<Longrightarrow> combine (Ks @ [ \<zero> ]) Us = combine Ks Us"
proof (induct Ks arbitrary: Us)
  case Nil thus ?case by (induct Us) (auto)
next
  case Cons thus ?case by (cases Us) (auto)
qed

lemma combine_prepend_replicate:
  "\<lbrakk> set Ks \<subseteq> carrier R; set Us \<subseteq> carrier R \<rbrakk> \<Longrightarrow>
     combine ((replicate n \<zero>) @ Ks) Us = combine Ks (drop n Us)"
proof (induct n arbitrary: Us, simp)
  case (Suc n) thus ?case
    by (cases Us) (auto, meson combine_in_carrier ring_simprules(8) set_drop_subset subset_trans)
qed

lemma combine_append_replicate:
  "set Us \<subseteq> carrier R \<Longrightarrow> combine (Ks @ (replicate n \<zero>)) Us = combine Ks Us"
  by (induct n) (auto, metis append.assoc combine_append_zero replicate_append_same)

lemma combine_append:
  assumes "length Ks = length Us"
    and "set Ks  \<subseteq> carrier R" "set Us \<subseteq> carrier R"
    and "set Ks' \<subseteq> carrier R" "set Vs \<subseteq> carrier R"
  shows "(combine Ks Us) \<oplus> (combine Ks' Vs) = combine (Ks @ Ks') (Us @ Vs)"
  using assms
proof (induct Ks arbitrary: Us)
  case Nil thus ?case by auto
next
  case (Cons k Ks)
  then obtain u Us' where Us: "Us = u # Us'"
    by (metis length_Suc_conv)
  hence u: "u \<in> carrier R" and Us': "set Us' \<subseteq> carrier R"
    using Cons(4) by auto
  then show ?case
    using combine_in_carrier[OF _ Us', of Ks] Cons
          combine_in_carrier[OF Cons(5-6)] unfolding Us
    by (auto, simp add: add.m_assoc)
qed

lemma combine_add:
  assumes "length Ks = length Us" and "length Ks' = length Us"
    and "set Ks  \<subseteq> carrier R" "set Ks'  \<subseteq> carrier R" "set Us \<subseteq> carrier R"
  shows "(combine Ks Us) \<oplus> (combine Ks' Us) = combine (map2 (\<oplus>) Ks Ks') Us"
  using assms
proof (induct Us arbitrary: Ks Ks')
  case Nil thus ?case by simp
next
  case (Cons u Us)
  then obtain c c' Cs Cs' where Ks: "Ks = c # Cs" and Ks': "Ks' = c' # Cs'"
    by (metis length_Suc_conv)
  hence in_carrier:
    "c  \<in> carrier R" "set Cs  \<subseteq> carrier R"
    "c' \<in> carrier R" "set Cs' \<subseteq> carrier R"
    "u  \<in> carrier R" "set Us  \<subseteq> carrier R"
    using Cons(4-6) by auto
  hence lc_in_carrier: "combine Cs Us \<in> carrier R" "combine Cs' Us \<in> carrier R"
    using combine_in_carrier by auto
  have "combine Ks (u # Us) \<oplus> combine Ks' (u # Us) =
        ((c \<otimes> u) \<oplus> combine Cs Us) \<oplus> ((c' \<otimes> u) \<oplus> combine Cs' Us)"
    unfolding Ks Ks' by auto
  also have " ... = ((c \<oplus> c') \<otimes> u \<oplus> (combine Cs Us \<oplus> combine Cs' Us))"
    using lc_in_carrier in_carrier(1,3,5) by (simp add: l_distr ring_simprules(7,22))
  also have " ... = combine (map2 (\<oplus>) Ks Ks') (u # Us)"
    using Cons unfolding Ks Ks' by auto
  finally show ?case .
qed

lemma combine_normalize:
  assumes "set Ks \<subseteq> carrier R" "set Us \<subseteq> carrier R" "combine Ks Us = a" 
  obtains Ks'
  where "set (take (length Us) Ks) \<subseteq> set Ks'" "set Ks' \<subseteq> set (take (length Us) Ks) \<union> { \<zero> }"
    and "length Ks' = length Us" "combine Ks' Us = a"
proof -
  define Ks'
    where "Ks' = (if length Ks \<le> length Us
                  then Ks @ (replicate (length Us - length Ks) \<zero>) else take (length Us) Ks)"
  hence "set (take (length Us) Ks) \<subseteq> set Ks'" "set Ks' \<subseteq> set (take (length Us) Ks) \<union> { \<zero> }"
        "length Ks' = length Us" "a = combine Ks' Us"
    using combine_append_replicate[OF assms(2)] combine_take assms(3) by auto
  thus thesis
    using that by blast
qed

lemma line_extension_mem_iff: "u \<in> line_extension K a E \<longleftrightarrow> (\<exists>k \<in> K. \<exists>v \<in> E. u = k \<otimes> a \<oplus> v)"
  unfolding line_extension_def set_add_def'[of R "K #> a" E] unfolding r_coset_def by blast

lemma line_extension_in_carrier:
  assumes "K \<subseteq> carrier R" "a \<in> carrier R" "E \<subseteq> carrier R"
  shows "line_extension K a E \<subseteq> carrier R"
  using set_add_closed[OF r_coset_subset_G[OF assms(1-2)] assms(3)]
  by (simp add: line_extension_def)

lemma Span_in_carrier:
  assumes "K \<subseteq> carrier R" "set Us \<subseteq> carrier R"
  shows "Span K Us \<subseteq> carrier R"
  using assms by (induct Us) (auto simp add: line_extension_in_carrier)


subsection \<open>Some Basic Properties of Linear Independence\<close>

lemma independent_in_carrier: "independent K Us \<Longrightarrow> set Us \<subseteq> carrier R"
  by (induct Us rule: independent.induct) (simp_all)

lemma independent_backwards:
  "independent K (u # Us) \<Longrightarrow> u \<notin> Span K Us"
  "independent K (u # Us) \<Longrightarrow> independent K Us"
  "independent K (u # Us) \<Longrightarrow> u \<in> carrier R"
  by (cases rule: independent.cases, auto)+

lemma dimension_independent [intro]: "independent K Us \<Longrightarrow> dimension (length Us) K (Span K Us)"
proof (induct Us)
  case Nil thus ?case by simp
next
  case Cons thus ?case
    using Suc_dim independent_backwards[OF Cons(2)] by auto 
qed


text \<open>Now, we fix K, a subfield of the ring. Many lemmas would also be true for weaker
      structures, but our interest is to work with subfields, so generalization could
      be the subject of a future work.\<close>

context
  fixes K :: "'a set" assumes K: "subfield K R"
begin


subsection \<open>Basic Properties - Second Part\<close>

lemmas subring_props [simp] =
  subringE[OF subfieldE(1)[OF K]]

lemma line_extension_is_subgroup:
  assumes "subgroup E (add_monoid R)" "a \<in> carrier R"
  shows "subgroup (line_extension K a E) (add_monoid R)"
proof (rule add.subgroupI)
  show "line_extension K a E \<subseteq> carrier R"
    by (simp add: assms add.subgroupE(1) line_extension_def r_coset_subset_G set_add_closed)
next
  have "\<zero> = \<zero> \<otimes> a \<oplus> \<zero>"
    using assms(2) by simp
  hence "\<zero> \<in> line_extension K a E"
    using line_extension_mem_iff subgroup.one_closed[OF assms(1)] by auto
  thus "line_extension K a E \<noteq> {}" by auto
next
  fix u1 u2
  assume "u1 \<in> line_extension K a E" and "u2 \<in> line_extension K a E"
  then obtain k1 k2 v1 v2
    where u1: "k1 \<in> K" "v1 \<in> E" "u1 = (k1 \<otimes> a) \<oplus> v1"
      and u2: "k2 \<in> K" "v2 \<in> E" "u2 = (k2 \<otimes> a) \<oplus> v2"
      and in_carr: "k1 \<in> carrier R" "v1 \<in> carrier R" "k2 \<in> carrier R" "v2 \<in> carrier R"
    using line_extension_mem_iff by (meson add.subgroupE(1)[OF assms(1)] subring_props(1) subsetCE)

  hence "u1 \<oplus> u2 = ((k1 \<oplus> k2) \<otimes> a) \<oplus> (v1 \<oplus> v2)"
    using assms(2) by algebra
  moreover have "k1 \<oplus> k2 \<in> K" and "v1 \<oplus> v2 \<in> E"
    using add.subgroupE(4)[OF assms(1)] u1 u2 by auto
  ultimately show "u1 \<oplus> u2 \<in> line_extension K a E"
    using line_extension_mem_iff by auto

  have "\<ominus> u1 = ((\<ominus> k1) \<otimes> a) \<oplus> (\<ominus> v1)"
    using in_carr(1-2) u1(3) assms(2) by algebra
  moreover have "\<ominus> k1 \<in> K" and "\<ominus> v1 \<in> E"
    using add.subgroupE(3)[OF assms(1)] u1 by auto
  ultimately show "(\<ominus> u1) \<in> line_extension K a E"
    using line_extension_mem_iff by auto
qed

corollary Span_is_add_subgroup:
  "set Us \<subseteq> carrier R \<Longrightarrow> subgroup (Span K Us) (add_monoid R)"
  using line_extension_is_subgroup normal_imp_subgroup[OF add.one_is_normal] by (induct Us) (auto)

lemma line_extension_smult_closed:
  assumes "\<And>k v. \<lbrakk> k \<in> K; v \<in> E \<rbrakk> \<Longrightarrow> k \<otimes> v \<in> E" and "E \<subseteq> carrier R" "a \<in> carrier R"
  shows "\<And>k u. \<lbrakk> k \<in> K; u \<in> line_extension K a E \<rbrakk> \<Longrightarrow> k \<otimes> u \<in> line_extension K a E"
proof -
  fix k u assume A: "k \<in> K" "u \<in> line_extension K a E"
  then obtain k' v'
    where u: "k' \<in> K" "v' \<in> E" "u = k' \<otimes> a \<oplus> v'"
      and in_carr: "k \<in> carrier R" "k' \<in> carrier R" "v' \<in> carrier R"
    using line_extension_mem_iff assms(2) by (meson subring_props(1) subsetCE)
  hence "k \<otimes> u = (k \<otimes> k') \<otimes> a \<oplus> (k \<otimes> v')"
    using assms(3) by algebra
  thus "k \<otimes> u \<in> line_extension K a E"
    using assms(1)[OF A(1) u(2)] line_extension_mem_iff u(1) A(1) by auto
qed

lemma Span_subgroup_props [simp]:
  assumes "set Us \<subseteq> carrier R"
  shows "Span K Us \<subseteq> carrier R"
    and "\<zero> \<in> Span K Us"
    and "\<And>v1 v2. \<lbrakk> v1 \<in> Span K Us; v2 \<in> Span K Us \<rbrakk> \<Longrightarrow> (v1 \<oplus> v2) \<in> Span K Us"
    and "\<And>v. v \<in> Span K Us \<Longrightarrow> (\<ominus> v) \<in> Span K Us"
  using add.subgroupE subgroup.one_closed[of _ "add_monoid R"]
        Span_is_add_subgroup[OF assms(1)] by auto

lemma Span_smult_closed [simp]:
  assumes "set Us \<subseteq> carrier R"
  shows "\<And>k v. \<lbrakk> k \<in> K; v \<in> Span K Us \<rbrakk> \<Longrightarrow> k \<otimes> v \<in> Span K Us"
  using assms
proof (induct Us)
  case Nil thus ?case
    using r_null subring_props(1) by (auto, blast)
next
  case Cons thus ?case
    using Span_subgroup_props(1) line_extension_smult_closed by auto
qed

lemma Span_m_inv_simprule [simp]:
  assumes "set Us \<subseteq> carrier R"
  shows "\<lbrakk> k \<in> K - { \<zero> }; a \<in> carrier R \<rbrakk> \<Longrightarrow> k \<otimes> a \<in> Span K Us \<Longrightarrow> a \<in> Span K Us"
proof -
  assume k: "k \<in> K - { \<zero> }" and a: "a \<in> carrier R" and ka: "k \<otimes> a \<in> Span K Us"
  have inv_k: "inv k \<in> K" "inv k \<otimes> k = \<one>"
    using subfield_m_inv[OF K k] by simp+
  hence "inv k \<otimes> (k \<otimes> a) \<in> Span K Us"
    using Span_smult_closed[OF assms _ ka] by simp
  thus ?thesis
    using inv_k subring_props(1)a k
    by (metis (no_types, lifting) DiffE l_one m_assoc subset_iff)
qed


subsection \<open>Span as Linear Combinations\<close>

text \<open>We show that Span is the set of linear combinations\<close>

lemma line_extension_of_combine_set:
  assumes "u \<in> carrier R"
  shows "line_extension K u { combine Ks Us | Ks. set Ks \<subseteq> K } =
                { combine Ks (u # Us) | Ks. set Ks \<subseteq> K }"
  (is "?line_extension = ?combinations")
proof
  show "?line_extension \<subseteq> ?combinations"
  proof
    fix v assume "v \<in> ?line_extension"
    then obtain k Ks
      where "k \<in> K" "set Ks \<subseteq> K" and "v = combine (k # Ks) (u # Us)"
      using line_extension_mem_iff by auto
    thus "v \<in> ?combinations"
      by (metis (mono_tags, lifting) insert_subset list.simps(15) mem_Collect_eq)
  qed
next
  show "?combinations \<subseteq> ?line_extension"
  proof
    fix v assume "v \<in> ?combinations"
    then obtain Ks where v: "set Ks \<subseteq> K" "v = combine Ks (u # Us)"
      by auto
    thus "v \<in> ?line_extension"
    proof (cases Ks)
      case Cons thus ?thesis
        using v line_extension_mem_iff by auto
    next
      case Nil
      hence "v = \<zero>"
        using v by simp
      moreover have "combine [] Us = \<zero>" by simp
      hence "\<zero> \<in> { combine Ks Us | Ks. set Ks \<subseteq> K }"
        by (metis (mono_tags, lifting) local.Nil mem_Collect_eq v(1))
      hence "(\<zero> \<otimes> u) \<oplus> \<zero> \<in> ?line_extension"
        using line_extension_mem_iff subring_props(2) by blast
      hence "\<zero> \<in> ?line_extension"
        using assms by auto
      ultimately show ?thesis by auto
    qed
  qed
qed

lemma Span_eq_combine_set:
  assumes "set Us \<subseteq> carrier R" shows "Span K Us = { combine Ks Us | Ks. set Ks \<subseteq> K }"
  using assms line_extension_of_combine_set
  by (induct Us) (auto, metis empty_set empty_subsetI)

lemma line_extension_of_combine_set_length_version:
  assumes "u \<in> carrier R"
  shows "line_extension K u { combine Ks Us | Ks. length Ks = length Us \<and> set Ks \<subseteq> K } =
                      { combine Ks (u # Us) | Ks. length Ks = length (u # Us) \<and> set Ks \<subseteq> K }"
  (is "?line_extension = ?combinations")
proof
  show "?line_extension \<subseteq> ?combinations"
  proof
    fix v assume "v \<in> ?line_extension"
    then obtain k Ks
      where "v = combine (k # Ks) (u # Us)" "length (k # Ks) = length (u # Us)" "set (k # Ks) \<subseteq> K"
      using line_extension_mem_iff by auto
    thus "v \<in> ?combinations" by blast
  qed
next
  show "?combinations \<subseteq> ?line_extension"
  proof
    fix c assume "c \<in> ?combinations"
    then obtain Ks where c: "c = combine Ks (u # Us)" "length Ks = length (u # Us)" "set Ks \<subseteq> K"
      by blast
    then obtain k Ks' where k: "Ks = k # Ks'"
      by (metis length_Suc_conv)
    thus "c \<in> ?line_extension"
      using c line_extension_mem_iff unfolding k by auto
  qed
qed

lemma Span_eq_combine_set_length_version:
  assumes "set Us \<subseteq> carrier R"
  shows "Span K Us = { combine Ks Us | Ks. length Ks = length Us \<and> set Ks \<subseteq> K }"
  using assms line_extension_of_combine_set_length_version by (induct Us) (auto)


subsubsection \<open>Corollaries\<close>

corollary Span_mem_iff_length_version:
  assumes "set Us \<subseteq> carrier R"
  shows "a \<in> Span K Us \<longleftrightarrow> (\<exists>Ks. set Ks \<subseteq> K \<and> length Ks = length Us \<and> a = combine Ks Us)"
  using Span_eq_combine_set_length_version[OF assms] by blast

corollary Span_mem_imp_non_trivial_combine:
  assumes "set Us \<subseteq> carrier R" and "a \<in> Span K Us"
  obtains k Ks
  where "k \<in> K - { \<zero> }" "set Ks \<subseteq> K" "length Ks = length Us" "combine (k # Ks) (a # Us) = \<zero>"
proof -
  obtain Ks where Ks: "set Ks \<subseteq> K" "length Ks = length Us" "a = combine Ks Us"
    using Span_mem_iff_length_version[OF assms(1)] assms(2) by auto
  hence "((\<ominus> \<one>) \<otimes> a) \<oplus> a = combine ((\<ominus> \<one>) # Ks) (a # Us)"
    by auto
  moreover have "((\<ominus> \<one>) \<otimes> a) \<oplus> a = \<zero>"
    using assms(2) Span_subgroup_props(1)[OF assms(1)] l_minus l_neg by auto  
  moreover have "\<ominus> \<one> \<noteq> \<zero>"
    using subfieldE(6)[OF K] l_neg by force 
  ultimately show ?thesis
    using that subring_props(3,5) Ks(1-2) by (force simp del: combine.simps)
qed

corollary Span_mem_iff:
  assumes "set Us \<subseteq> carrier R" and "a \<in> carrier R"
  shows "a \<in> Span K Us \<longleftrightarrow> (\<exists>k \<in> K - { \<zero> }. \<exists>Ks. set Ks \<subseteq> K \<and> combine (k # Ks) (a # Us) = \<zero>)"
         (is "?in_Span \<longleftrightarrow> ?exists_combine")
proof
  assume "?in_Span"
  then obtain Ks where Ks: "set Ks \<subseteq> K" "a = combine Ks Us"
    using Span_eq_combine_set[OF assms(1)] by auto
  hence "((\<ominus> \<one>) \<otimes> a) \<oplus> a = combine ((\<ominus> \<one>) # Ks) (a # Us)"
    by auto
  moreover have "((\<ominus> \<one>) \<otimes> a) \<oplus> a = \<zero>"
    using assms(2) l_minus l_neg by auto
  moreover have "\<ominus> \<one> \<noteq> \<zero>"
    using subfieldE(6)[OF K] l_neg by force
  ultimately show "?exists_combine"
    using subring_props(3,5) Ks(1) by (force simp del: combine.simps)
next
  assume "?exists_combine"
  then obtain k Ks
    where k: "k \<in> K" "k \<noteq> \<zero>" and Ks: "set Ks \<subseteq> K" and a: "(k \<otimes> a) \<oplus> combine Ks Us = \<zero>"
    by auto
  hence "combine Ks Us \<in> Span K Us"
    using Span_eq_combine_set[OF assms(1)] by auto
  hence "k \<otimes> a \<in> Span K Us"
    using Span_subgroup_props[OF assms(1)] k Ks a
    by (metis (no_types, lifting) assms(2) contra_subsetD m_closed minus_equality subring_props(1))
  thus "?in_Span"
    using Span_m_inv_simprule[OF assms(1) _ assms(2), of k] k by auto
qed


subsection \<open>Span as the minimal subgroup that contains \<^term>\<open>K <#> (set Us)\<close>\<close>

text \<open>Now we show the link between Span and Group.generate\<close>

lemma mono_Span:
  assumes "set Us \<subseteq> carrier R" and "u \<in> carrier R"
  shows "Span K Us \<subseteq> Span K (u # Us)"
proof
  fix v assume v: "v \<in> Span K Us"
  hence "(\<zero> \<otimes> u) \<oplus> v \<in> Span K (u # Us)"
    using line_extension_mem_iff by auto
  thus "v \<in> Span K (u # Us)"
    using Span_subgroup_props(1)[OF assms(1)] assms(2) v
    by (auto simp del: Span.simps)
qed

lemma Span_min:
  assumes "set Us \<subseteq> carrier R" and "subgroup E (add_monoid R)"
  shows "K <#> (set Us) \<subseteq> E \<Longrightarrow> Span K Us \<subseteq> E"
proof -
  assume "K <#> (set Us) \<subseteq> E" show "Span K Us \<subseteq> E"
  proof
    fix v assume "v \<in> Span K Us"
    then obtain Ks where v: "set Ks \<subseteq> K" "v = combine Ks Us"
      using Span_eq_combine_set[OF assms(1)] by auto
    from \<open>set Ks \<subseteq> K\<close> \<open>set Us \<subseteq> carrier R\<close> and \<open>K <#> (set Us) \<subseteq> E\<close>
    show "v \<in> E" unfolding v(2)
    proof (induct Ks Us rule: combine.induct)
      case (1 k Ks u Us)
      hence "k \<in> K" and "u \<in> set (u # Us)" by auto
      hence "k \<otimes> u \<in> E"
        using 1(4) unfolding set_mult_def by auto
      moreover have "K <#> set Us \<subseteq> E"
        using 1(4) unfolding set_mult_def by auto
      hence "combine Ks Us \<in> E"
        using 1 by auto
      ultimately show ?case
        using add.subgroupE(4)[OF assms(2)] by auto
    next
      case "2_1" thus ?case
        using subgroup.one_closed[OF assms(2)] by auto
    next
      case  "2_2" thus ?case
        using subgroup.one_closed[OF assms(2)] by auto
    qed
  qed
qed

lemma Span_eq_generate:
  assumes "set Us \<subseteq> carrier R" shows "Span K Us = generate (add_monoid R) (K <#> (set Us))"
proof (rule add.generateI)
  show "subgroup (Span K Us) (add_monoid R)"
    using Span_is_add_subgroup[OF assms] .
next
  show "\<And>E. \<lbrakk> subgroup E (add_monoid R); K <#> set Us \<subseteq> E \<rbrakk> \<Longrightarrow> Span K Us \<subseteq> E"
    using Span_min assms by blast
next
  show "K <#> set Us \<subseteq> Span K Us"
  using assms
  proof (induct Us)
    case Nil thus ?case
      unfolding set_mult_def by auto
  next
    case (Cons u Us)
    have "K <#> set (u # Us) = (K <#> { u }) \<union> (K <#> set Us)"
      unfolding set_mult_def by auto
    moreover have "\<And>k. k \<in> K \<Longrightarrow> k \<otimes> u \<in> Span K (u # Us)"
    proof -
      fix k assume k: "k \<in> K"
      hence "combine [ k ] (u # Us) \<in> Span K (u # Us)"
        using Span_eq_combine_set[OF Cons(2)] by (auto simp del: combine.simps)
      moreover have "k \<in> carrier R" and "u \<in> carrier R"
        using Cons(2) k subring_props(1) by (blast, auto)
      ultimately show "k \<otimes> u \<in> Span K (u # Us)"
        by (auto simp del: Span.simps)
    qed
    hence "K <#> { u } \<subseteq> Span K (u # Us)"
      unfolding set_mult_def by auto
    moreover have "K <#> set Us \<subseteq> Span K (u # Us)"
      using mono_Span[of Us u] Cons by (auto simp del: Span.simps)
    ultimately show ?case
      using Cons by (auto simp del: Span.simps)
  qed
qed


subsubsection \<open>Corollaries\<close>

corollary Span_same_set:
  assumes "set Us \<subseteq> carrier R"
  shows "set Us = set Vs \<Longrightarrow> Span K Us = Span K Vs"
  using Span_eq_generate assms by auto

corollary Span_incl: "set Us \<subseteq> carrier R \<Longrightarrow> K <#> (set Us) \<subseteq> Span K Us"
  using Span_eq_generate generate.incl[of _ _ "add_monoid R"] by auto

corollary Span_base_incl: "set Us \<subseteq> carrier R \<Longrightarrow> set Us \<subseteq> Span K Us"
proof -
  assume A: "set Us \<subseteq> carrier R"
  hence "{ \<one> } <#> set Us = set Us"
    unfolding set_mult_def by force
  moreover have "{ \<one> } <#> set Us \<subseteq> K <#> set Us"
    using subring_props(3) unfolding set_mult_def by blast
  ultimately show ?thesis
    using Span_incl[OF A] by auto
qed

corollary mono_Span_sublist:
  assumes "set Us \<subseteq> set Vs" "set Vs \<subseteq> carrier R"
  shows "Span K Us \<subseteq> Span K Vs"
  using add.mono_generate[OF mono_set_mult[OF _ assms(1), of K K R]]
        Span_eq_generate[OF assms(2)] Span_eq_generate[of Us] assms by auto

corollary mono_Span_append:
  assumes "set Us \<subseteq> carrier R" "set Vs \<subseteq> carrier R"
  shows "Span K Us \<subseteq> Span K (Us @ Vs)"
    and "Span K Us \<subseteq> Span K (Vs @ Us)"
  using mono_Span_sublist[of Us "Us @ Vs"] assms
        Span_same_set[of "Us @ Vs" "Vs @ Us"] by auto

corollary mono_Span_subset:
  assumes "set Us \<subseteq> Span K Vs" "set Vs \<subseteq> carrier R"
  shows "Span K Us \<subseteq> Span K Vs"
proof (rule Span_min[OF _ Span_is_add_subgroup[OF assms(2)]])
  show "set Us \<subseteq> carrier R"
    using Span_subgroup_props(1)[OF assms(2)] assms by auto
  show "K <#> set Us \<subseteq> Span K Vs"
    using Span_smult_closed[OF assms(2)] assms(1) unfolding set_mult_def by blast
qed

lemma Span_strict_incl:
  assumes "set Us \<subseteq> carrier R" "set Vs \<subseteq> carrier R"
  shows "Span K Us \<subset> Span K Vs \<Longrightarrow> (\<exists>v \<in> set Vs. v \<notin> Span K Us)"
proof -
  assume "Span K Us \<subset> Span K Vs" show "\<exists>v \<in> set Vs. v \<notin> Span K Us"
  proof (rule ccontr)
    assume "\<not> (\<exists>v \<in> set Vs. v \<notin> Span K Us)"
    hence "Span K Vs \<subseteq> Span K Us"
      using mono_Span_subset[OF _ assms(1), of Vs] by auto
    from \<open>Span K Us \<subset> Span K Vs\<close> and \<open>Span K Vs \<subseteq> Span K Us\<close>
    show False by simp
  qed
qed

lemma Span_append_eq_set_add:
  assumes "set Us \<subseteq> carrier R" and "set Vs \<subseteq> carrier R"
  shows "Span K (Us @ Vs) = (Span K Us <+>\<^bsub>R\<^esub> Span K Vs)"
  using assms
proof (induct Us)
  case Nil thus ?case
    using Span_subgroup_props(1)[OF Nil(2)] unfolding set_add_def' by force
next
  case (Cons u Us)
  hence in_carrier:
    "u \<in> carrier R" "set Us \<subseteq> carrier R" "set Vs \<subseteq> carrier R"
    by auto

  have "line_extension K u (Span K Us <+>\<^bsub>R\<^esub> Span K Vs) = (Span K (u # Us) <+>\<^bsub>R\<^esub> Span K Vs)"
  proof
    show "line_extension K u (Span K Us <+>\<^bsub>R\<^esub> Span K Vs) \<subseteq> (Span K (u # Us) <+>\<^bsub>R\<^esub> Span K Vs)"
    proof
      fix v assume "v \<in> line_extension K u (Span K Us <+>\<^bsub>R\<^esub> Span K Vs)"
      then obtain k u' v'
        where v: "k \<in> K" "u' \<in> Span K Us" "v' \<in> Span K Vs" "v = k \<otimes> u \<oplus> (u' \<oplus> v')"
        using line_extension_mem_iff[of v _ u "Span K Us <+>\<^bsub>R\<^esub> Span K Vs"]
        unfolding set_add_def' by blast
      hence "v = (k \<otimes> u \<oplus> u') \<oplus> v'"
        using in_carrier(2-3)[THEN Span_subgroup_props(1)] in_carrier(1) subring_props(1)
        by (metis (no_types, lifting) rev_subsetD ring_simprules(7) semiring_simprules(3))
      moreover have "k \<otimes> u \<oplus> u' \<in> Span K (u # Us)"
        using line_extension_mem_iff v(1-2) by auto
      ultimately show "v \<in> Span K (u # Us) <+>\<^bsub>R\<^esub> Span K Vs"
        unfolding set_add_def' using v(3) by auto
    qed
  next
    show "Span K (u # Us) <+>\<^bsub>R\<^esub> Span K Vs \<subseteq> line_extension K u (Span K Us <+>\<^bsub>R\<^esub> Span K Vs)"
    proof
      fix v assume "v \<in> Span K (u # Us) <+>\<^bsub>R\<^esub> Span K Vs"
      then obtain k u' v'
        where v: "k \<in> K" "u' \<in> Span K Us" "v' \<in> Span K Vs" "v = (k \<otimes> u \<oplus> u') \<oplus> v'"
        using line_extension_mem_iff[of _ _ u "Span K Us"] unfolding set_add_def' by auto
      hence "v = (k \<otimes> u) \<oplus> (u' \<oplus> v')"
        using in_carrier(2-3)[THEN Span_subgroup_props(1)] in_carrier(1) subring_props(1)
        by (metis (no_types, lifting) rev_subsetD ring_simprules(5,7))
      thus "v \<in> line_extension K u (Span K Us <+>\<^bsub>R\<^esub> Span K Vs)"
        using line_extension_mem_iff[of "(k \<otimes> u) \<oplus> (u' \<oplus> v')" K u "Span K Us <+>\<^bsub>R\<^esub> Span K Vs"]
        unfolding set_add_def' using v by auto
    qed
  qed
  thus ?case
    using Cons by auto
qed


subsection \<open>Characterisation of Linearly Independent "Sets"\<close>

declare independent_backwards [intro]
declare independent_in_carrier [intro]

lemma independent_distinct: "independent K Us \<Longrightarrow> distinct Us"
proof (induct Us rule: list.induct)
  case Nil thus ?case by simp
next
  case Cons thus ?case
    using independent_backwards[OF Cons(2)]
          independent_in_carrier[OF Cons(2)]
          Span_base_incl
    by auto
qed

lemma independent_strict_incl:
  assumes "independent K (u # Us)" shows "Span K Us \<subset> Span K (u # Us)"
proof -
  have "u \<in> Span K (u # Us)"
    using Span_base_incl[OF independent_in_carrier[OF assms]] by auto
  moreover have "Span K Us \<subseteq> Span K (u # Us)"
    using mono_Span independent_in_carrier[OF assms] by auto
  ultimately show ?thesis
    using independent_backwards(1)[OF assms] by auto
qed

corollary independent_replacement:
  assumes "independent K (u # Us)" and "independent K Vs"
  shows "Span K (u # Us) \<subseteq> Span K Vs \<Longrightarrow> (\<exists>v \<in> set Vs. independent K (v # Us))"
proof -
  assume "Span K (u # Us) \<subseteq> Span K Vs"
  hence "Span K Us \<subset> Span K Vs"
    using independent_strict_incl[OF assms(1)] by auto
  then obtain v where v: "v \<in> set Vs" "v \<notin> Span K Us"
    using Span_strict_incl[of Us Vs] assms[THEN independent_in_carrier] by auto
  thus ?thesis
    using li_Cons[of v K Us] assms independent_in_carrier[OF assms(2)] by auto
qed

lemma independent_split:
  assumes "independent K (Us @ Vs)"
  shows "independent K Vs"
    and "independent K Us"
    and "Span K Us \<inter> Span K Vs = { \<zero> }"
proof -
  from assms show "independent K Vs"
    by (induct Us) (auto)
next
  from assms show "independent K Us"
  proof (induct Us)
    case Nil thus ?case by simp
  next
    case (Cons u Us')
    hence u: "u \<in> carrier R" and "set Us' \<subseteq> carrier R" "set Vs \<subseteq> carrier R"
      using independent_in_carrier[of K "(u # Us') @ Vs"] by auto
    hence "Span K Us' \<subseteq> Span K (Us' @ Vs)"
      using mono_Span_append(1) by simp
    thus ?case
      using independent_backwards[of K u "Us' @ Vs"] Cons li_Cons[OF u] by auto
  qed
next
  from assms show "Span K Us \<inter> Span K Vs = { \<zero> }"
  proof (induct Us rule: list.induct)
    case Nil thus ?case
      using Span_subgroup_props(2)[OF independent_in_carrier[of K Vs]] by simp
  next
    case (Cons u Us)
    hence IH: "Span K Us \<inter> Span K Vs = {\<zero>}" by auto
    have in_carrier:
      "u \<in> carrier R" "set Us \<subseteq> carrier R" "set Vs \<subseteq> carrier R" "set (u # Us) \<subseteq> carrier R"
      using Cons(2)[THEN independent_in_carrier] by auto
    hence "{ \<zero> } \<subseteq> Span K (u # Us) \<inter> Span K Vs"
      using in_carrier(3-4)[THEN Span_subgroup_props(2)] by auto

    moreover have "Span K (u # Us) \<inter> Span K Vs \<subseteq> { \<zero> }"
    proof (rule ccontr)
      assume "\<not> Span K (u # Us) \<inter> Span K Vs \<subseteq> {\<zero>}"
      hence "\<exists>a. a \<noteq> \<zero> \<and> a \<in> Span K (u # Us) \<and> a \<in> Span K Vs" by auto
      then obtain k u' v'
        where u': "u' \<in> Span K Us" "u' \<in> carrier R"
          and v': "v' \<in> Span K Vs" "v' \<in> carrier R" "v' \<noteq> \<zero>"
          and k: "k \<in> K" "(k \<otimes> u \<oplus> u') = v'"
        using line_extension_mem_iff[of _ _ u "Span K Us"] in_carrier(2-3)[THEN Span_subgroup_props(1)]
              subring_props(1) by force
      hence "v' = \<zero>" if "k = \<zero>"
        using in_carrier(1) that IH by auto
      hence diff_zero: "k \<noteq> \<zero>" using v'(3) by auto

      have "k \<in> carrier R"
        using subring_props(1) k(1) by blast
      hence "k \<otimes> u = (\<ominus> u') \<oplus> v'"
        using in_carrier(1) k(2) u'(2) v'(2) add.m_comm r_neg1 by auto
      hence "k \<otimes> u \<in> Span K (Us @ Vs)"
        using Span_subgroup_props(4)[OF in_carrier(2) u'(1)] v'(1)
              Span_append_eq_set_add[OF in_carrier(2-3)] unfolding set_add_def' by blast
      hence "u \<in> Span K (Us @ Vs)"
        using Cons(2) Span_m_inv_simprule[OF _ _ in_carrier(1), of "Us @ Vs" k]
              diff_zero k(1) in_carrier(2-3) by auto
      moreover have "u \<notin> Span K (Us @ Vs)"
        using independent_backwards(1)[of K u "Us @ Vs"] Cons(2) by auto
      ultimately show False by simp
    qed

    ultimately show ?case by auto
  qed
qed

lemma independent_append:
  assumes "independent K Us" and "independent K Vs" and "Span K Us \<inter> Span K Vs = { \<zero> }"
  shows "independent K (Us @ Vs)"
  using assms
proof (induct Us rule: list.induct)
  case Nil thus ?case by simp
next
  case (Cons u Us)
  hence in_carrier:
    "u \<in> carrier R" "set Us \<subseteq> carrier R" "set Vs \<subseteq> carrier R" "set (u # Us) \<subseteq> carrier R"
    using Cons(2-3)[THEN independent_in_carrier] by auto
  hence "Span K Us \<subseteq> Span K (u # Us)"
    using mono_Span by auto
  hence "Span K Us \<inter> Span K Vs = { \<zero> }"
    using Cons(4) Span_subgroup_props(2)[OF in_carrier(2)] by auto
  hence "independent K (Us @ Vs)"
    using Cons by auto
  moreover have "u \<notin> Span K (Us @ Vs)"
  proof (rule ccontr)
    assume "\<not> u \<notin> Span K (Us @ Vs)"
    then obtain u' v'
      where u': "u' \<in> Span K Us" "u' \<in> carrier R"
        and v': "v' \<in> Span K Vs" "v' \<in> carrier R" and u:"u = u' \<oplus> v'"
      using Span_append_eq_set_add[OF in_carrier(2-3)] in_carrier(2-3)[THEN Span_subgroup_props(1)]
      unfolding set_add_def' by blast
    hence "u \<oplus> (\<ominus> u') = v'"
      using in_carrier(1) by algebra
    moreover have "u \<in> Span K (u # Us)" and "u' \<in> Span K (u # Us)"
      using Span_base_incl[OF in_carrier(4)] mono_Span[OF in_carrier(2,1)] u'(1)
      by (auto simp del: Span.simps)
    hence "u \<oplus> (\<ominus> u') \<in> Span K (u # Us)"
      using Span_subgroup_props(3-4)[OF in_carrier(4)] by (auto simp del: Span.simps)
    ultimately have "u \<oplus> (\<ominus> u') = \<zero>"
      using Cons(4) v'(1) by auto
    hence "u = u'"
      using Cons(4) v'(1) in_carrier(1) u'(2) \<open>u \<oplus> \<ominus> u' = v'\<close> u by auto
    thus False
      using u'(1) independent_backwards(1)[OF Cons(2)] by simp
  qed
  ultimately show ?case
    using in_carrier(1) li_Cons by simp
qed

lemma independent_imp_trivial_combine:
  assumes "independent K Us"
  shows "\<And>Ks. \<lbrakk> set Ks \<subseteq> K; combine Ks Us = \<zero> \<rbrakk> \<Longrightarrow> set (take (length Us) Ks) \<subseteq> { \<zero> }"
  using assms
proof (induct Us rule: list.induct)
  case Nil thus ?case by simp
next
  case (Cons u Us) thus ?case
  proof (cases "Ks = []")
    assume "Ks = []" thus ?thesis by auto
  next
    assume "Ks \<noteq> []"
    then obtain k Ks' where k: "k \<in> K" and Ks': "set Ks' \<subseteq> K" and Ks: "Ks = k # Ks'"
      using Cons(2) by (metis insert_subset list.exhaust_sel list.simps(15))
    hence Us: "set Us \<subseteq> carrier R" and u: "u \<in> carrier R"
      using independent_in_carrier[OF Cons(4)] by auto
    have "u \<in> Span K Us" if "k \<noteq> \<zero>"
      using that Span_mem_iff[OF Us u] Cons(3-4) Ks' k unfolding Ks by blast
    hence k_zero: "k = \<zero>"
      using independent_backwards[OF Cons(4)] by blast
    hence "combine Ks' Us = \<zero>"
      using combine_in_carrier[OF _ Us, of Ks'] Ks' u Cons(3) subring_props(1) unfolding Ks by auto
    hence "set (take (length Us) Ks') \<subseteq> { \<zero> }"
      using Cons(1)[OF Ks' _ independent_backwards(2)[OF Cons(4)]] by simp
    thus ?thesis
      using k_zero unfolding Ks by auto
  qed
qed

lemma non_trivial_combine_imp_dependent:
  assumes "set Ks \<subseteq> K" and "combine Ks Us = \<zero>" and "\<not> set (take (length Us) Ks) \<subseteq> { \<zero> }"
  shows "dependent K Us"
  using independent_imp_trivial_combine[OF _ assms(1-2)] assms(3) by blast  

lemma trivial_combine_imp_independent:
  assumes "set Us \<subseteq> carrier R"
    and "\<And>Ks. \<lbrakk> set Ks \<subseteq> K; combine Ks Us = \<zero> \<rbrakk> \<Longrightarrow> set (take (length Us) Ks) \<subseteq> { \<zero> }"
  shows "independent K Us"
  using assms
proof (induct Us)
  case Nil thus ?case by simp
next
  case (Cons u Us)
  hence Us: "set Us \<subseteq> carrier R" and u: "u \<in> carrier R" by auto

  have "\<And>Ks. \<lbrakk> set Ks \<subseteq> K; combine Ks Us = \<zero> \<rbrakk> \<Longrightarrow> set (take (length Us) Ks) \<subseteq> { \<zero> }"
  proof -
    fix Ks assume Ks: "set Ks \<subseteq> K" and lin_c: "combine Ks Us = \<zero>"
    hence "combine (\<zero> # Ks) (u # Us) = \<zero>"
      using u subring_props(1) combine_in_carrier[OF _ Us] by auto
    hence "set (take (length (u # Us)) (\<zero> # Ks)) \<subseteq> { \<zero> }"
      using Cons(3)[of "\<zero> # Ks"] subring_props(2) Ks by auto
    thus "set (take (length Us) Ks) \<subseteq> { \<zero> }" by auto
  qed
  hence "independent K Us"
    using Cons(1)[OF Us] by simp

  moreover have "u \<notin> Span K Us"
  proof (rule ccontr)
    assume "\<not> u \<notin> Span K Us"
    then obtain k Ks where k: "k \<in> K" "k \<noteq> \<zero>" and Ks: "set Ks \<subseteq> K" and u: "combine (k # Ks) (u # Us) = \<zero>"
      using Span_mem_iff[OF Us u] by auto
    have "set (take (length (u # Us)) (k # Ks)) \<subseteq> { \<zero> }"
      using Cons(3)[OF _ u] k(1) Ks by auto
    hence "k = \<zero>" by auto
    from \<open>k = \<zero>\<close> and \<open>k \<noteq> \<zero>\<close> show False by simp
  qed

  ultimately show ?case
    using li_Cons[OF u] by simp
qed

corollary dependent_imp_non_trivial_combine:
  assumes "set Us \<subseteq> carrier R" and "dependent K Us"
  obtains Ks where "length Ks = length Us" "combine Ks Us = \<zero>" "set Ks \<subseteq> K" "set Ks \<noteq> { \<zero> }"
proof -
  obtain Ks
    where Ks: "set Ks \<subseteq> carrier R" "set Ks \<subseteq> K" "combine Ks Us = \<zero>" "\<not> set (take (length Us) Ks) \<subseteq> { \<zero> }"
    using trivial_combine_imp_independent[OF assms(1)] assms(2) subring_props(1) by blast
  obtain Ks'
    where Ks': "set (take (length Us) Ks) \<subseteq> set Ks'" "set Ks' \<subseteq> set (take (length Us) Ks) \<union> { \<zero> }"
               "length Ks' = length Us" "combine Ks' Us = \<zero>"
    using combine_normalize[OF Ks(1) assms(1) Ks(3)] by metis
  have "set (take (length Us) Ks) \<subseteq> set Ks"
    by (simp add: set_take_subset) 
  hence "set Ks' \<subseteq> K"
    using Ks(2) Ks'(2) subring_props(2) Un_commute by blast
  moreover have "set Ks' \<noteq> { \<zero> }"
    using Ks'(1) Ks(4) by auto
  ultimately show thesis
    using that Ks' by blast
qed

corollary unique_decomposition:
  assumes "independent K Us"
  shows "a \<in> Span K Us \<Longrightarrow> \<exists>!Ks. set Ks \<subseteq> K \<and> length Ks = length Us \<and> a = combine Ks Us"
proof -
  note in_carrier = independent_in_carrier[OF assms]

  assume "a \<in> Span K Us"
  then obtain Ks where Ks: "set Ks \<subseteq> K" "length Ks = length Us" "a = combine Ks Us"
    using Span_mem_iff_length_version[OF in_carrier] by blast

  moreover
  have "\<And>Ks'. \<lbrakk> set Ks' \<subseteq> K; length Ks' = length Us; a = combine Ks' Us \<rbrakk> \<Longrightarrow> Ks = Ks'"
  proof -
    fix Ks' assume Ks': "set Ks' \<subseteq> K" "length Ks' = length Us" "a = combine Ks' Us"
    hence set_Ks: "set Ks \<subseteq> carrier R" and set_Ks': "set Ks' \<subseteq> carrier R"
      using subring_props(1) Ks(1) by blast+
    have same_length: "length Ks = length Ks'"
      using Ks Ks' by simp

    have "(combine Ks Us) \<oplus> ((\<ominus> \<one>) \<otimes> (combine Ks' Us)) = \<zero>"
      using combine_in_carrier[OF set_Ks  in_carrier]
            combine_in_carrier[OF set_Ks' in_carrier] Ks(3) Ks'(3) by algebra
    hence "(combine Ks Us) \<oplus> (combine (map ((\<otimes>) (\<ominus> \<one>)) Ks') Us) = \<zero>"
      using combine_r_distr[OF set_Ks' in_carrier, of "\<ominus> \<one>"] subring_props by auto
    moreover have set_map: "set (map ((\<otimes>) (\<ominus> \<one>)) Ks') \<subseteq> K"
      using Ks'(1) subring_props by (induct Ks') (auto)
    hence "set (map ((\<otimes>) (\<ominus> \<one>)) Ks') \<subseteq> carrier R"
      using subring_props(1) by blast
    ultimately have "combine (map2 (\<oplus>) Ks (map ((\<otimes>) (\<ominus> \<one>)) Ks')) Us = \<zero>"
      using combine_add[OF Ks(2) _ set_Ks _ in_carrier, of "map ((\<otimes>) (\<ominus> \<one>)) Ks'"] Ks'(2) by auto
    moreover have "set (map2 (\<oplus>) Ks (map ((\<otimes>) (\<ominus> \<one>)) Ks')) \<subseteq> K"
      using Ks(1) set_map subring_props(7)
      by (induct Ks) (auto, metis contra_subsetD in_set_zipE local.set_map set_ConsD subring_props(7))
    ultimately have "set (take (length Us) (map2 (\<oplus>) Ks (map ((\<otimes>) (\<ominus> \<one>)) Ks'))) \<subseteq> { \<zero> }"
      using independent_imp_trivial_combine[OF assms] by auto
    hence "set (map2 (\<oplus>) Ks (map ((\<otimes>) (\<ominus> \<one>)) Ks')) \<subseteq> { \<zero> }"
      using Ks(2) Ks'(2) by auto
    thus "Ks = Ks'"
      using set_Ks set_Ks' same_length
    proof (induct Ks arbitrary: Ks')
      case Nil thus?case by simp
    next
      case (Cons k Ks)
      then obtain k' Ks'' where k': "Ks' = k' # Ks''"
        by (metis Suc_length_conv)
      have "Ks = Ks''"
        using Cons unfolding k' by auto
      moreover have "k = k'"
        using Cons(2-4) l_minus minus_equality unfolding k' by (auto, fastforce)
      ultimately show ?case
        unfolding k' by simp
    qed
  qed

  ultimately show ?thesis by blast
qed


subsection \<open>Replacement Theorem\<close>

lemma independent_rotate1_aux:
  "independent K (u # Us @ Vs) \<Longrightarrow> independent K ((Us @ [u]) @ Vs)"
proof -
  assume "independent K (u # Us @ Vs)"
  hence li: "independent K [u]" "independent K Us" "independent K Vs"
    and inter: "Span K [u] \<inter> Span K Us = { \<zero> }"
               "Span K (u # Us) \<inter> Span K Vs = { \<zero> }"
    using independent_split[of "u # Us" Vs] independent_split[of "[u]" Us] by auto
  hence "independent K (Us @ [u])"
    using independent_append[OF li(2,1)] by auto
  moreover have "Span K (Us @ [u]) \<inter> Span K Vs = { \<zero> }"
    using Span_same_set[of "u # Us" "Us @ [u]"] li(1-2)[THEN independent_in_carrier] inter(2) by auto
  ultimately show "independent K ((Us @ [u]) @ Vs)"
    using independent_append[OF _ li(3), of "Us @ [u]"] by simp
qed

corollary independent_rotate1:
  "independent K (Us @ Vs) \<Longrightarrow> independent K ((rotate1 Us) @ Vs)"
  using independent_rotate1_aux by (cases Us) (auto)

(*
corollary independent_rotate:
  "independent K (Us @ Vs) \<Longrightarrow> independent K ((rotate n Us) @ Vs)"
  using independent_rotate1 by (induct n) auto

lemma rotate_append: "rotate (length l) (l @ q) = q @ l"
  by (induct l arbitrary: q) (auto simp add: rotate1_rotate_swap)
*)

corollary independent_same_set:
  assumes "set Us = set Vs" and "length Us = length Vs"
  shows "independent K Us \<Longrightarrow> independent K Vs"
proof -
  assume "independent K Us" thus ?thesis
    using assms
  proof (induct Us arbitrary: Vs rule: list.induct)
    case Nil thus ?case by simp
  next
    case (Cons u Us)
    then obtain Vs' Vs'' where Vs: "Vs = Vs' @ (u # Vs'')"
      by (metis list.set_intros(1) split_list)

    have in_carrier: "u \<in> carrier R" "set Us \<subseteq> carrier R"
      using independent_in_carrier[OF Cons(2)] by auto

    have "distinct Vs"
      using Cons(3-4) independent_distinct[OF Cons(2)]
      by (metis card_distinct distinct_card)
    hence "u \<notin> set (Vs' @ Vs'')" and "u \<notin> set Us"
      using independent_distinct[OF Cons(2)] unfolding Vs by auto
    hence set_eq: "set Us = set (Vs' @ Vs'')" and "length (Vs' @ Vs'') = length Us"
      using Cons(3-4) unfolding Vs by auto
    hence "independent K (Vs' @ Vs'')"
      using Cons(1)[OF independent_backwards(2)[OF Cons(2)]] unfolding Vs by simp
    hence "independent K (u # (Vs' @ Vs''))"
      using li_Cons Span_same_set[OF _ set_eq] independent_backwards(1)[OF Cons(2)] in_carrier by auto
    hence "independent K (Vs' @ (u # Vs''))"
      using independent_rotate1[of "u # Vs'" Vs''] by auto
    thus ?case unfolding Vs .
  qed
qed

lemma replacement_theorem:
  assumes "independent K (Us' @ Us)" and "independent K Vs"
    and "Span K (Us' @ Us) \<subseteq> Span K Vs"
  shows "\<exists>Vs'. set Vs' \<subseteq> set Vs \<and> length Vs' = length Us' \<and> independent K (Vs' @ Us)"
  using assms
proof (induct "length Us'" arbitrary: Us' Us)
  case 0 thus ?case by auto
next
  case (Suc n)
  then obtain u Us'' where Us'': "Us' = Us'' @ [u]"
    by (metis list.size(3) nat.simps(3) rev_exhaust)
  then obtain Vs' where Vs': "set Vs' \<subseteq> set Vs" "length Vs' = n" "independent K (Vs' @ (u # Us))"
    using Suc(1)[of Us'' "u # Us"] Suc(2-5) by auto
  hence li: "independent K ((u # Vs') @ Us)"
    using independent_same_set[OF _ _ Vs'(3), of "(u # Vs') @ Us"] by auto
  moreover have in_carrier:
    "u \<in> carrier R" "set Us \<subseteq> carrier R" "set Us' \<subseteq> carrier R" "set Vs \<subseteq> carrier R"
    using Suc(3-4)[THEN independent_in_carrier] Us'' by auto
  moreover have "Span K ((u # Vs') @ Us) \<subseteq> Span K Vs"
  proof -
    have "set Us \<subseteq> Span K Vs" "u \<in> Span K Vs"
      using Suc(5) Span_base_incl[of "Us' @ Us"] Us'' in_carrier(2-3) by auto
    moreover have "set Vs' \<subseteq> Span K Vs"
      using Span_base_incl[OF in_carrier(4)] Vs'(1) by auto
    ultimately have "set ((u # Vs') @ Us) \<subseteq> Span K Vs" by auto
    thus ?thesis
      using mono_Span_subset[OF _ in_carrier(4)] by (simp del: Span.simps)
  qed
  ultimately obtain v where "v \<in> set Vs" "independent K ((v # Vs') @ Us)"
    using independent_replacement[OF _ Suc(4), of u "Vs' @ Us"] by auto
  thus ?case
    using Vs'(1-2) Suc(2)
    by (metis (mono_tags, lifting) insert_subset length_Cons list.simps(15))
qed

corollary independent_length_le:
  assumes "independent K Us" and "independent K Vs"
  shows "set Us \<subseteq> Span K Vs \<Longrightarrow> length Us \<le> length Vs"
proof -
  assume "set Us \<subseteq> Span K Vs"
  hence "Span K Us \<subseteq> Span K Vs"
    using mono_Span_subset[OF _ independent_in_carrier[OF assms(2)]] by simp
  then obtain Vs' where Vs': "set Vs' \<subseteq> set Vs" "length Vs' = length Us" "independent K Vs'"
    using replacement_theorem[OF _ assms(2), of Us "[]"] assms(1) by auto
  hence "card (set Vs') \<le> card (set Vs)"
    by (simp add: card_mono)
  thus "length Us \<le> length Vs"
    using independent_distinct assms(2) Vs'(2-3) by (simp add: distinct_card)
qed


subsection \<open>Dimension\<close>

lemma exists_base:
  assumes "dimension n K E"
  shows "\<exists>Vs. set Vs \<subseteq> carrier R \<and> independent K Vs \<and> length Vs = n \<and> Span K Vs = E"
    (is "\<exists>Vs. ?base K Vs E n")
  using assms
proof (induct E rule: dimension.induct)
  case zero_dim thus ?case by auto
next
  case (Suc_dim v E n K)
  then obtain Vs where Vs: "set Vs \<subseteq> carrier R" "independent K Vs" "length Vs = n" "Span K Vs = E"
    by auto
  hence "?base K (v # Vs) (line_extension K v E) (Suc n)"
    using Suc_dim li_Cons by auto
  thus ?case by blast
qed

lemma dimension_zero: "dimension 0 K E \<Longrightarrow> E = { \<zero> }"
proof -
  assume "dimension 0 K E"
  then obtain Vs where "length Vs = 0" "Span K Vs = E"
    using exists_base by blast
  thus ?thesis
    by auto
qed

lemma dimension_one [iff]: "dimension 1 K K"
proof -
  have "K = Span K [ \<one> ]"
    using line_extension_mem_iff[of _ K \<one> "{ \<zero> }"] subfieldE(3)[OF K] by (auto simp add: rev_subsetD)
  thus ?thesis
    using dimension.Suc_dim[OF one_closed _ dimension.zero_dim, of K] subfieldE(6)[OF K] by auto 
qed

lemma dimensionI:
  assumes "independent K Us" "Span K Us = E"
  shows "dimension (length Us) K E"
  using dimension_independent[OF assms(1)] assms(2) by simp

lemma space_subgroup_props:
  assumes "dimension n K E"
  shows "E \<subseteq> carrier R"
    and "\<zero> \<in> E"
    and "\<And>v1 v2. \<lbrakk> v1 \<in> E; v2 \<in> E \<rbrakk> \<Longrightarrow> (v1 \<oplus> v2) \<in> E"
    and "\<And>v. v \<in> E \<Longrightarrow> (\<ominus> v) \<in> E"
    and "\<And>k v. \<lbrakk> k \<in> K; v \<in> E \<rbrakk> \<Longrightarrow> k \<otimes> v \<in> E"
    and "\<lbrakk> k \<in> K - { \<zero> }; a \<in> carrier R \<rbrakk> \<Longrightarrow> k \<otimes> a \<in> E \<Longrightarrow> a \<in> E"
  using exists_base[OF assms] Span_subgroup_props Span_smult_closed Span_m_inv_simprule by auto

lemma independent_length_le_dimension:
  assumes "dimension n K E" and "independent K Us" "set Us \<subseteq> E"
  shows "length Us \<le> n"
proof -
  obtain Vs where Vs: "set Vs \<subseteq> carrier R" "independent K Vs" "length Vs = n" "Span K Vs = E"
    using exists_base[OF assms(1)] by auto
  thus ?thesis
    using independent_length_le assms(2-3) by auto
qed

lemma dimension_is_inj:
  assumes "dimension n K E" and "dimension m K E"
  shows "n = m"
proof -
  { fix n m assume n: "dimension n K E" and m: "dimension m K E"
    then obtain Vs
      where Vs: "set Vs \<subseteq> carrier R" "independent K Vs" "length Vs = n" "Span K Vs = E"
      using exists_base by meson
    hence "n \<le> m"
      using independent_length_le_dimension[OF m Vs(2)] Span_base_incl[OF Vs(1)] by auto
  } note aux_lemma = this

  show ?thesis
    using aux_lemma[OF assms] aux_lemma[OF assms(2,1)] by simp
qed

corollary independent_length_eq_dimension:
  assumes "dimension n K E" and "independent K Us" "set Us \<subseteq> E"
  shows "length Us = n \<longleftrightarrow> Span K Us = E"
proof
  assume len: "length Us = n" show "Span K Us = E"
  proof (rule ccontr)
    assume "Span K Us \<noteq> E"
    hence "Span K Us \<subset> E"
      using mono_Span_subset[of Us] exists_base[OF assms(1)] assms(3) by blast
    then obtain v where v: "v \<in> E" "v \<notin> Span K Us"
      using Span_strict_incl exists_base[OF assms(1)] space_subgroup_props(1)[OF assms(1)] assms by blast
    hence "independent K (v # Us)"
      using li_Cons[OF _ _ assms(2)] space_subgroup_props(1)[OF assms(1)] by auto
    hence "length (v # Us) \<le> n"
      using independent_length_le_dimension[OF assms(1)] v(1) assms(2-3) by fastforce
    moreover have "length (v # Us) = Suc n"
      using len by simp
    ultimately show False by simp
  qed
next
  assume "Span K Us = E"
  hence "dimension (length Us) K E"
    using dimensionI assms by auto
  thus "length Us = n"
    using dimension_is_inj[OF assms(1)] by auto
qed

lemma complete_base:
  assumes "dimension n K E" and "independent K Us" "set Us \<subseteq> E"
  shows "\<exists>Vs. length (Vs @ Us) = n \<and> independent K (Vs @ Us) \<and> Span K (Vs @ Us) = E"
proof -
  { fix Us k assume "k \<le> n" "independent K Us" "set Us \<subseteq> E" "length Us = k"
    hence "\<exists>Vs. length (Vs @ Us) = n \<and> independent K (Vs @ Us) \<and> Span K (Vs @ Us) = E"
    proof (induct arbitrary: Us rule: inc_induct)
      case base thus ?case
        using independent_length_eq_dimension[OF assms(1) base(1-2)] by auto
    next
      case (step m)
      have "Span K Us \<subseteq> E"
        using mono_Span_subset step(4-6) exists_base[OF assms(1)] by blast
      hence "Span K Us \<subset> E"
        using independent_length_eq_dimension[OF assms(1) step(4-5)] step(2,6) assms(1) by blast
      then obtain v where v: "v \<in> E" "v \<notin> Span K Us"
        using Span_strict_incl exists_base[OF assms(1)] by blast
      hence "independent K (v # Us)"
        using space_subgroup_props(1)[OF assms(1)] li_Cons[OF _ v(2) step(4)] by auto
      then obtain Vs
        where "length (Vs @ (v # Us)) = n" "independent K (Vs @ (v # Us))" "Span K (Vs @ (v # Us)) = E"
        using step(3)[of "v # Us"] step(1-2,4-6) v by auto
      thus ?case
        by (metis append.assoc append_Cons append_Nil)
    qed } note aux_lemma = this

  have "length Us \<le> n"
    using independent_length_le_dimension[OF assms] .
  thus ?thesis
    using aux_lemma[OF _ assms(2-3)] by auto
qed

lemma filter_base:
  assumes "set Us \<subseteq> carrier R"
  obtains Vs where "set Vs \<subseteq> carrier R" and "independent K Vs" and "Span K Vs = Span K Us"
proof -
  from \<open>set Us \<subseteq> carrier R\<close> have "\<exists>Vs. independent K Vs \<and> Span K Vs = Span K Us"
  proof (induction Us)
    case Nil thus ?case by auto
  next
    case (Cons u Us)
    then obtain Vs where Vs: "independent K Vs" "Span K Vs = Span K Us"
      by auto
    show ?case
    proof (cases "u \<in> Span K Us")
      case True
      hence "Span K (u # Us) = Span K Us"
        using Span_base_incl mono_Span_subset
        by (metis Cons.prems insert_subset list.simps(15) subset_antisym)
      thus ?thesis
        using Vs by blast
    next
      case False
      hence "Span K (u # Vs) = Span K (u # Us)" and "independent K (u # Vs)"
        using li_Cons[of u K Vs] Cons(2) Vs by auto
      thus ?thesis
        by blast
    qed
  qed
  thus ?thesis
    using independent_in_carrier that by auto
qed

lemma dimension_backwards:
  "dimension (Suc n) K E \<Longrightarrow> \<exists>v \<in> carrier R. \<exists>E'. dimension n K E' \<and> v \<notin> E' \<and> E = line_extension K v E'"
  by (cases rule: dimension.cases) (auto)

lemma dimension_direct_sum_space:
  assumes "dimension n K E" and "dimension m K F" and "E \<inter> F = { \<zero> }"
  shows "dimension (n + m) K (E <+>\<^bsub>R\<^esub> F)"
proof -
  obtain Us Vs
    where Vs: "set Vs \<subseteq> carrier R" "independent K Vs" "length Vs = n" "Span K Vs = E"
      and Us: "set Us \<subseteq> carrier R" "independent K Us" "length Us = m" "Span K Us = F"
    using assms(1-2)[THEN exists_base] by auto
  hence "Span K (Vs @ Us) = E <+>\<^bsub>R\<^esub> F"
    using Span_append_eq_set_add by auto
  moreover have "independent K (Vs @ Us)"
    using assms(3) independent_append[OF Vs(2) Us(2)] unfolding Vs(4) Us(4) by simp
  ultimately show "dimension (n + m) K (E <+>\<^bsub>R\<^esub> F)"
    using dimensionI[of "Vs @ Us"] Vs(3) Us(3) by auto
qed

lemma dimension_sum_space:
  assumes "dimension n K E" and "dimension m K F" and "dimension k K (E \<inter> F)"
  shows "dimension (n + m - k) K (E <+>\<^bsub>R\<^esub> F)"
proof -
  obtain Bs
    where Bs: "set Bs \<subseteq> carrier R" "length Bs = k" "independent K Bs" "Span K Bs = E \<inter> F"
    using exists_base[OF assms(3)] by blast
  then obtain Us Vs
    where Us: "length (Us @ Bs) = n" "independent K (Us @ Bs)" "Span K (Us @ Bs) = E"
      and Vs: "length (Vs @ Bs) = m" "independent K (Vs @ Bs)" "Span K (Vs @ Bs) = F"
    using Span_base_incl[OF Bs(1)] assms(1-2)[THEN complete_base] by (metis le_infE)
  hence in_carrier: "set Us \<subseteq> carrier R" "set (Vs @ Bs) \<subseteq> carrier R"
    using independent_in_carrier[OF Us(2)] independent_in_carrier[OF Vs(2)] by auto
  hence "Span K Us \<inter> (Span K (Vs @ Bs)) \<subseteq> Span K Bs"
    using Bs(4) Us(3) Vs(3) mono_Span_append(1)[OF _ Bs(1), of Us] by auto
  hence "Span K Us \<inter> (Span K (Vs @ Bs)) \<subseteq> { \<zero> }"
    using independent_split(3)[OF Us(2)] by blast
  hence "Span K Us \<inter> (Span K (Vs @ Bs)) = { \<zero> }"
    using in_carrier[THEN Span_subgroup_props(2)] by auto

  hence dim: "dimension (n + m - k) K (Span K (Us @ (Vs @ Bs)))"
    using independent_append[OF independent_split(2)[OF Us(2)] Vs(2)] Us(1) Vs(1) Bs(2)
          dimension_independent[of K "Us @ (Vs @ Bs)"] by auto

  have "(Span K Us) <+>\<^bsub>R\<^esub> F \<subseteq> E <+>\<^bsub>R\<^esub> F"
    using mono_Span_append(1)[OF in_carrier(1) Bs(1)] Us(3) unfolding set_add_def' by auto
  moreover have "E <+>\<^bsub>R\<^esub> F \<subseteq> (Span K Us) <+>\<^bsub>R\<^esub> F"
  proof
    fix v assume "v \<in> E <+>\<^bsub>R\<^esub> F"
    then obtain u' v' where v: "u' \<in> E" "v' \<in> F" "v = u' \<oplus> v'"
      unfolding set_add_def' by auto
    then obtain u1' u2' where u1': "u1' \<in> Span K Us" and u2': "u2' \<in> Span K Bs" and u': "u' = u1' \<oplus> u2'"
      using Span_append_eq_set_add[OF in_carrier(1) Bs(1)] Us(3) unfolding set_add_def' by blast

    have "v = u1' \<oplus> (u2' \<oplus> v')"
      using Span_subgroup_props(1)[OF Bs(1)] Span_subgroup_props(1)[OF in_carrier(1)]
            space_subgroup_props(1)[OF assms(2)] u' v u1' u2' a_assoc[of u1' u2' v'] by auto
    moreover have "u2' \<oplus> v' \<in> F"
      using space_subgroup_props(3)[OF assms(2) _ v(2)] u2' Bs(4) by auto
    ultimately show "v \<in> (Span K Us) <+>\<^bsub>R\<^esub> F"
      using u1' unfolding set_add_def' by auto
  qed
  ultimately have "Span K (Us @ (Vs @ Bs)) = E <+>\<^bsub>R\<^esub> F"
    using Span_append_eq_set_add[OF in_carrier] Vs(3) by auto

  thus ?thesis using dim by simp
qed

end (* of fixed K context. *)

end (* of ring context. *)


lemma (in ring) telescopic_base_aux:
  assumes "subfield K R" "subfield F R"
    and "dimension n K F" and "dimension 1 F E"
  shows "dimension n K E"
proof -
  obtain Us u
    where Us: "set Us \<subseteq> carrier R" "length Us = n" "independent K Us" "Span K Us = F"
      and u: "u \<in> carrier R" "independent F [u]" "Span F [u] = E"
    using exists_base[OF assms(2,4)] exists_base[OF assms(1,3)] independent_backwards(3) assms(2)
    by (metis One_nat_def length_0_conv length_Suc_conv)
  have in_carrier: "set (map (\<lambda>u'. u' \<otimes> u) Us) \<subseteq> carrier R"
    using Us(1) u(1) by (induct Us) (auto)

  have li: "independent K (map (\<lambda>u'. u' \<otimes> u) Us)"
  proof (rule trivial_combine_imp_independent[OF assms(1) in_carrier])
    fix Ks assume Ks: "set Ks \<subseteq> K" and "combine Ks (map (\<lambda>u'. u' \<otimes> u) Us) = \<zero>"
    hence "(combine Ks Us) \<otimes> u = \<zero>"
      using combine_l_distr[OF _ Us(1) u(1)] subring_props(1)[OF assms(1)] by auto
    hence "combine [ combine Ks Us ] [ u ] = \<zero>"
      by simp
    moreover have "combine Ks Us \<in> F"
      using Us(4) Ks(1) Span_eq_combine_set[OF assms(1) Us(1)] by auto
    ultimately have "combine Ks Us = \<zero>"
      using independent_imp_trivial_combine[OF assms(2) u(2), of "[ combine Ks Us ]"] by auto
    hence "set (take (length Us) Ks) \<subseteq> { \<zero> }"
      using independent_imp_trivial_combine[OF assms(1) Us(3) Ks(1)] by simp
    thus "set (take (length (map (\<lambda>u'. u' \<otimes> u) Us)) Ks) \<subseteq> { \<zero> }" by simp
  qed

  have "E \<subseteq> Span K (map (\<lambda>u'. u' \<otimes> u) Us)"
  proof
    fix v assume "v \<in> E"
    then obtain f where f: "f \<in> F" "v = f \<otimes> u \<oplus> \<zero>"
      using u(1,3) line_extension_mem_iff by auto
    then obtain Ks where Ks: "set Ks \<subseteq> K" "f = combine Ks Us"
      using Span_eq_combine_set[OF assms(1) Us(1)] Us(4) by auto
    have "v = f \<otimes> u"
      using subring_props(1)[OF assms(2)] f u(1) by auto
    hence "v = combine Ks (map (\<lambda>u'. u' \<otimes> u) Us)"
      using combine_l_distr[OF _ Us(1) u(1), of Ks] Ks(1-2)
            subring_props(1)[OF assms(1)] by blast
    thus "v \<in> Span K (map (\<lambda>u'. u' \<otimes> u) Us)"
      unfolding Span_eq_combine_set[OF assms(1) in_carrier] using Ks(1) by blast
  qed
  moreover have "Span K (map (\<lambda>u'. u' \<otimes> u) Us) \<subseteq> E"
  proof
    fix v assume "v \<in> Span K (map (\<lambda>u'. u' \<otimes> u) Us)"
    then obtain Ks where Ks: "set Ks \<subseteq> K" "v = combine Ks (map (\<lambda>u'. u' \<otimes> u) Us)"
      unfolding Span_eq_combine_set[OF assms(1) in_carrier] by blast
    hence "v = (combine Ks Us) \<otimes> u"
      using combine_l_distr[OF _ Us(1) u(1), of Ks] subring_props(1)[OF assms(1)] by auto
    moreover have "combine Ks Us \<in> F"
      using Us(4) Span_eq_combine_set[OF assms(1) Us(1)] Ks(1) by blast
    ultimately have "v = (combine Ks Us) \<otimes> u \<oplus> \<zero>" and "combine Ks Us \<in> F"
      using subring_props(1)[OF assms(2)] u(1) by auto
    thus "v \<in> E"
      using u(3) line_extension_mem_iff by auto
  qed
  ultimately have "Span K (map (\<lambda>u'. u' \<otimes> u) Us) = E" by auto
  thus ?thesis
    using dimensionI[OF assms(1) li] Us(2) by simp
qed

lemma (in ring) telescopic_base:
  assumes "subfield K R" "subfield F R"
    and "dimension n K F" and "dimension m F E"
  shows "dimension (n * m) K E"
  using assms(4)
proof (induct m arbitrary: E)
  case 0 thus ?case
    using dimension_zero[OF assms(2)] zero_dim by auto
next
  case (Suc m)
  obtain Vs
    where Vs: "set Vs \<subseteq> carrier R" "length Vs = Suc m" "independent F Vs" "Span F Vs = E"
    using exists_base[OF assms(2) Suc(2)] by blast
  then obtain v Vs' where v: "Vs = v # Vs'"
    by (meson length_Suc_conv)
  hence li: "independent F [ v ]" "independent F Vs'" and inter: "Span F [ v ] \<inter> Span F Vs' = { \<zero> }"
    using Vs(3) independent_split[OF assms(2), of "[ v ]" Vs'] by auto
  have "dimension n K (Span F [ v ])"
    using dimension_independent[OF li(1)] telescopic_base_aux[OF assms(1-3)] by simp
  moreover have "dimension (n * m) K (Span F Vs')"
    using Suc(1) dimension_independent[OF li(2)] Vs(2) unfolding v by auto
  ultimately have "dimension (n * Suc m) K (Span F [ v ] <+>\<^bsub>R\<^esub> Span F Vs')"
    using dimension_direct_sum_space[OF assms(1) _ _ inter] by auto
  thus "dimension (n * Suc m) K E"
    using Span_append_eq_set_add[OF assms(2) li[THEN independent_in_carrier]] Vs(4) v by auto
qed


context ring_hom_ring
begin

lemma combine_hom:
  "\<lbrakk> set Ks \<subseteq> carrier R; set Us \<subseteq> carrier R \<rbrakk> \<Longrightarrow> combine (map h Ks) (map h Us) = h (R.combine Ks Us)"
  by (induct Ks Us rule: R.combine.induct) (auto)

lemma line_extension_hom:
  assumes "K \<subseteq> carrier R" "a \<in> carrier R" "E \<subseteq> carrier R"
  shows "line_extension (h ` K) (h a) (h ` E) = h ` R.line_extension K a E"
  using set_add_hom[OF homh R.r_coset_subset_G[OF assms(1-2)] assms(3)]
        coset_hom(2)[OF ring_hom_in_hom(1)[OF homh] assms(1-2)]
  unfolding R.line_extension_def S.line_extension_def
  by simp

lemma Span_hom:
  assumes "K \<subseteq> carrier R" "set Us \<subseteq> carrier R"
  shows "Span (h ` K) (map h Us) = h ` R.Span K Us"
  using assms line_extension_hom R.Span_in_carrier by (induct Us) (auto)

lemma inj_on_subgroup_iff_trivial_ker:
  assumes "subgroup H (add_monoid R)"
  shows "inj_on h H \<longleftrightarrow> a_kernel (R \<lparr> carrier := H \<rparr>) S h = { \<zero> }"
  using group_hom.inj_on_subgroup_iff_trivial_ker[OF a_group_hom assms]
  unfolding a_kernel_def[of "R \<lparr> carrier := H \<rparr>" S h] by simp

corollary inj_on_Span_iff_trivial_ker:
  assumes "subfield K R" "set Us \<subseteq> carrier R"
  shows "inj_on h (R.Span K Us) \<longleftrightarrow> a_kernel (R \<lparr> carrier := R.Span K Us \<rparr>) S h = { \<zero> }"
  using inj_on_subgroup_iff_trivial_ker[OF R.Span_is_add_subgroup[OF assms]] .


context
  fixes K :: "'a set" assumes K: "subfield K R" and one_zero: "\<one>\<^bsub>S\<^esub> \<noteq> \<zero>\<^bsub>S\<^esub>"
begin

lemma inj_hom_preserves_independent:
  assumes "inj_on h (R.Span K Us)"
  and "R.independent K Us" shows "independent (h ` K) (map h Us)"
proof (rule ccontr)
  have in_carrier: "set Us \<subseteq> carrier R" "set (map h Us) \<subseteq> carrier S"
    using R.independent_in_carrier[OF assms(2)] by auto 

  assume ld: "dependent (h ` K) (map h Us)"
  obtain Ks :: "'c list"
    where Ks: "length Ks = length Us" "combine Ks (map h Us) = \<zero>\<^bsub>S\<^esub>" "set Ks \<subseteq> h ` K" "set Ks \<noteq> { \<zero>\<^bsub>S\<^esub> }"
    using dependent_imp_non_trivial_combine[OF img_is_subfield(2)[OF K one_zero] in_carrier(2) ld]
    by (metis length_map)
  obtain Ks' where Ks': "set Ks' \<subseteq> K" "Ks = map h Ks'"
    using Ks(3) by (induct Ks) (auto, metis insert_subset list.simps(15,9))
  hence "h (R.combine Ks' Us) = \<zero>\<^bsub>S\<^esub>"
    using combine_hom[OF _ in_carrier(1)] Ks(2) subfieldE(3)[OF K] by (metis subset_trans)
  moreover have "R.combine Ks' Us \<in> R.Span K Us"
    using R.Span_eq_combine_set[OF K in_carrier(1)] Ks'(1) by auto
  ultimately have "R.combine Ks' Us = \<zero>"
    using assms hom_zero R.Span_subgroup_props(2)[OF K in_carrier(1)] by (auto simp add: inj_on_def)
  hence "set Ks' \<subseteq> { \<zero> }"
    using R.independent_imp_trivial_combine[OF K assms(2)] Ks' Ks(1)
    by (metis length_map order_refl take_all)
  hence "set Ks \<subseteq> { \<zero>\<^bsub>S\<^esub> }"
    unfolding Ks' using hom_zero by (induct Ks') (auto)
  hence "Ks = []"
    using Ks(4) by (metis set_empty2 subset_singletonD)
  hence "independent (h ` K) (map h Us)"
    using independent.li_Nil Ks(1) by simp
  from \<open>dependent (h ` K) (map h Us)\<close> and this show False by simp
qed

corollary inj_hom_dimension:
  assumes "inj_on h E"
  and "R.dimension n K E" shows "dimension n (h ` K) (h ` E)"
proof -
  obtain Us
    where Us: "set Us \<subseteq> carrier R" "R.independent K Us" "length Us = n" "R.Span K Us = E"
    using R.exists_base[OF K assms(2)] by blast
  hence "dimension n (h ` K) (Span (h ` K) (map h Us))"
    using dimension_independent[OF inj_hom_preserves_independent[OF _ Us(2)]] assms(1) by auto
  thus ?thesis
    using Span_hom[OF subfieldE(3)[OF K] Us(1)] Us(4) by simp
qed

corollary rank_nullity_theorem:
  assumes "R.dimension n K E" and "R.dimension m K (a_kernel (R \<lparr> carrier := E \<rparr>) S h)"
  shows "dimension (n - m) (h ` K) (h ` E)"
proof -
  obtain Us
    where Us: "set Us \<subseteq> carrier R" "R.independent K Us" "length Us = m"
              "R.Span K Us = a_kernel (R \<lparr> carrier := E \<rparr>) S h"
    using R.exists_base[OF K assms(2)] by blast
  obtain Vs
    where Vs: "R.independent K (Vs @ Us)" "length (Vs @ Us) = n" "R.Span K (Vs @ Us) = E" 
    using R.complete_base[OF K assms(1) Us(2)] R.Span_base_incl[OF K Us(1)] Us(4)
    unfolding a_kernel_def' by auto
  have set_Vs: "set Vs \<subseteq> carrier R"
    using R.independent_in_carrier[OF Vs(1)] by auto
  have "R.Span K Vs \<inter> a_kernel (R \<lparr> carrier := E \<rparr>) S h = { \<zero> }"
    using R.independent_split[OF K Vs(1)] Us(4) by simp
  moreover have "R.Span K Vs \<subseteq> E"
    using R.mono_Span_append(1)[OF K set_Vs Us(1)] Vs(3) by auto
  ultimately have "a_kernel (R \<lparr> carrier := R.Span K Vs \<rparr>) S h \<subseteq> { \<zero> }"
    unfolding a_kernel_def' by (simp del: R.Span.simps, blast)
  hence "a_kernel (R \<lparr> carrier := R.Span K Vs \<rparr>) S h = { \<zero> }"
    using R.Span_subgroup_props(2)[OF K set_Vs]
    unfolding a_kernel_def' by (auto simp del: R.Span.simps)
  hence "inj_on h (R.Span K Vs)"
    using inj_on_Span_iff_trivial_ker[OF K set_Vs] by simp
  moreover have "R.dimension (n - m) K (R.Span K Vs)"
    using R.dimension_independent[OF R.independent_split(2)[OF K Vs(1)]] Vs(2) Us(3) by auto
  ultimately have "dimension (n - m) (h ` K) (h ` (R.Span K Vs))"
    using assms(1) inj_hom_dimension by simp

  have "h ` E = h ` (R.Span K Vs <+>\<^bsub>R\<^esub> R.Span K Us)"
    using R.Span_append_eq_set_add[OF K set_Vs Us(1)] Vs(3) by simp
  hence "h ` E = h ` (R.Span K Vs) <+>\<^bsub>S\<^esub> h ` (R.Span K Us)"
    using R.Span_subgroup_props(1)[OF K] set_Vs Us(1) set_add_hom[OF homh] by auto
  moreover have "h ` (R.Span K Us) = { \<zero>\<^bsub>S\<^esub> }"
    using R.space_subgroup_props(2)[OF K assms(1)] unfolding Us(4) a_kernel_def' by force
  ultimately have "h ` E = h ` (R.Span K Vs) <+>\<^bsub>S\<^esub> { \<zero>\<^bsub>S\<^esub> }"
    by simp
  hence "h ` E = h ` (R.Span K Vs)"
    using R.Span_subgroup_props(1-2)[OF K set_Vs] unfolding set_add_def' by force

  from \<open>dimension (n - m) (h ` K) (h ` (R.Span K Vs))\<close> and this show ?thesis by simp
qed

end (* of fixed K context. *)

end (* of ring_hom_ring context. *)

lemma (in ring_hom_ring)
  assumes "subfield K R" and "set Us \<subseteq> carrier R" and "\<one>\<^bsub>S\<^esub> \<noteq> \<zero>\<^bsub>S\<^esub>"
    and "independent (h ` K) (map h Us)" shows "R.independent K Us"
proof (rule ccontr)
  assume "R.dependent K Us"
  then obtain Ks
    where "length Ks = length Us" and "R.combine Ks Us = \<zero>" and "set Ks \<subseteq> K" and "set Ks \<noteq> { \<zero> }"
    using R.dependent_imp_non_trivial_combine[OF assms(1-2)] by metis
  hence "combine (map h Ks) (map h Us) = \<zero>\<^bsub>S\<^esub>"
    using combine_hom[OF _ assms(2), of Ks] subfieldE(3)[OF assms(1)] by simp
  moreover from \<open>set Ks \<subseteq> K\<close> have "set (map h Ks) \<subseteq> h ` K"
    by (induction Ks) (auto)
  moreover have "\<not> set (map h Ks) \<subseteq> { h \<zero> }"
  proof (rule ccontr)
    assume "\<not> \<not> set (map h Ks) \<subseteq> { h \<zero> }" then have "set (map h Ks) \<subseteq> { h \<zero> }"
      by simp
    moreover from \<open>R.dependent K Us\<close> and \<open>length Ks = length Us\<close> have "Ks \<noteq> []"
      by auto
    ultimately have "set (map h Ks) = { h \<zero> }"
      using subset_singletonD by fastforce
    with \<open>set Ks \<subseteq> K\<close> have "set Ks = { \<zero> }"
      using inj_onD[OF _ _ _ subringE(2)[OF subfieldE(1)[OF assms(1)]], of h]
            img_is_subfield(1)[OF assms(1,3)] subset_singletonD
      by (induction Ks) (auto simp add: subset_singletonD, fastforce)
    with \<open>set Ks \<noteq> { \<zero> }\<close> show False
      by simp
  qed
  with \<open>length Ks = length Us\<close> have "\<not> set (take (length (map h Us)) (map h Ks)) \<subseteq> { h \<zero> }"
    by auto
  ultimately have "dependent (h ` K) (map h Us)"
    using non_trivial_combine_imp_dependent[OF img_is_subfield(2)[OF assms(1,3)], of "map h Ks"] by simp
  with \<open>independent (h ` K) (map h Us)\<close> show False
    by simp
qed


subsection \<open>Finite Dimension\<close>

definition (in ring) finite_dimension :: "'a set \<Rightarrow> 'a set \<Rightarrow> bool"
  where "finite_dimension K E \<longleftrightarrow> (\<exists>n. dimension n K E)"

abbreviation (in ring) infinite_dimension :: "'a set \<Rightarrow> 'a set \<Rightarrow> bool"
  where "infinite_dimension K E \<equiv> \<not> finite_dimension K E"

definition (in ring) dim :: "'a set \<Rightarrow> 'a set \<Rightarrow> nat"
  where "dim K E = (THE n. dimension n K E)"

locale subalgebra = subgroup V "add_monoid R" for K and V and R (structure) +
  assumes smult_closed: "\<lbrakk> k \<in> K; v \<in> V \<rbrakk> \<Longrightarrow> k \<otimes> v \<in> V"


subsubsection \<open>Basic Properties\<close>

lemma (in ring) unique_dimension:
  assumes "subfield K R" and "finite_dimension K E" shows "\<exists>!n. dimension n K E"
  using assms(2) dimension_is_inj[OF assms(1)] unfolding finite_dimension_def by auto

lemma (in ring) finite_dimensionI:
  assumes "dimension n K E" shows "finite_dimension K E"
  using assms unfolding finite_dimension_def by auto

lemma (in ring) finite_dimensionE:
  assumes "subfield K R" and "finite_dimension K E" shows "dimension ((dim over K) E) K E"
  using theI'[OF unique_dimension[OF assms]] unfolding over_def dim_def by simp

lemma (in ring) dimI:
  assumes "subfield K R" and "dimension n K E" shows "(dim over K) E = n"
  using finite_dimensionE[OF assms(1) finite_dimensionI] dimension_is_inj[OF assms(1)] assms(2)
  unfolding over_def dim_def by auto

lemma (in ring) finite_dimensionE' [elim]:
  assumes "finite_dimension K E" and "\<And>n. dimension n K E \<Longrightarrow> P" shows P
  using assms unfolding finite_dimension_def by auto

lemma (in ring) Span_finite_dimension:
  assumes "subfield K R" and "set Us \<subseteq> carrier R"
  shows "finite_dimension K (Span K Us)"
  using filter_base[OF assms] finite_dimensionI[OF dimension_independent[of K]] by metis

lemma (in ring) carrier_is_subalgebra:
  assumes "K \<subseteq> carrier R" shows "subalgebra K (carrier R) R"
  using assms subalgebra.intro[OF add.group_incl_imp_subgroup[of "carrier R"], of K] add.group_axioms
  unfolding subalgebra_axioms_def by auto

lemma (in ring) subalgebra_in_carrier:
  assumes "subalgebra K V R" shows "V \<subseteq> carrier R"
  using subgroup.subset[OF subalgebra.axioms(1)[OF assms]] by simp

lemma (in ring) subalgebra_inter:
  assumes "subalgebra K V R" and "subalgebra K V' R" shows "subalgebra K (V \<inter> V') R"
  using add.subgroups_Inter_pair assms unfolding subalgebra_def subalgebra_axioms_def by auto

lemma (in ring_hom_ring) img_is_subalgebra:
  assumes "K \<subseteq> carrier R" and "subalgebra K V R" shows "subalgebra (h ` K) (h ` V) S"
proof (intro subalgebra.intro)
  have "group_hom (add_monoid R) (add_monoid S) h"
    using ring_hom_in_hom(2)[OF homh] R.add.group_axioms add.group_axioms
    unfolding group_hom_def group_hom_axioms_def by auto
  thus "subgroup (h ` V) (add_monoid S)"
    using group_hom.subgroup_img_is_subgroup[OF _ subalgebra.axioms(1)[OF assms(2)]] by force
next
  show "subalgebra_axioms (h ` K) (h ` V) S"
    using R.subalgebra_in_carrier[OF assms(2)] subalgebra.axioms(2)[OF assms(2)] assms(1)
    unfolding subalgebra_axioms_def
    by (auto, metis hom_mult image_eqI subset_iff)
qed

lemma (in ring) ideal_is_subalgebra:
  assumes "K \<subseteq> carrier R" "ideal I R" shows "subalgebra K I R"
  using ideal.axioms(1)[OF assms(2)] ideal.I_l_closed[OF assms(2)] assms(1)
  unfolding subalgebra_def subalgebra_axioms_def additive_subgroup_def by auto

lemma (in ring) Span_is_subalgebra:
  assumes "subfield K R" "set Us \<subseteq> carrier R" shows "subalgebra K (Span K Us) R"
  using Span_smult_closed[OF assms] Span_is_add_subgroup[OF assms]
  unfolding subalgebra_def subalgebra_axioms_def by auto

lemma (in ring) finite_dimension_imp_subalgebra:
  assumes "subfield K R" "finite_dimension K E" shows "subalgebra K E R"
  using exists_base[OF assms(1) finite_dimensionE[OF assms]] Span_is_subalgebra[OF assms(1)] by auto

lemma (in ring) subalgebra_Span_incl:
  assumes "subfield K R" and "subalgebra K V R" "set Us \<subseteq> V" shows "Span K Us \<subseteq> V"
proof -
  have "K <#> (set Us) \<subseteq> V"
    using subalgebra.smult_closed[OF assms(2)] assms(3) unfolding set_mult_def by blast
  moreover have "set Us \<subseteq> carrier R"
    using subalgebra_in_carrier[OF assms(2)] assms(3) by auto
  ultimately show ?thesis
    using subalgebra.axioms(1)[OF assms(2)] Span_min[OF assms(1)] by blast
qed

lemma (in ring) Span_subalgebra_minimal:
  assumes "subfield K R" "set Us \<subseteq> carrier R"
  shows "Span K Us = \<Inter> { V. subalgebra K V R \<and> set Us \<subseteq> V }"
  using Span_is_subalgebra[OF assms] Span_base_incl[OF assms] subalgebra_Span_incl[OF assms(1)]
  by blast

lemma (in ring) Span_subalgebraI:
  assumes "subfield K R"
    and "subalgebra K E R" "set Us \<subseteq> E"
    and "\<And>V. \<lbrakk> subalgebra K V R; set Us \<subseteq> V \<rbrakk> \<Longrightarrow> E \<subseteq> V"
  shows "E = Span K Us"
proof -
  have "\<Inter> { V. subalgebra K V R \<and> set Us \<subseteq> V } = E"
    using assms(2-4) by auto
  thus "E = Span K Us"
    using Span_subalgebra_minimal subalgebra_in_carrier[of K E] assms by auto
qed

lemma (in ring) subalbegra_incl_imp_finite_dimension:
  assumes "subfield K R" and "finite_dimension K E"
  and "subalgebra K V R" "V \<subseteq> E" shows "finite_dimension K V"
proof -
  obtain n where n: "dimension n K E"
    using assms(2) by auto

  define S where "S = { Us. set Us \<subseteq> V \<and> independent K Us }"
  have "length ` S \<subseteq> {..n}"
    unfolding S_def using independent_length_le_dimension[OF assms(1) n] assms(4) by auto
  moreover have "[] \<in> S"
    unfolding S_def by simp
  hence "length ` S \<noteq> {}" by blast
  ultimately obtain m where m: "m \<in> length ` S" and greatest: "\<And>k. k \<in> length ` S \<Longrightarrow> k \<le> m"
    by (meson Max_ge Max_in finite_atMost rev_finite_subset)
  then obtain Us where Us: "set Us \<subseteq> V" "independent K Us" "m = length Us"
      unfolding S_def by auto
  have "Span K Us = V"
  proof (rule ccontr)
    assume "\<not> Span K Us = V" then have "Span K Us \<subset> V"
      using subalgebra_Span_incl[OF assms(1,3) Us(1)] by blast
    then obtain v where v:"v \<in> V" "v \<notin> Span K Us"
      by blast
    hence "independent K (v # Us)"
      using independent.li_Cons[OF _ _ Us(2)] subalgebra_in_carrier[OF assms(3)] by auto
    hence "(v # Us) \<in> S"
      unfolding S_def using Us(1) v(1) by auto
    hence "length (v # Us) \<le> m"
      using greatest by blast
    moreover have "length (v # Us) = Suc m"
      using Us(3) by auto
    ultimately show False by simp
  qed
  thus ?thesis
    using finite_dimensionI[OF dimension_independent[OF Us(2)]] by simp
qed

lemma (in ring_hom_ring) infinite_dimension_hom:
  assumes "subfield K R" and "\<one>\<^bsub>S\<^esub> \<noteq> \<zero>\<^bsub>S\<^esub>" and "inj_on h E" and "subalgebra K E R"
  shows "R.infinite_dimension K E \<Longrightarrow> infinite_dimension (h ` K) (h ` E)"
proof -
  note subfield = img_is_subfield(2)[OF assms(1-2)]

  assume "R.infinite_dimension K E"
  show "infinite_dimension (h ` K) (h ` E)"
  proof (rule ccontr)
    assume "\<not> infinite_dimension (h ` K) (h ` E)"
    then obtain Vs where "set Vs \<subseteq> carrier S" and "Span (h ` K) Vs = h ` E"
      using exists_base[OF subfield] by blast
    hence "set Vs \<subseteq> h ` E"
      using Span_base_incl[OF subfield] by blast
    hence "\<exists>Us. set Us \<subseteq> E \<and> Vs = map h Us"
      by (induct Vs) (auto, metis insert_subset list.simps(9,15))
    then obtain Us where "set Us \<subseteq> E" and "Vs = map h Us"
      by blast
    with \<open>Span (h ` K) Vs = h ` E\<close> have "h ` (R.Span K Us) = h ` E"
      using R.subalgebra_in_carrier[OF assms(4)] Span_hom assms(1) by auto
    moreover from \<open>set Us \<subseteq> E\<close> have "R.Span K Us \<subseteq> E"
      using R.subalgebra_Span_incl assms(1-4) by blast
    ultimately have "R.Span K Us = E"
    proof (auto simp del: R.Span.simps)
      fix a assume "a \<in> E"
      with \<open>h ` (R.Span K Us) = h ` E\<close> obtain b where "b \<in> R.Span K Us" and "h a = h b"
        by auto
      with \<open>R.Span K Us \<subseteq> E\<close> and \<open>a \<in> E\<close> have "a = b"
        using inj_onD[OF assms(3)] by auto
      with \<open>b \<in> R.Span K Us\<close> show "a \<in> R.Span K Us"
        by simp
    qed
    with \<open>set Us \<subseteq> E\<close> have "R.finite_dimension K E"
      using R.Span_finite_dimension[OF assms(1)] R.subalgebra_in_carrier[OF assms(4)] by auto
    with \<open>R.infinite_dimension K E\<close> show False
      by simp
  qed
qed


subsubsection \<open>Reformulation of some lemmas in this new language.\<close>

lemma (in ring) sum_space_dim:
  assumes "subfield K R" "finite_dimension K E" "finite_dimension K F"
  shows "finite_dimension K (E <+>\<^bsub>R\<^esub> F)"
    and "((dim over K) (E <+>\<^bsub>R\<^esub> F)) = ((dim over K) E) + ((dim over K) F) - ((dim over K) (E \<inter> F))"
proof -
  obtain n m k where n: "dimension n K E" and m: "dimension m K F" and k: "dimension k K (E \<inter> F)"
    using assms(2-3) subalbegra_incl_imp_finite_dimension[OF assms(1-2)
          subalgebra_inter[OF assms(2-3)[THEN finite_dimension_imp_subalgebra[OF assms(1)]]]]
    by (meson inf_le1 finite_dimension_def)
  hence "dimension (n + m - k) K (E <+>\<^bsub>R\<^esub> F)"
    using dimension_sum_space[OF assms(1)] by simp
  thus "finite_dimension K (E <+>\<^bsub>R\<^esub> F)"
   and "((dim over K) (E <+>\<^bsub>R\<^esub> F)) = ((dim over K) E) + ((dim over K) F) - ((dim over K) (E \<inter> F))"
    using finite_dimensionI dimI[OF assms(1)] n m k by auto
qed

lemma (in ring) telescopic_base_dim:
  assumes "subfield K R" "subfield F R" and "finite_dimension K F" and "finite_dimension F E"
  shows "finite_dimension K E" and "(dim over K) E = ((dim over K) F) * ((dim over F) E)"
  using telescopic_base[OF assms(1-2)
        finite_dimensionE[OF assms(1,3)]
        finite_dimensionE[OF assms(2,4)]]
        dimI[OF assms(1)] finite_dimensionI
  by auto

end