src/HOL/Computational_Algebra/Euclidean_Algorithm.thy
author wenzelm
Fri Apr 07 21:17:18 2017 +0200 (2017-04-07)
changeset 65435 378175f44328
parent 65417 fc41a5650fb1
child 66817 0b12755ccbb2
permissions -rw-r--r--
tuned headers;
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(*  Title:      HOL/Computational_Algebra/Euclidean_Algorithm.thy
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    Author:     Manuel Eberl, TU Muenchen
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*)
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section \<open>Abstract euclidean algorithm in euclidean (semi)rings\<close>
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theory Euclidean_Algorithm
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  imports Factorial_Ring
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begin
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subsection \<open>Generic construction of the (simple) euclidean algorithm\<close>
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context euclidean_semiring
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begin
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context
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begin
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qualified function gcd :: "'a \<Rightarrow> 'a \<Rightarrow> 'a"
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  where "gcd a b = (if b = 0 then normalize a else gcd b (a mod b))"
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  by pat_completeness simp
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termination
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  by (relation "measure (euclidean_size \<circ> snd)") (simp_all add: mod_size_less)
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declare gcd.simps [simp del]
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lemma eucl_induct [case_names zero mod]:
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  assumes H1: "\<And>b. P b 0"
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  and H2: "\<And>a b. b \<noteq> 0 \<Longrightarrow> P b (a mod b) \<Longrightarrow> P a b"
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  shows "P a b"
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proof (induct a b rule: gcd.induct)
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  case (1 a b)
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  show ?case
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  proof (cases "b = 0")
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    case True then show "P a b" by simp (rule H1)
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  next
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    case False
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    then have "P b (a mod b)"
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      by (rule "1.hyps")
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    with \<open>b \<noteq> 0\<close> show "P a b"
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      by (blast intro: H2)
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  qed
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qed
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qualified lemma gcd_0:
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  "gcd a 0 = normalize a"
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  by (simp add: gcd.simps [of a 0])
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qualified lemma gcd_mod:
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  "a \<noteq> 0 \<Longrightarrow> gcd a (b mod a) = gcd b a"
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  by (simp add: gcd.simps [of b 0] gcd.simps [of b a])
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qualified definition lcm :: "'a \<Rightarrow> 'a \<Rightarrow> 'a"
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  where "lcm a b = normalize (a * b) div gcd a b"
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qualified definition Lcm :: "'a set \<Rightarrow> 'a" \<comment>
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    \<open>Somewhat complicated definition of Lcm that has the advantage of working
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    for infinite sets as well\<close>
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  where
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  [code del]: "Lcm A = (if \<exists>l. l \<noteq> 0 \<and> (\<forall>a\<in>A. a dvd l) then
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     let l = SOME l. l \<noteq> 0 \<and> (\<forall>a\<in>A. a dvd l) \<and> euclidean_size l =
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       (LEAST n. \<exists>l. l \<noteq> 0 \<and> (\<forall>a\<in>A. a dvd l) \<and> euclidean_size l = n)
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       in normalize l 
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      else 0)"
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qualified definition Gcd :: "'a set \<Rightarrow> 'a"
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  where [code del]: "Gcd A = Lcm {d. \<forall>a\<in>A. d dvd a}"
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end    
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lemma semiring_gcd:
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  "class.semiring_gcd one zero times gcd lcm
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    divide plus minus unit_factor normalize"
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proof
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  show "gcd a b dvd a"
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    and "gcd a b dvd b" for a b
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    by (induct a b rule: eucl_induct)
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      (simp_all add: local.gcd_0 local.gcd_mod dvd_mod_iff)
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next
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  show "c dvd a \<Longrightarrow> c dvd b \<Longrightarrow> c dvd gcd a b" for a b c
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  proof (induct a b rule: eucl_induct)
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    case (zero a) from \<open>c dvd a\<close> show ?case
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      by (rule dvd_trans) (simp add: local.gcd_0)
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  next
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    case (mod a b)
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    then show ?case
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      by (simp add: local.gcd_mod dvd_mod_iff)
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  qed
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next
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  show "normalize (gcd a b) = gcd a b" for a b
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    by (induct a b rule: eucl_induct)
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      (simp_all add: local.gcd_0 local.gcd_mod)
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next
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  show "lcm a b = normalize (a * b) div gcd a b" for a b
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    by (fact local.lcm_def)
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qed
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interpretation semiring_gcd one zero times gcd lcm
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  divide plus minus unit_factor normalize
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  by (fact semiring_gcd)
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lemma semiring_Gcd:
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  "class.semiring_Gcd one zero times gcd lcm Gcd Lcm
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    divide plus minus unit_factor normalize"
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proof -
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  show ?thesis
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  proof
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    have "(\<forall>a\<in>A. a dvd Lcm A) \<and> (\<forall>b. (\<forall>a\<in>A. a dvd b) \<longrightarrow> Lcm A dvd b)" for A
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    proof (cases "\<exists>l. l \<noteq>  0 \<and> (\<forall>a\<in>A. a dvd l)")
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      case False
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      then have "Lcm A = 0"
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        by (auto simp add: local.Lcm_def)
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      with False show ?thesis
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        by auto
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    next
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      case True
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      then obtain l\<^sub>0 where l\<^sub>0_props: "l\<^sub>0 \<noteq> 0" "\<forall>a\<in>A. a dvd l\<^sub>0" by blast
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      define n where "n = (LEAST n. \<exists>l. l \<noteq> 0 \<and> (\<forall>a\<in>A. a dvd l) \<and> euclidean_size l = n)"
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      define l where "l = (SOME l. l \<noteq> 0 \<and> (\<forall>a\<in>A. a dvd l) \<and> euclidean_size l = n)"
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      have "\<exists>l. l \<noteq> 0 \<and> (\<forall>a\<in>A. a dvd l) \<and> euclidean_size l = n"
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        apply (subst n_def)
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        apply (rule LeastI [of _ "euclidean_size l\<^sub>0"])
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        apply (rule exI [of _ l\<^sub>0])
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        apply (simp add: l\<^sub>0_props)
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        done
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      from someI_ex [OF this] have "l \<noteq> 0" and "\<forall>a\<in>A. a dvd l"
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        and "euclidean_size l = n" 
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        unfolding l_def by simp_all
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      {
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        fix l' assume "\<forall>a\<in>A. a dvd l'"
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        with \<open>\<forall>a\<in>A. a dvd l\<close> have "\<forall>a\<in>A. a dvd gcd l l'"
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          by (auto intro: gcd_greatest)
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        moreover from \<open>l \<noteq> 0\<close> have "gcd l l' \<noteq> 0"
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          by simp
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        ultimately have "\<exists>b. b \<noteq> 0 \<and> (\<forall>a\<in>A. a dvd b) \<and> 
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          euclidean_size b = euclidean_size (gcd l l')"
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          by (intro exI [of _ "gcd l l'"], auto)
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        then have "euclidean_size (gcd l l') \<ge> n"
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          by (subst n_def) (rule Least_le)
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        moreover have "euclidean_size (gcd l l') \<le> n"
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        proof -
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          have "gcd l l' dvd l"
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            by simp
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          then obtain a where "l = gcd l l' * a" ..
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          with \<open>l \<noteq> 0\<close> have "a \<noteq> 0"
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            by auto
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          hence "euclidean_size (gcd l l') \<le> euclidean_size (gcd l l' * a)"
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            by (rule size_mult_mono)
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          also have "gcd l l' * a = l" using \<open>l = gcd l l' * a\<close> ..
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          also note \<open>euclidean_size l = n\<close>
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          finally show "euclidean_size (gcd l l') \<le> n" .
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        qed
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        ultimately have *: "euclidean_size l = euclidean_size (gcd l l')" 
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          by (intro le_antisym, simp_all add: \<open>euclidean_size l = n\<close>)
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        from \<open>l \<noteq> 0\<close> have "l dvd gcd l l'"
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          by (rule dvd_euclidean_size_eq_imp_dvd) (auto simp add: *)
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        hence "l dvd l'" by (rule dvd_trans [OF _ gcd_dvd2])
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      }
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      with \<open>\<forall>a\<in>A. a dvd l\<close> and \<open>l \<noteq> 0\<close>
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        have "(\<forall>a\<in>A. a dvd normalize l) \<and> 
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          (\<forall>l'. (\<forall>a\<in>A. a dvd l') \<longrightarrow> normalize l dvd l')"
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        by auto
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      also from True have "normalize l = Lcm A"
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        by (simp add: local.Lcm_def Let_def n_def l_def)
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      finally show ?thesis .
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    qed
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    then show dvd_Lcm: "a \<in> A \<Longrightarrow> a dvd Lcm A"
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      and Lcm_least: "(\<And>a. a \<in> A \<Longrightarrow> a dvd b) \<Longrightarrow> Lcm A dvd b" for A and a b
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      by auto
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    show "a \<in> A \<Longrightarrow> Gcd A dvd a" for A and a
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      by (auto simp add: local.Gcd_def intro: Lcm_least)
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    show "(\<And>a. a \<in> A \<Longrightarrow> b dvd a) \<Longrightarrow> b dvd Gcd A" for A and b
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      by (auto simp add: local.Gcd_def intro: dvd_Lcm)
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    show [simp]: "normalize (Lcm A) = Lcm A" for A
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      by (simp add: local.Lcm_def)
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    show "normalize (Gcd A) = Gcd A" for A
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      by (simp add: local.Gcd_def)
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  qed
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qed
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interpretation semiring_Gcd one zero times gcd lcm Gcd Lcm
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    divide plus minus unit_factor normalize
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  by (fact semiring_Gcd)
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subclass factorial_semiring
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proof -
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  show "class.factorial_semiring divide plus minus zero times one
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     unit_factor normalize"
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  proof (standard, rule factorial_semiring_altI_aux) \<comment> \<open>FIXME rule\<close>
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    fix x assume "x \<noteq> 0"
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    thus "finite {p. p dvd x \<and> normalize p = p}"
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    proof (induction "euclidean_size x" arbitrary: x rule: less_induct)
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      case (less x)
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      show ?case
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      proof (cases "\<exists>y. y dvd x \<and> \<not>x dvd y \<and> \<not>is_unit y")
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        case False
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        have "{p. p dvd x \<and> normalize p = p} \<subseteq> {1, normalize x}"
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        proof
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          fix p assume p: "p \<in> {p. p dvd x \<and> normalize p = p}"
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          with False have "is_unit p \<or> x dvd p" by blast
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          thus "p \<in> {1, normalize x}"
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          proof (elim disjE)
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            assume "is_unit p"
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            hence "normalize p = 1" by (simp add: is_unit_normalize)
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            with p show ?thesis by simp
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          next
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            assume "x dvd p"
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            with p have "normalize p = normalize x" by (intro associatedI) simp_all
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            with p show ?thesis by simp
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          qed
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        qed
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        moreover have "finite \<dots>" by simp
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        ultimately show ?thesis by (rule finite_subset)
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      next
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        case True
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        then obtain y where y: "y dvd x" "\<not>x dvd y" "\<not>is_unit y" by blast
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        define z where "z = x div y"
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        let ?fctrs = "\<lambda>x. {p. p dvd x \<and> normalize p = p}"
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        from y have x: "x = y * z" by (simp add: z_def)
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        with less.prems have "y \<noteq> 0" "z \<noteq> 0" by auto
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        have normalized_factors_product:
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          "{p. p dvd a * b \<and> normalize p = p} = 
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             (\<lambda>(x,y). x * y) ` ({p. p dvd a \<and> normalize p = p} \<times> {p. p dvd b \<and> normalize p = p})" for a b
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        proof safe
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          fix p assume p: "p dvd a * b" "normalize p = p"
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          from dvd_productE[OF p(1)] guess x y . note xy = this
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          define x' y' where "x' = normalize x" and "y' = normalize y"
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          have "p = x' * y'"
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            by (subst p(2) [symmetric]) (simp add: xy x'_def y'_def normalize_mult)
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          moreover from xy have "normalize x' = x'" "normalize y' = y'" "x' dvd a" "y' dvd b" 
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            by (simp_all add: x'_def y'_def)
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          ultimately show "p \<in> (\<lambda>(x, y). x * y) ` 
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            ({p. p dvd a \<and> normalize p = p} \<times> {p. p dvd b \<and> normalize p = p})"
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            by blast
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        qed (auto simp: normalize_mult mult_dvd_mono)
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        from x y have "\<not>is_unit z" by (auto simp: mult_unit_dvd_iff)
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        have "?fctrs x = (\<lambda>(p,p'). p * p') ` (?fctrs y \<times> ?fctrs z)"
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          by (subst x) (rule normalized_factors_product)
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        also have "\<not>y * z dvd y * 1" "\<not>y * z dvd 1 * z"
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          by (subst dvd_times_left_cancel_iff dvd_times_right_cancel_iff; fact)+
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        hence "finite ((\<lambda>(p,p'). p * p') ` (?fctrs y \<times> ?fctrs z))"
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          by (intro finite_imageI finite_cartesian_product less dvd_proper_imp_size_less)
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             (auto simp: x)
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        finally show ?thesis .
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      qed
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    qed
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  next
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    fix p
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    assume "irreducible p"
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    then show "prime_elem p"
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      by (rule irreducible_imp_prime_elem_gcd)
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  qed
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qed
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lemma Gcd_eucl_set [code]:
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  "Gcd (set xs) = fold gcd xs 0"
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  by (fact Gcd_set_eq_fold)
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lemma Lcm_eucl_set [code]:
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  "Lcm (set xs) = fold lcm xs 1"
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  by (fact Lcm_set_eq_fold)
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end
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hide_const (open) gcd lcm Gcd Lcm
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lemma prime_elem_int_abs_iff [simp]:
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  fixes p :: int
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  shows "prime_elem \<bar>p\<bar> \<longleftrightarrow> prime_elem p"
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  using prime_elem_normalize_iff [of p] by simp
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lemma prime_elem_int_minus_iff [simp]:
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  fixes p :: int
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  shows "prime_elem (- p) \<longleftrightarrow> prime_elem p"
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  using prime_elem_normalize_iff [of "- p"] by simp
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lemma prime_int_iff:
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  fixes p :: int
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  shows "prime p \<longleftrightarrow> p > 0 \<and> prime_elem p"
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  by (auto simp add: prime_def dest: prime_elem_not_zeroI)
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subsection \<open>The (simple) euclidean algorithm as gcd computation\<close>
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class euclidean_semiring_gcd = euclidean_semiring + gcd + Gcd +
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  assumes gcd_eucl: "Euclidean_Algorithm.gcd = GCD.gcd"
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    and lcm_eucl: "Euclidean_Algorithm.lcm = GCD.lcm"
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  assumes Gcd_eucl: "Euclidean_Algorithm.Gcd = GCD.Gcd"
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    and Lcm_eucl: "Euclidean_Algorithm.Lcm = GCD.Lcm"
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begin
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subclass semiring_gcd
haftmann@64786
   293
  unfolding gcd_eucl [symmetric] lcm_eucl [symmetric]
haftmann@64786
   294
  by (fact semiring_gcd)
haftmann@58023
   295
eberlm@62422
   296
subclass semiring_Gcd
haftmann@64786
   297
  unfolding  gcd_eucl [symmetric] lcm_eucl [symmetric]
haftmann@64786
   298
    Gcd_eucl [symmetric] Lcm_eucl [symmetric]
haftmann@64786
   299
  by (fact semiring_Gcd)
eberlm@63498
   300
eberlm@63498
   301
subclass factorial_semiring_gcd
eberlm@63498
   302
proof
haftmann@64786
   303
  show "gcd a b = gcd_factorial a b" for a b
haftmann@64786
   304
    apply (rule sym)
haftmann@64786
   305
    apply (rule gcdI)
haftmann@64786
   306
       apply (fact gcd_lcm_factorial)+
haftmann@64786
   307
    done
haftmann@64786
   308
  then show "lcm a b = lcm_factorial a b" for a b
eberlm@63498
   309
    by (simp add: lcm_factorial_gcd_factorial lcm_gcd)
haftmann@64786
   310
  show "Gcd A = Gcd_factorial A" for A
haftmann@64786
   311
    apply (rule sym)
haftmann@64786
   312
    apply (rule GcdI)
haftmann@64786
   313
       apply (fact gcd_lcm_factorial)+
haftmann@64786
   314
    done
haftmann@64786
   315
  show "Lcm A = Lcm_factorial A" for A
haftmann@64786
   316
    apply (rule sym)
haftmann@64786
   317
    apply (rule LcmI)
haftmann@64786
   318
       apply (fact gcd_lcm_factorial)+
haftmann@64786
   319
    done
eberlm@63498
   320
qed
eberlm@63498
   321
haftmann@64786
   322
lemma gcd_mod_right [simp]:
haftmann@64786
   323
  "a \<noteq> 0 \<Longrightarrow> gcd a (b mod a) = gcd a b"
haftmann@64786
   324
  unfolding gcd.commute [of a b]
haftmann@64786
   325
  by (simp add: gcd_eucl [symmetric] local.gcd_mod)
haftmann@58023
   326
haftmann@64786
   327
lemma gcd_mod_left [simp]:
haftmann@64786
   328
  "b \<noteq> 0 \<Longrightarrow> gcd (a mod b) b = gcd a b"
haftmann@64786
   329
  by (drule gcd_mod_right [of _ a]) (simp add: gcd.commute)
haftmann@58023
   330
haftmann@58023
   331
lemma euclidean_size_gcd_le1 [simp]:
haftmann@58023
   332
  assumes "a \<noteq> 0"
haftmann@58023
   333
  shows "euclidean_size (gcd a b) \<le> euclidean_size a"
haftmann@58023
   334
proof -
haftmann@64786
   335
  from gcd_dvd1 obtain c where A: "a = gcd a b * c" ..
haftmann@64786
   336
  with assms have "c \<noteq> 0"
haftmann@64786
   337
    by auto
haftmann@64786
   338
  moreover from this
haftmann@64786
   339
  have "euclidean_size (gcd a b) \<le> euclidean_size (gcd a b * c)"
haftmann@64786
   340
    by (rule size_mult_mono)
haftmann@64786
   341
  with A show ?thesis
haftmann@64786
   342
    by simp
haftmann@58023
   343
qed
haftmann@58023
   344
haftmann@58023
   345
lemma euclidean_size_gcd_le2 [simp]:
haftmann@58023
   346
  "b \<noteq> 0 \<Longrightarrow> euclidean_size (gcd a b) \<le> euclidean_size b"
haftmann@58023
   347
  by (subst gcd.commute, rule euclidean_size_gcd_le1)
haftmann@58023
   348
haftmann@58023
   349
lemma euclidean_size_gcd_less1:
haftmann@64786
   350
  assumes "a \<noteq> 0" and "\<not> a dvd b"
haftmann@58023
   351
  shows "euclidean_size (gcd a b) < euclidean_size a"
haftmann@58023
   352
proof (rule ccontr)
haftmann@58023
   353
  assume "\<not>euclidean_size (gcd a b) < euclidean_size a"
eberlm@62422
   354
  with \<open>a \<noteq> 0\<close> have A: "euclidean_size (gcd a b) = euclidean_size a"
haftmann@58023
   355
    by (intro le_antisym, simp_all)
eberlm@62422
   356
  have "a dvd gcd a b"
eberlm@62422
   357
    by (rule dvd_euclidean_size_eq_imp_dvd) (simp_all add: assms A)
eberlm@62422
   358
  hence "a dvd b" using dvd_gcdD2 by blast
haftmann@64786
   359
  with \<open>\<not> a dvd b\<close> show False by contradiction
haftmann@58023
   360
qed
haftmann@58023
   361
haftmann@58023
   362
lemma euclidean_size_gcd_less2:
haftmann@64786
   363
  assumes "b \<noteq> 0" and "\<not> b dvd a"
haftmann@58023
   364
  shows "euclidean_size (gcd a b) < euclidean_size b"
haftmann@58023
   365
  using assms by (subst gcd.commute, rule euclidean_size_gcd_less1)
haftmann@58023
   366
haftmann@58023
   367
lemma euclidean_size_lcm_le1: 
haftmann@58023
   368
  assumes "a \<noteq> 0" and "b \<noteq> 0"
haftmann@58023
   369
  shows "euclidean_size a \<le> euclidean_size (lcm a b)"
haftmann@58023
   370
proof -
haftmann@60690
   371
  have "a dvd lcm a b" by (rule dvd_lcm1)
haftmann@60690
   372
  then obtain c where A: "lcm a b = a * c" ..
eberlm@62429
   373
  with \<open>a \<noteq> 0\<close> and \<open>b \<noteq> 0\<close> have "c \<noteq> 0" by (auto simp: lcm_eq_0_iff)
haftmann@58023
   374
  then show ?thesis by (subst A, intro size_mult_mono)
haftmann@58023
   375
qed
haftmann@58023
   376
haftmann@58023
   377
lemma euclidean_size_lcm_le2:
haftmann@58023
   378
  "a \<noteq> 0 \<Longrightarrow> b \<noteq> 0 \<Longrightarrow> euclidean_size b \<le> euclidean_size (lcm a b)"
haftmann@58023
   379
  using euclidean_size_lcm_le1 [of b a] by (simp add: ac_simps)
haftmann@58023
   380
haftmann@58023
   381
lemma euclidean_size_lcm_less1:
haftmann@64786
   382
  assumes "b \<noteq> 0" and "\<not> b dvd a"
haftmann@58023
   383
  shows "euclidean_size a < euclidean_size (lcm a b)"
haftmann@58023
   384
proof (rule ccontr)
haftmann@58023
   385
  from assms have "a \<noteq> 0" by auto
haftmann@58023
   386
  assume "\<not>euclidean_size a < euclidean_size (lcm a b)"
wenzelm@60526
   387
  with \<open>a \<noteq> 0\<close> and \<open>b \<noteq> 0\<close> have "euclidean_size (lcm a b) = euclidean_size a"
haftmann@58023
   388
    by (intro le_antisym, simp, intro euclidean_size_lcm_le1)
haftmann@58023
   389
  with assms have "lcm a b dvd a" 
eberlm@62429
   390
    by (rule_tac dvd_euclidean_size_eq_imp_dvd) (auto simp: lcm_eq_0_iff)
eberlm@62422
   391
  hence "b dvd a" by (rule lcm_dvdD2)
wenzelm@60526
   392
  with \<open>\<not>b dvd a\<close> show False by contradiction
haftmann@58023
   393
qed
haftmann@58023
   394
haftmann@58023
   395
lemma euclidean_size_lcm_less2:
haftmann@64786
   396
  assumes "a \<noteq> 0" and "\<not> a dvd b"
haftmann@58023
   397
  shows "euclidean_size b < euclidean_size (lcm a b)"
haftmann@58023
   398
  using assms euclidean_size_lcm_less1 [of a b] by (simp add: ac_simps)
haftmann@58023
   399
haftmann@58023
   400
end
haftmann@58023
   401
haftmann@64786
   402
lemma factorial_euclidean_semiring_gcdI:
haftmann@64786
   403
  "OFCLASS('a::{factorial_semiring_gcd, euclidean_semiring}, euclidean_semiring_gcd_class)"
haftmann@64786
   404
proof
haftmann@64786
   405
  interpret semiring_Gcd 1 0 times
haftmann@64786
   406
    Euclidean_Algorithm.gcd Euclidean_Algorithm.lcm
haftmann@64786
   407
    Euclidean_Algorithm.Gcd Euclidean_Algorithm.Lcm
haftmann@64848
   408
    divide plus minus unit_factor normalize
haftmann@64786
   409
    rewrites "dvd.dvd op * = Rings.dvd"
haftmann@64786
   410
    by (fact semiring_Gcd) (simp add: dvd.dvd_def dvd_def fun_eq_iff)
haftmann@64786
   411
  show [simp]: "Euclidean_Algorithm.gcd = (gcd :: 'a \<Rightarrow> _)"
haftmann@64786
   412
  proof (rule ext)+
haftmann@64786
   413
    fix a b :: 'a
haftmann@64786
   414
    show "Euclidean_Algorithm.gcd a b = gcd a b"
haftmann@64786
   415
    proof (induct a b rule: eucl_induct)
haftmann@64786
   416
      case zero
haftmann@64786
   417
      then show ?case
haftmann@64786
   418
        by simp
haftmann@64786
   419
    next
haftmann@64786
   420
      case (mod a b)
haftmann@64786
   421
      moreover have "gcd b (a mod b) = gcd b a"
haftmann@64786
   422
        using GCD.gcd_add_mult [of b "a div b" "a mod b", symmetric]
haftmann@64786
   423
          by (simp add: div_mult_mod_eq)
haftmann@64786
   424
      ultimately show ?case
haftmann@64786
   425
        by (simp add: Euclidean_Algorithm.gcd_mod ac_simps)
haftmann@64786
   426
    qed
haftmann@64786
   427
  qed
haftmann@64786
   428
  show [simp]: "Euclidean_Algorithm.Lcm = (Lcm :: 'a set \<Rightarrow> _)"
haftmann@64786
   429
    by (auto intro!: Lcm_eqI GCD.dvd_Lcm GCD.Lcm_least)
haftmann@64786
   430
  show "Euclidean_Algorithm.lcm = (lcm :: 'a \<Rightarrow> _)"
haftmann@64786
   431
    by (simp add: fun_eq_iff Euclidean_Algorithm.lcm_def semiring_gcd_class.lcm_gcd)
haftmann@64786
   432
  show "Euclidean_Algorithm.Gcd = (Gcd :: 'a set \<Rightarrow> _)"
haftmann@64786
   433
    by (simp add: fun_eq_iff Euclidean_Algorithm.Gcd_def semiring_Gcd_class.Gcd_Lcm)
haftmann@64786
   434
qed
eberlm@63498
   435
haftmann@58023
   436
haftmann@64786
   437
subsection \<open>The extended euclidean algorithm\<close>
haftmann@64786
   438
  
haftmann@58023
   439
class euclidean_ring_gcd = euclidean_semiring_gcd + idom
haftmann@58023
   440
begin
haftmann@58023
   441
haftmann@58023
   442
subclass euclidean_ring ..
haftmann@60439
   443
subclass ring_gcd ..
eberlm@63498
   444
subclass factorial_ring_gcd ..
haftmann@60439
   445
haftmann@64786
   446
function euclid_ext_aux :: "'a \<Rightarrow> 'a \<Rightarrow> 'a \<Rightarrow> 'a \<Rightarrow> 'a \<Rightarrow> 'a \<Rightarrow> ('a \<times> 'a) \<times> 'a"
haftmann@64786
   447
  where "euclid_ext_aux s' s t' t r' r = (
haftmann@64786
   448
     if r = 0 then let c = 1 div unit_factor r' in ((s' * c, t' * c), normalize r')
haftmann@64786
   449
     else let q = r' div r
haftmann@64786
   450
          in euclid_ext_aux s (s' - q * s) t (t' - q * t) r (r' mod r))"
haftmann@64786
   451
  by auto
haftmann@64786
   452
termination
haftmann@64786
   453
  by (relation "measure (\<lambda>(_, _, _, _, _, b). euclidean_size b)")
haftmann@64786
   454
    (simp_all add: mod_size_less)
eberlm@62442
   455
haftmann@64786
   456
abbreviation (input) euclid_ext :: "'a \<Rightarrow> 'a \<Rightarrow> ('a \<times> 'a) \<times> 'a"
haftmann@64786
   457
  where "euclid_ext \<equiv> euclid_ext_aux 1 0 0 1"
haftmann@64786
   458
    
haftmann@64786
   459
lemma
haftmann@64786
   460
  assumes "gcd r' r = gcd a b"
haftmann@64786
   461
  assumes "s' * a + t' * b = r'"
haftmann@64786
   462
  assumes "s * a + t * b = r"
haftmann@64786
   463
  assumes "euclid_ext_aux s' s t' t r' r = ((x, y), c)"
haftmann@64786
   464
  shows euclid_ext_aux_eq_gcd: "c = gcd a b"
haftmann@64786
   465
    and euclid_ext_aux_bezout: "x * a + y * b = gcd a b"
haftmann@64786
   466
proof -
haftmann@64786
   467
  have "case euclid_ext_aux s' s t' t r' r of ((x, y), c) \<Rightarrow> 
haftmann@64786
   468
    x * a + y * b = c \<and> c = gcd a b" (is "?P (euclid_ext_aux s' s t' t r' r)")
haftmann@64786
   469
    using assms(1-3)
haftmann@64786
   470
  proof (induction s' s t' t r' r rule: euclid_ext_aux.induct)
haftmann@64786
   471
    case (1 s' s t' t r' r)
haftmann@64786
   472
    show ?case
haftmann@64786
   473
    proof (cases "r = 0")
haftmann@64786
   474
      case True
haftmann@64786
   475
      hence "euclid_ext_aux s' s t' t r' r = 
haftmann@64786
   476
               ((s' div unit_factor r', t' div unit_factor r'), normalize r')"
haftmann@64786
   477
        by (subst euclid_ext_aux.simps) (simp add: Let_def)
haftmann@64786
   478
      also have "?P \<dots>"
haftmann@64786
   479
      proof safe
haftmann@64786
   480
        have "s' div unit_factor r' * a + t' div unit_factor r' * b = 
haftmann@64786
   481
                (s' * a + t' * b) div unit_factor r'"
haftmann@64786
   482
          by (cases "r' = 0") (simp_all add: unit_div_commute)
haftmann@64786
   483
        also have "s' * a + t' * b = r'" by fact
haftmann@64786
   484
        also have "\<dots> div unit_factor r' = normalize r'" by simp
haftmann@64786
   485
        finally show "s' div unit_factor r' * a + t' div unit_factor r' * b = normalize r'" .
haftmann@64786
   486
      next
haftmann@64786
   487
        from "1.prems" True show "normalize r' = gcd a b"
haftmann@64786
   488
          by simp
haftmann@64786
   489
      qed
haftmann@64786
   490
      finally show ?thesis .
haftmann@64786
   491
    next
haftmann@64786
   492
      case False
haftmann@64786
   493
      hence "euclid_ext_aux s' s t' t r' r = 
haftmann@64786
   494
             euclid_ext_aux s (s' - r' div r * s) t (t' - r' div r * t) r (r' mod r)"
haftmann@64786
   495
        by (subst euclid_ext_aux.simps) (simp add: Let_def)
haftmann@64786
   496
      also from "1.prems" False have "?P \<dots>"
haftmann@64786
   497
      proof (intro "1.IH")
haftmann@64786
   498
        have "(s' - r' div r * s) * a + (t' - r' div r * t) * b =
haftmann@64786
   499
              (s' * a + t' * b) - r' div r * (s * a + t * b)" by (simp add: algebra_simps)
haftmann@64786
   500
        also have "s' * a + t' * b = r'" by fact
haftmann@64786
   501
        also have "s * a + t * b = r" by fact
haftmann@64786
   502
        also have "r' - r' div r * r = r' mod r" using div_mult_mod_eq [of r' r]
haftmann@64786
   503
          by (simp add: algebra_simps)
haftmann@64786
   504
        finally show "(s' - r' div r * s) * a + (t' - r' div r * t) * b = r' mod r" .
haftmann@64786
   505
      qed (auto simp: gcd_mod_right algebra_simps minus_mod_eq_div_mult [symmetric] gcd.commute)
haftmann@64786
   506
      finally show ?thesis .
haftmann@64786
   507
    qed
haftmann@64786
   508
  qed
haftmann@64786
   509
  with assms(4) show "c = gcd a b" "x * a + y * b = gcd a b"
haftmann@64786
   510
    by simp_all
haftmann@64786
   511
qed
haftmann@60572
   512
haftmann@64786
   513
declare euclid_ext_aux.simps [simp del]
haftmann@64786
   514
haftmann@64786
   515
definition bezout_coefficients :: "'a \<Rightarrow> 'a \<Rightarrow> 'a \<times> 'a"
haftmann@64786
   516
  where [code]: "bezout_coefficients a b = fst (euclid_ext a b)"
haftmann@64786
   517
haftmann@64786
   518
lemma bezout_coefficients_0: 
haftmann@64786
   519
  "bezout_coefficients a 0 = (1 div unit_factor a, 0)"
haftmann@64786
   520
  by (simp add: bezout_coefficients_def euclid_ext_aux.simps)
haftmann@64786
   521
haftmann@64786
   522
lemma bezout_coefficients_left_0: 
haftmann@64786
   523
  "bezout_coefficients 0 a = (0, 1 div unit_factor a)"
haftmann@64786
   524
  by (simp add: bezout_coefficients_def euclid_ext_aux.simps)
haftmann@64786
   525
haftmann@64786
   526
lemma bezout_coefficients:
haftmann@64786
   527
  assumes "bezout_coefficients a b = (x, y)"
haftmann@64786
   528
  shows "x * a + y * b = gcd a b"
haftmann@64786
   529
  using assms by (simp add: bezout_coefficients_def
haftmann@64786
   530
    euclid_ext_aux_bezout [of a b a b 1 0 0 1 x y] prod_eq_iff)
haftmann@64786
   531
haftmann@64786
   532
lemma bezout_coefficients_fst_snd:
haftmann@64786
   533
  "fst (bezout_coefficients a b) * a + snd (bezout_coefficients a b) * b = gcd a b"
haftmann@64786
   534
  by (rule bezout_coefficients) simp
haftmann@64786
   535
haftmann@64786
   536
lemma euclid_ext_eq [simp]:
haftmann@64786
   537
  "euclid_ext a b = (bezout_coefficients a b, gcd a b)" (is "?p = ?q")
haftmann@64786
   538
proof
haftmann@64786
   539
  show "fst ?p = fst ?q"
haftmann@64786
   540
    by (simp add: bezout_coefficients_def)
haftmann@64786
   541
  have "snd (euclid_ext_aux 1 0 0 1 a b) = gcd a b"
haftmann@64786
   542
    by (rule euclid_ext_aux_eq_gcd [of a b a b 1 0 0 1])
haftmann@64786
   543
      (simp_all add: prod_eq_iff)
haftmann@64786
   544
  then show "snd ?p = snd ?q"
haftmann@64786
   545
    by simp
haftmann@64786
   546
qed
haftmann@64786
   547
haftmann@64786
   548
declare euclid_ext_eq [symmetric, code_unfold]
haftmann@60572
   549
haftmann@60572
   550
end
haftmann@58023
   551
haftmann@58023
   552
haftmann@60572
   553
subsection \<open>Typical instances\<close>
haftmann@58023
   554
eberlm@62422
   555
instance nat :: euclidean_semiring_gcd
eberlm@62422
   556
proof
haftmann@64786
   557
  interpret semiring_Gcd 1 0 times
haftmann@64786
   558
    "Euclidean_Algorithm.gcd" "Euclidean_Algorithm.lcm"
haftmann@64786
   559
    "Euclidean_Algorithm.Gcd" "Euclidean_Algorithm.Lcm"
haftmann@64848
   560
    divide plus minus unit_factor normalize
haftmann@64786
   561
    rewrites "dvd.dvd op * = Rings.dvd"
haftmann@64786
   562
    by (fact semiring_Gcd) (simp add: dvd.dvd_def dvd_def fun_eq_iff)
haftmann@64786
   563
  show [simp]: "(Euclidean_Algorithm.gcd :: nat \<Rightarrow> _) = gcd"
haftmann@64786
   564
  proof (rule ext)+
haftmann@64786
   565
    fix m n :: nat
haftmann@64786
   566
    show "Euclidean_Algorithm.gcd m n = gcd m n"
haftmann@64786
   567
    proof (induct m n rule: eucl_induct)
haftmann@64786
   568
      case zero
haftmann@64786
   569
      then show ?case
haftmann@64786
   570
        by simp
haftmann@64786
   571
    next
haftmann@64786
   572
      case (mod m n)
haftmann@64786
   573
      then have "gcd n (m mod n) = gcd n m"
haftmann@64786
   574
        using gcd_nat.simps [of m n] by (simp add: ac_simps)
haftmann@64786
   575
      with mod show ?case
haftmann@64786
   576
        by (simp add: Euclidean_Algorithm.gcd_mod ac_simps)
haftmann@64786
   577
    qed
haftmann@64786
   578
  qed
haftmann@64786
   579
  show [simp]: "(Euclidean_Algorithm.Lcm :: nat set \<Rightarrow> _) = Lcm"
haftmann@64786
   580
    by (auto intro!: ext Lcm_eqI)
haftmann@64786
   581
  show "(Euclidean_Algorithm.lcm :: nat \<Rightarrow> _) = lcm"
haftmann@64786
   582
    by (simp add: fun_eq_iff Euclidean_Algorithm.lcm_def semiring_gcd_class.lcm_gcd)
haftmann@64786
   583
  show "(Euclidean_Algorithm.Gcd :: nat set \<Rightarrow> _) = Gcd"
haftmann@64786
   584
    by (simp add: fun_eq_iff Euclidean_Algorithm.Gcd_def semiring_Gcd_class.Gcd_Lcm)
eberlm@62422
   585
qed
eberlm@62422
   586
eberlm@62422
   587
instance int :: euclidean_ring_gcd
eberlm@62422
   588
proof
haftmann@64786
   589
  interpret semiring_Gcd 1 0 times
haftmann@64786
   590
    "Euclidean_Algorithm.gcd" "Euclidean_Algorithm.lcm"
haftmann@64786
   591
    "Euclidean_Algorithm.Gcd" "Euclidean_Algorithm.Lcm"
haftmann@64848
   592
    divide plus minus unit_factor normalize
haftmann@64786
   593
    rewrites "dvd.dvd op * = Rings.dvd"
haftmann@64786
   594
    by (fact semiring_Gcd) (simp add: dvd.dvd_def dvd_def fun_eq_iff)
haftmann@64786
   595
  show [simp]: "(Euclidean_Algorithm.gcd :: int \<Rightarrow> _) = gcd"
haftmann@64786
   596
  proof (rule ext)+
haftmann@64786
   597
    fix k l :: int
haftmann@64786
   598
    show "Euclidean_Algorithm.gcd k l = gcd k l"
haftmann@64786
   599
    proof (induct k l rule: eucl_induct)
haftmann@64786
   600
      case zero
haftmann@64786
   601
      then show ?case
haftmann@64786
   602
        by simp
haftmann@64786
   603
    next
haftmann@64786
   604
      case (mod k l)
haftmann@64786
   605
      have "gcd l (k mod l) = gcd l k"
haftmann@64786
   606
      proof (cases l "0::int" rule: linorder_cases)
haftmann@64786
   607
        case less
haftmann@64786
   608
        then show ?thesis
haftmann@64786
   609
          using gcd_non_0_int [of "- l" "- k"] by (simp add: ac_simps)
haftmann@64786
   610
      next
haftmann@64786
   611
        case equal
haftmann@64786
   612
        with mod show ?thesis
haftmann@64786
   613
          by simp
haftmann@64786
   614
      next
haftmann@64786
   615
        case greater
haftmann@64786
   616
        then show ?thesis
haftmann@64786
   617
          using gcd_non_0_int [of l k] by (simp add: ac_simps)
haftmann@64786
   618
      qed
haftmann@64786
   619
      with mod show ?case
haftmann@64786
   620
        by (simp add: Euclidean_Algorithm.gcd_mod ac_simps)
haftmann@64786
   621
    qed
haftmann@64786
   622
  qed
haftmann@64786
   623
  show [simp]: "(Euclidean_Algorithm.Lcm :: int set \<Rightarrow> _) = Lcm"
haftmann@64786
   624
    by (auto intro!: ext Lcm_eqI)
haftmann@64786
   625
  show "(Euclidean_Algorithm.lcm :: int \<Rightarrow> _) = lcm"
haftmann@64786
   626
    by (simp add: fun_eq_iff Euclidean_Algorithm.lcm_def semiring_gcd_class.lcm_gcd)
haftmann@64786
   627
  show "(Euclidean_Algorithm.Gcd :: int set \<Rightarrow> _) = Gcd"
haftmann@64786
   628
    by (simp add: fun_eq_iff Euclidean_Algorithm.Gcd_def semiring_Gcd_class.Gcd_Lcm)
eberlm@62422
   629
qed
eberlm@62422
   630
haftmann@63924
   631
end