src/HOL/NumberTheory/Factorization.thy

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(*  Title:      HOL/NumberTheory/Factorization.thy
    ID:         $Id$
    Author:     Thomas Marthedal Rasmussen
    Copyright   2000  University of Cambridge
*)

header {* Fundamental Theorem of Arithmetic (unique factorization into primes) *}

theory Factorization = Primes + Permutation:


subsection {* Definitions *}

consts
  primel :: "nat list => bool "
  nondec :: "nat list => bool "
  prod :: "nat list => nat"
  oinsert :: "nat => nat list => nat list"
  sort :: "nat list => nat list"

defs
  primel_def: "primel xs == set xs \<subseteq> prime"

primrec
  "nondec [] = True"
  "nondec (x # xs) = (case xs of [] => True | y # ys => x \<le> y \<and> nondec xs)"

primrec
  "prod [] = Suc 0"
  "prod (x # xs) = x * prod xs"

primrec
  "oinsert x [] = [x]"
  "oinsert x (y # ys) = (if x \<le> y then x # y # ys else y # oinsert x ys)"

primrec
  "sort [] = []"
  "sort (x # xs) = oinsert x (sort xs)"


subsection {* Arithmetic *}

lemma one_less_m: "(m::nat) \<noteq> m * k ==> m \<noteq> Suc 0 ==> Suc 0 < m"
  apply (case_tac m)
   apply auto
  done

lemma one_less_k: "(m::nat) \<noteq> m * k ==> Suc 0 < m * k ==> Suc 0 < k"
  apply (case_tac k)
   apply auto
  done

lemma mult_left_cancel: "(0::nat) < k ==> k * n = k * m ==> n = m"
  apply auto
  done

lemma mn_eq_m_one: "(0::nat) < m ==> m * n = m ==> n = Suc 0"
  apply (case_tac n)
   apply auto
  done

lemma prod_mn_less_k:
    "(0::nat) < n ==> 0 < k ==> Suc 0 < m ==> m * n = k ==> n < k"
  apply (induct m)
   apply auto
  done


subsection {* Prime list and product *}

lemma prod_append: "prod (xs @ ys) = prod xs * prod ys"
  apply (induct xs)
   apply (simp_all add: mult_assoc)
  done

lemma prod_xy_prod:
    "prod (x # xs) = prod (y # ys) ==> x * prod xs = y * prod ys"
  apply auto
  done

lemma primel_append: "primel (xs @ ys) = (primel xs \<and> primel ys)"
  apply (unfold primel_def)
  apply auto
  done

lemma prime_primel: "n \<in> prime ==> primel [n] \<and> prod [n] = n"
  apply (unfold primel_def)
  apply auto
  done

lemma prime_nd_one: "p \<in> prime ==> \<not> p dvd Suc 0"
  apply (unfold prime_def dvd_def)
  apply auto
  done

lemma hd_dvd_prod: "prod (x # xs) = prod ys ==> x dvd (prod ys)"
  apply (unfold dvd_def)
  apply (rule exI)
  apply (rule sym)
  apply simp
  done

lemma primel_tl: "primel (x # xs) ==> primel xs"
  apply (unfold primel_def)
  apply auto
  done

lemma primel_hd_tl: "(primel (x # xs)) = (x \<in> prime \<and> primel xs)"
  apply (unfold primel_def)
  apply auto
  done

lemma primes_eq: "p \<in> prime ==> q \<in> prime ==> p dvd q ==> p = q"
  apply (unfold prime_def)
  apply auto
  done

lemma primel_one_empty: "primel xs ==> prod xs = Suc 0 ==> xs = []"
  apply (unfold primel_def prime_def)
  apply (case_tac xs)
   apply simp_all
  done

lemma prime_g_one: "p \<in> prime ==> Suc 0 < p"
  apply (unfold prime_def)
  apply auto
  done

lemma prime_g_zero: "p \<in> prime ==> 0 < p"
  apply (unfold prime_def)
  apply auto
  done

lemma primel_nempty_g_one [rule_format]:
    "primel xs --> xs \<noteq> [] --> Suc 0 < prod xs"
  apply (unfold primel_def prime_def)
  apply (induct xs)
   apply (auto elim: one_less_mult)
  done

lemma primel_prod_gz: "primel xs ==> 0 < prod xs"
  apply (unfold primel_def prime_def)
  apply (induct xs)
   apply auto
  done


subsection {* Sorting *}

lemma nondec_oinsert [rule_format]: "nondec xs --> nondec (oinsert x xs)"
  apply (induct xs)
   apply (case_tac [2] list)
    apply (simp_all cong del: list.weak_case_cong)
  done

lemma nondec_sort: "nondec (sort xs)"
  apply (induct xs)
   apply simp_all
  apply (erule nondec_oinsert)
  done

lemma x_less_y_oinsert: "x \<le> y ==> l = y # ys ==> x # l = oinsert x l"
  apply simp_all
  done

lemma nondec_sort_eq [rule_format]: "nondec xs --> xs = sort xs"
  apply (induct xs)
   apply safe
    apply simp_all
   apply (case_tac list)
    apply simp_all
  apply (case_tac list)
   apply simp
  apply (rule_tac y = aa and ys = lista in x_less_y_oinsert)
   apply simp_all
  done

lemma oinsert_x_y: "oinsert x (oinsert y l) = oinsert y (oinsert x l)"
  apply (induct l)
  apply auto
  done


subsection {* Permutation *}

lemma perm_primel [rule_format]: "xs <~~> ys ==> primel xs --> primel ys"
  apply (unfold primel_def)
  apply (erule perm.induct)
     apply simp_all
  done

lemma perm_prod [rule_format]: "xs <~~> ys ==> prod xs = prod ys"
  apply (erule perm.induct)
     apply (simp_all add: mult_ac)
  done

lemma perm_subst_oinsert: "xs <~~> ys ==> oinsert a xs <~~> oinsert a ys"
  apply (erule perm.induct)
     apply auto
  done

lemma perm_oinsert: "x # xs <~~> oinsert x xs"
  apply (induct xs)
   apply auto
  done

lemma perm_sort: "xs <~~> sort xs"
  apply (induct xs)
  apply (auto intro: perm_oinsert elim: perm_subst_oinsert)
  done

lemma perm_sort_eq: "xs <~~> ys ==> sort xs = sort ys"
  apply (erule perm.induct)
     apply (simp_all add: oinsert_x_y)
  done


subsection {* Existence *}

lemma ex_nondec_lemma:
    "primel xs ==> \<exists>ys. primel ys \<and> nondec ys \<and> prod ys = prod xs"
  apply (blast intro: nondec_sort perm_prod perm_primel perm_sort perm_sym)
  done

lemma not_prime_ex_mk:
  "Suc 0 < n \<and> n \<notin> prime ==>
    \<exists>m k. Suc 0 < m \<and> Suc 0 < k \<and> m < n \<and> k < n \<and> n = m * k"
  apply (unfold prime_def dvd_def)
  apply (auto intro: n_less_m_mult_n n_less_n_mult_m one_less_m one_less_k)
  done

lemma split_primel:
    "primel xs ==> primel ys ==> \<exists>l. primel l \<and> prod l = prod xs * prod ys"
  apply (rule exI)
  apply safe
   apply (rule_tac [2] prod_append)
  apply (simp add: primel_append)
  done

lemma factor_exists [rule_format]: "Suc 0 < n --> (\<exists>l. primel l \<and> prod l = n)"
  apply (induct n rule: nat_less_induct)
  apply (rule impI)
  apply (case_tac "n \<in> prime")
   apply (rule exI)
   apply (erule prime_primel)
  apply (cut_tac n = n in not_prime_ex_mk)
   apply (auto intro!: split_primel)
  done

lemma nondec_factor_exists: "Suc 0 < n ==> \<exists>l. primel l \<and> nondec l \<and> prod l = n"
  apply (erule factor_exists [THEN exE])
  apply (blast intro!: ex_nondec_lemma)
  done


subsection {* Uniqueness *}

lemma prime_dvd_mult_list [rule_format]:
    "p \<in> prime ==> p dvd (prod xs) --> (\<exists>m. m:set xs \<and> p dvd m)"
  apply (induct xs)
   apply (force simp add: prime_def)
   apply (force dest: prime_dvd_mult)
  done

lemma hd_xs_dvd_prod:
  "primel (x # xs) ==> primel ys ==> prod (x # xs) = prod ys
    ==> \<exists>m. m \<in> set ys \<and> x dvd m"
  apply (rule prime_dvd_mult_list)
   apply (simp add: primel_hd_tl)
  apply (erule hd_dvd_prod)
  done

lemma prime_dvd_eq: "primel (x # xs) ==> primel ys ==> m \<in> set ys ==> x dvd m ==> x = m"
  apply (rule primes_eq)
    apply (auto simp add: primel_def primel_hd_tl)
  done

lemma hd_xs_eq_prod:
  "primel (x # xs) ==>
    primel ys ==> prod (x # xs) = prod ys ==> x \<in> set ys"
  apply (frule hd_xs_dvd_prod)
    apply auto
  apply (drule prime_dvd_eq)
     apply auto
  done

lemma perm_primel_ex:
  "primel (x # xs) ==>
    primel ys ==> prod (x # xs) = prod ys ==> \<exists>l. ys <~~> (x # l)"
  apply (rule exI)
  apply (rule perm_remove)
  apply (erule hd_xs_eq_prod)
   apply simp_all
  done

lemma primel_prod_less:
  "primel (x # xs) ==>
    primel ys ==> prod (x # xs) = prod ys ==> prod xs < prod ys"
  apply (auto intro: prod_mn_less_k prime_g_one primel_prod_gz simp add: primel_hd_tl)
  done

lemma prod_one_empty:
    "primel xs ==> p * prod xs = p ==> p \<in> prime ==> xs = []"
  apply (auto intro: primel_one_empty simp add: prime_def)
  done

lemma uniq_ex_aux:
  "\<forall>m. m < prod ys --> (\<forall>xs ys. primel xs \<and> primel ys \<and>
      prod xs = prod ys \<and> prod xs = m --> xs <~~> ys) ==>
    primel list ==> primel x ==> prod list = prod x ==> prod x < prod ys
    ==> x <~~> list"
  apply simp
  done

lemma factor_unique [rule_format]:
  "\<forall>xs ys. primel xs \<and> primel ys \<and> prod xs = prod ys \<and> prod xs = n
    --> xs <~~> ys"
  apply (induct n rule: nat_less_induct)
  apply safe
  apply (case_tac xs)
   apply (force intro: primel_one_empty)
  apply (rule perm_primel_ex [THEN exE])
     apply simp_all
  apply (rule perm.trans [THEN perm_sym])
  apply assumption
  apply (rule perm.Cons)
  apply (case_tac "x = []")
   apply (simp add: perm_sing_eq primel_hd_tl)
   apply (rule_tac p = a in prod_one_empty)
     apply simp_all
  apply (erule uniq_ex_aux)
     apply (auto intro: primel_tl perm_primel simp add: primel_hd_tl)
   apply (rule_tac k = a and n = "prod list" and m = "prod x" in mult_left_cancel)
    apply (rule_tac [3] x = a in primel_prod_less)
      apply (rule_tac [2] prod_xy_prod)
      apply (rule_tac [2] s = "prod ys" in HOL.trans)
       apply (erule_tac [3] perm_prod)
      apply (erule_tac [5] perm_prod [symmetric])
     apply (auto intro: perm_primel prime_g_zero)
  done

lemma perm_nondec_unique:
    "xs <~~> ys ==> nondec xs ==> nondec ys ==> xs = ys"
  apply (rule HOL.trans)
   apply (rule HOL.trans)
    apply (erule nondec_sort_eq)
   apply (erule perm_sort_eq)
  apply (erule nondec_sort_eq [symmetric])
  done

lemma unique_prime_factorization [rule_format]:
    "\<forall>n. Suc 0 < n --> (\<exists>!l. primel l \<and> nondec l \<and> prod l = n)"
  apply safe
   apply (erule nondec_factor_exists)
  apply (rule perm_nondec_unique)
    apply (rule factor_unique)
    apply simp_all
  done

end