src/HOL/Computational_Algebra/Primes.thy

HOL-Library: Changed notation for sum_mset

(*  Title:      HOL/Computational_Algebra/Primes.thy
    Author:     Christophe Tabacznyj
    Author:     Lawrence C. Paulson
    Author:     Amine Chaieb
    Author:     Thomas M. Rasmussen
    Author:     Jeremy Avigad
    Author:     Tobias Nipkow
    Author:     Manuel Eberl

This theory deals with properties of primes. Definitions and lemmas are
proved uniformly for the natural numbers and integers.

This file combines and revises a number of prior developments.

The original theories "GCD" and "Primes" were by Christophe Tabacznyj
and Lawrence C. Paulson, based on @{cite davenport92}. They introduced
gcd, lcm, and prime for the natural numbers.

The original theory "IntPrimes" was by Thomas M. Rasmussen, and
extended gcd, lcm, primes to the integers. Amine Chaieb provided
another extension of the notions to the integers, and added a number
of results to "Primes" and "GCD". IntPrimes also defined and developed
the congruence relations on the integers. The notion was extended to
the natural numbers by Chaieb.

Jeremy Avigad combined all of these, made everything uniform for the
natural numbers and the integers, and added a number of new theorems.

Tobias Nipkow cleaned up a lot.

Florian Haftmann and Manuel Eberl put primality and prime factorisation
onto an algebraic foundation and thus generalised these concepts to 
other rings, such as polynomials. (see also the Factorial_Ring theory).

There were also previous formalisations of unique factorisation by 
Thomas Marthedal Rasmussen, Jeremy Avigad, and David Gray.
*)

section \<open>Primes\<close>

theory Primes
imports Euclidean_Algorithm
begin

subsection \<open>Primes on \<^typ>\<open>nat\<close> and \<^typ>\<open>int\<close>\<close>

lemma Suc_0_not_prime_nat [simp]: "\<not> prime (Suc 0)"
  using not_prime_1 [where ?'a = nat] by simp

lemma prime_ge_2_nat:
  "p \<ge> 2" if "prime p" for p :: nat
proof -
  from that have "p \<noteq> 0" and "p \<noteq> 1"
    by (auto dest: prime_elem_not_zeroI prime_elem_not_unit)
  then show ?thesis
    by simp
qed

lemma prime_ge_2_int:
  "p \<ge> 2" if "prime p" for p :: int
proof -
  from that have "prime_elem p" and "\<bar>p\<bar> = p"
    by (auto dest: normalize_prime)
  then have "p \<noteq> 0" and "\<bar>p\<bar> \<noteq> 1" and "p \<ge> 0"
    by (auto dest: prime_elem_not_zeroI prime_elem_not_unit)
  then show ?thesis
    by simp
qed

lemma prime_ge_0_int: "prime p \<Longrightarrow> p \<ge> (0::int)"
  using prime_ge_2_int [of p] by simp

lemma prime_gt_0_nat: "prime p \<Longrightarrow> p > (0::nat)"
  using prime_ge_2_nat [of p] by simp

(* As a simp or intro rule,

     prime p \<Longrightarrow> p > 0

   wreaks havoc here. When the premise includes \<forall>x \<in># M. prime x, it
   leads to the backchaining

     x > 0
     prime x
     x \<in># M   which is, unfortunately,
     count M x > 0  FIXME no, this is obsolete
*)

lemma prime_gt_0_int: "prime p \<Longrightarrow> p > (0::int)"
  using prime_ge_2_int [of p] by simp

lemma prime_ge_1_nat: "prime p \<Longrightarrow> p \<ge> (1::nat)"
  using prime_ge_2_nat [of p] by simp

lemma prime_ge_Suc_0_nat: "prime p \<Longrightarrow> p \<ge> Suc 0"
  using prime_ge_1_nat [of p] by simp

lemma prime_ge_1_int: "prime p \<Longrightarrow> p \<ge> (1::int)"
  using prime_ge_2_int [of p] by simp

lemma prime_gt_1_nat: "prime p \<Longrightarrow> p > (1::nat)"
  using prime_ge_2_nat [of p] by simp

lemma prime_gt_Suc_0_nat: "prime p \<Longrightarrow> p > Suc 0"
  using prime_gt_1_nat [of p] by simp

lemma prime_gt_1_int: "prime p \<Longrightarrow> p > (1::int)"
  using prime_ge_2_int [of p] by simp

lemma prime_natI:
  "prime p" if "p \<ge> 2" and "\<And>m n. p dvd m * n \<Longrightarrow> p dvd m \<or> p dvd n" for p :: nat
  using that by (auto intro!: primeI prime_elemI)

lemma prime_intI:
  "prime p" if "p \<ge> 2" and "\<And>m n. p dvd m * n \<Longrightarrow> p dvd m \<or> p dvd n" for p :: int
  using that by (auto intro!: primeI prime_elemI)

lemma prime_elem_nat_iff [simp]:
  "prime_elem n \<longleftrightarrow> prime n" for n :: nat
  by (simp add: prime_def)

lemma prime_elem_iff_prime_abs [simp]:
  "prime_elem k \<longleftrightarrow> prime \<bar>k\<bar>" for k :: int
  by (auto intro: primeI)

lemma prime_nat_int_transfer [simp]:
  "prime (int n) \<longleftrightarrow> prime n" (is "?P \<longleftrightarrow> ?Q")
proof
  assume ?P
  then have "n \<ge> 2"
    by (auto dest: prime_ge_2_int)
  then show ?Q
  proof (rule prime_natI)
    fix r s
    assume "n dvd r * s"
    with of_nat_dvd_iff [of n "r * s"] have "int n dvd int r * int s"
      by simp
    with \<open>?P\<close> have "int n dvd int r \<or> int n dvd int s"
      using prime_dvd_mult_iff [of "int n" "int r" "int s"]
      by simp
    then show "n dvd r \<or> n dvd s"
      by simp
  qed
next
  assume ?Q
  then have "int n \<ge> 2"
    by (auto dest: prime_ge_2_nat)
  then show ?P
  proof (rule prime_intI)
    fix r s
    assume "int n dvd r * s"
    then have "n dvd nat \<bar>r * s\<bar>"
      by simp
    then have "n dvd nat \<bar>r\<bar> * nat \<bar>s\<bar>"
      by (simp add: nat_abs_mult_distrib)
    with \<open>?Q\<close> have "n dvd nat \<bar>r\<bar> \<or> n dvd nat \<bar>s\<bar>"
      using prime_dvd_mult_iff [of "n" "nat \<bar>r\<bar>" "nat \<bar>s\<bar>"]
      by simp
    then show "int n dvd r \<or> int n dvd s"
      by simp
  qed
qed

lemma prime_nat_iff_prime [simp]:
  "prime (nat k) \<longleftrightarrow> prime k"
proof (cases "k \<ge> 0")
  case True
  then show ?thesis
    using prime_nat_int_transfer [of "nat k"] by simp
next
  case False
  then show ?thesis
    by (auto dest: prime_ge_2_int)
qed

lemma prime_int_nat_transfer:
  "prime k \<longleftrightarrow> k \<ge> 0 \<and> prime (nat k)"
  by (auto dest: prime_ge_2_int)

lemma prime_nat_naiveI:
  "prime p" if "p \<ge> 2" and dvd: "\<And>n. n dvd p \<Longrightarrow> n = 1 \<or> n = p" for p :: nat
proof (rule primeI, rule prime_elemI)
  fix m n :: nat
  assume "p dvd m * n"
  then obtain r s where "p = r * s" "r dvd m" "s dvd n"
    by (blast dest: division_decomp)
  moreover have "r = 1 \<or> r = p"
    using \<open>r dvd m\<close> \<open>p = r * s\<close> dvd [of r] by simp
  ultimately show "p dvd m \<or> p dvd n"
    by auto
qed (use \<open>p \<ge> 2\<close> in simp_all)

lemma prime_int_naiveI:
  "prime p" if "p \<ge> 2" and dvd: "\<And>k. k dvd p \<Longrightarrow> \<bar>k\<bar> = 1 \<or> \<bar>k\<bar> = p" for p :: int
proof -
  from \<open>p \<ge> 2\<close> have "nat p \<ge> 2"
    by simp
  then have "prime (nat p)"
  proof (rule prime_nat_naiveI)
    fix n
    assume "n dvd nat p"
    with \<open>p \<ge> 2\<close> have "n dvd nat \<bar>p\<bar>"
      by simp
    then have "int n dvd p"
      by simp
    with dvd [of "int n"] show "n = 1 \<or> n = nat p"
      by auto
  qed
  then show ?thesis
    by simp
qed

lemma prime_nat_iff:
  "prime (n :: nat) \<longleftrightarrow> (1 < n \<and> (\<forall>m. m dvd n \<longrightarrow> m = 1 \<or> m = n))"
proof (safe intro!: prime_gt_1_nat)
  assume "prime n"
  then have *: "prime_elem n"
    by simp
  fix m assume m: "m dvd n" "m \<noteq> n"
  from * \<open>m dvd n\<close> have "n dvd m \<or> is_unit m"
    by (intro irreducibleD' prime_elem_imp_irreducible)
  with m show "m = 1" by (auto dest: dvd_antisym)
next
  assume "n > 1" "\<forall>m. m dvd n \<longrightarrow> m = 1 \<or> m = n"
  then show "prime n"
    using prime_nat_naiveI [of n] by auto
qed

lemma prime_int_iff:
  "prime (n::int) \<longleftrightarrow> (1 < n \<and> (\<forall>m. m \<ge> 0 \<and> m dvd n \<longrightarrow> m = 1 \<or> m = n))"
proof (intro iffI conjI allI impI; (elim conjE)?)
  assume *: "prime n"
  hence irred: "irreducible n" by (auto intro: prime_elem_imp_irreducible)
  from * have "n \<ge> 0" "n \<noteq> 0" "n \<noteq> 1"
    by (auto simp add: prime_ge_0_int)
  thus "n > 1" by presburger
  fix m assume "m dvd n" \<open>m \<ge> 0\<close>
  with irred have "m dvd 1 \<or> n dvd m" by (auto simp: irreducible_altdef)
  with \<open>m dvd n\<close> \<open>m \<ge> 0\<close> \<open>n > 1\<close> show "m = 1 \<or> m = n"
    using associated_iff_dvd[of m n] by auto
next
  assume n: "1 < n" "\<forall>m. m \<ge> 0 \<and> m dvd n \<longrightarrow> m = 1 \<or> m = n"
  hence "nat n > 1" by simp
  moreover have "\<forall>m. m dvd nat n \<longrightarrow> m = 1 \<or> m = nat n"
  proof (intro allI impI)
    fix m assume "m dvd nat n"
    with \<open>n > 1\<close> have "m dvd nat \<bar>n\<bar>"
      by simp
    then have "int m dvd n"
      by simp
    with n(2) have "int m = 1 \<or> int m = n"
      using of_nat_0_le_iff by blast
    thus "m = 1 \<or> m = nat n" by auto
  qed
  ultimately show "prime n" 
    unfolding prime_int_nat_transfer prime_nat_iff by auto
qed

lemma prime_nat_not_dvd:
  assumes "prime p" "p > n" "n \<noteq> (1::nat)"
  shows   "\<not>n dvd p"
proof
  assume "n dvd p"
  from assms(1) have "irreducible p" by (simp add: prime_elem_imp_irreducible)
  from irreducibleD'[OF this \<open>n dvd p\<close>] \<open>n dvd p\<close> \<open>p > n\<close> assms show False
    by (cases "n = 0") (auto dest!: dvd_imp_le)
qed

lemma prime_int_not_dvd:
  assumes "prime p" "p > n" "n > (1::int)"
  shows   "\<not>n dvd p"
proof
  assume "n dvd p"
  from assms(1) have "irreducible p" by (auto intro: prime_elem_imp_irreducible)
  from irreducibleD'[OF this \<open>n dvd p\<close>] \<open>n dvd p\<close> \<open>p > n\<close> assms show False
    by (auto dest!: zdvd_imp_le)
qed

lemma prime_odd_nat: "prime p \<Longrightarrow> p > (2::nat) \<Longrightarrow> odd p"
  by (intro prime_nat_not_dvd) auto

lemma prime_odd_int: "prime p \<Longrightarrow> p > (2::int) \<Longrightarrow> odd p"
  by (intro prime_int_not_dvd) auto

lemma prime_int_altdef:
  "prime p = (1 < p \<and> (\<forall>m::int. m \<ge> 0 \<longrightarrow> m dvd p \<longrightarrow>
    m = 1 \<or> m = p))"
  unfolding prime_int_iff by blast

lemma not_prime_eq_prod_nat:
  assumes "m > 1" "\<not> prime (m::nat)"
  shows   "\<exists>n k. n = m * k \<and> 1 < m \<and> m < n \<and> 1 < k \<and> k < n"
  using assms irreducible_altdef[of m]
  by (auto simp: prime_elem_iff_irreducible irreducible_altdef)

    
subsection \<open>Largest exponent of a prime factor\<close>

text\<open>Possibly duplicates other material, but avoid the complexities of multisets.\<close>
  
lemma prime_power_cancel_less:
  assumes "prime p" and eq: "m * (p ^ k) = m' * (p ^ k')" and less: "k < k'" and "\<not> p dvd m"
  shows False
proof -
  obtain l where l: "k' = k + l" and "l > 0"
    using less less_imp_add_positive by auto
  have "m = m * (p ^ k) div (p ^ k)"
    using \<open>prime p\<close> by simp
  also have "\<dots> = m' * (p ^ k') div (p ^ k)"
    using eq by simp
  also have "\<dots> = m' * (p ^ l) * (p ^ k) div (p ^ k)"
    by (simp add: l mult.commute mult.left_commute power_add)
  also have "... = m' * (p ^ l)"
    using \<open>prime p\<close> by simp
  finally have "p dvd m"
    using \<open>l > 0\<close> by simp
  with assms show False
    by simp
qed

lemma prime_power_cancel:
  assumes "prime p" and eq: "m * (p ^ k) = m' * (p ^ k')" and "\<not> p dvd m" "\<not> p dvd m'"
  shows "k = k'"
  using prime_power_cancel_less [OF \<open>prime p\<close>] assms
  by (metis linorder_neqE_nat)

lemma prime_power_cancel2:
  assumes "prime p" "m * (p ^ k) = m' * (p ^ k')" "\<not> p dvd m" "\<not> p dvd m'"
  obtains "m = m'" "k = k'"
  using prime_power_cancel [OF assms] assms by auto

lemma prime_power_canonical:
  fixes m :: nat
  assumes "prime p" "m > 0"
  shows "\<exists>k n. \<not> p dvd n \<and> m = n * p ^ k"
using \<open>m > 0\<close>
proof (induction m rule: less_induct)
  case (less m)
  show ?case
  proof (cases "p dvd m")
    case True
    then obtain m' where m': "m = p * m'"
      using dvdE by blast
    with \<open>prime p\<close> have "0 < m'" "m' < m"
      using less.prems prime_nat_iff by auto
    with m' less show ?thesis
      by (metis power_Suc mult.left_commute)
  next
    case False
    then show ?thesis
      by (metis mult.right_neutral power_0)
  qed
qed


subsubsection \<open>Make prime naively executable\<close>

lemma prime_nat_iff':
  "prime (p :: nat) \<longleftrightarrow> p > 1 \<and> (\<forall>n \<in> {2..<p}. \<not> n dvd p)"
proof safe
  assume "p > 1" and *: "\<forall>n\<in>{2..<p}. \<not>n dvd p"
  show "prime p" unfolding prime_nat_iff
  proof (intro conjI allI impI)
    fix m assume "m dvd p"
    with \<open>p > 1\<close> have "m \<noteq> 0" by (intro notI) auto
    hence "m \<ge> 1" by simp
    moreover from \<open>m dvd p\<close> and * have "m \<notin> {2..<p}" by blast
    with \<open>m dvd p\<close> and \<open>p > 1\<close> have "m \<le> 1 \<or> m = p" by (auto dest: dvd_imp_le)
    ultimately show "m = 1 \<or> m = p" by simp
  qed fact+
qed (auto simp: prime_nat_iff)

lemma prime_int_iff':
  "prime (p :: int) \<longleftrightarrow> p > 1 \<and> (\<forall>n \<in> {2..<p}. \<not> n dvd p)" (is "?P \<longleftrightarrow> ?Q")
proof (cases "p \<ge> 0")
  case True
  have "?P \<longleftrightarrow> prime (nat p)"
    by simp
  also have "\<dots> \<longleftrightarrow> p > 1 \<and> (\<forall>n\<in>{2..<nat p}. \<not> n dvd nat \<bar>p\<bar>)"
    using True by (simp add: prime_nat_iff')
  also have "{2..<nat p} = nat ` {2..<p}"
    using True int_eq_iff by fastforce 
  finally show "?P \<longleftrightarrow> ?Q" by simp
next
  case False
  then show ?thesis
    by (auto simp add: prime_ge_0_int) 
qed

lemma prime_int_numeral_eq [simp]:
  "prime (numeral m :: int) \<longleftrightarrow> prime (numeral m :: nat)"
  by (simp add: prime_int_nat_transfer)

lemma two_is_prime_nat [simp]: "prime (2::nat)"
  by (simp add: prime_nat_iff')

lemma prime_nat_numeral_eq [simp]:
  "prime (numeral m :: nat) \<longleftrightarrow>
    (1::nat) < numeral m \<and>
    (\<forall>n::nat \<in> set [2..<numeral m]. \<not> n dvd numeral m)"
  by (simp only: prime_nat_iff' set_upt)  \<comment> \<open>TODO Sieve Of Erathosthenes might speed this up\<close>


text\<open>A bit of regression testing:\<close>

lemma "prime(97::nat)" by simp
lemma "prime(97::int)" by simp

lemma prime_factor_nat: 
  "n \<noteq> (1::nat) \<Longrightarrow> \<exists>p. prime p \<and> p dvd n"
  using prime_divisor_exists[of n]
  by (cases "n = 0") (auto intro: exI[of _ "2::nat"])

lemma prime_factor_int:
  fixes k :: int
  assumes "\<bar>k\<bar> \<noteq> 1"
  obtains p where "prime p" "p dvd k"
proof (cases "k = 0")
  case True
  then have "prime (2::int)" and "2 dvd k"
    by simp_all
  with that show thesis
    by blast
next
  case False
  with assms prime_divisor_exists [of k] obtain p where "prime p" "p dvd k"
    by auto
  with that show thesis
    by blast
qed


subsection \<open>Infinitely many primes\<close>

lemma next_prime_bound: "\<exists>p::nat. prime p \<and> n < p \<and> p \<le> fact n + 1"
proof-
  have f1: "fact n + 1 \<noteq> (1::nat)" using fact_ge_1 [of n, where 'a=nat] by arith
  from prime_factor_nat [OF f1]
  obtain p :: nat where "prime p" and "p dvd fact n + 1" by auto
  then have "p \<le> fact n + 1" apply (intro dvd_imp_le) apply auto done
  { assume "p \<le> n"
    from \<open>prime p\<close> have "p \<ge> 1"
      by (cases p, simp_all)
    with \<open>p <= n\<close> have "p dvd fact n"
      by (intro dvd_fact)
    with \<open>p dvd fact n + 1\<close> have "p dvd fact n + 1 - fact n"
      by (rule dvd_diff_nat)
    then have "p dvd 1" by simp
    then have "p <= 1" by auto
    moreover from \<open>prime p\<close> have "p > 1"
      using prime_nat_iff by blast
    ultimately have False by auto}
  then have "n < p" by presburger
  with \<open>prime p\<close> and \<open>p <= fact n + 1\<close> show ?thesis by auto
qed

lemma bigger_prime: "\<exists>p. prime p \<and> p > (n::nat)"
  using next_prime_bound by auto

lemma primes_infinite: "\<not> (finite {(p::nat). prime p})"
proof
  assume "finite {(p::nat). prime p}"
  with Max_ge have "(\<exists>b. (\<forall>x \<in> {(p::nat). prime p}. x \<le> b))"
    by auto
  then obtain b where "\<forall>(x::nat). prime x \<longrightarrow> x \<le> b"
    by auto
  with bigger_prime [of b] show False
    by auto
qed

subsection \<open>Powers of Primes\<close>

text\<open>Versions for type nat only\<close>

lemma prime_product:
  fixes p::nat
  assumes "prime (p * q)"
  shows "p = 1 \<or> q = 1"
proof -
  from assms have
    "1 < p * q" and P: "\<And>m. m dvd p * q \<Longrightarrow> m = 1 \<or> m = p * q"
    unfolding prime_nat_iff by auto
  from \<open>1 < p * q\<close> have "p \<noteq> 0" by (cases p) auto
  then have Q: "p = p * q \<longleftrightarrow> q = 1" by auto
  have "p dvd p * q" by simp
  then have "p = 1 \<or> p = p * q" by (rule P)
  then show ?thesis by (simp add: Q)
qed

(* TODO: Generalise? *)
lemma prime_power_mult_nat:
  fixes p :: nat
  assumes p: "prime p" and xy: "x * y = p ^ k"
  shows "\<exists>i j. x = p ^ i \<and> y = p^ j"
using xy
proof(induct k arbitrary: x y)
  case 0 thus ?case apply simp by (rule exI[where x="0"], simp)
next
  case (Suc k x y)
  from Suc.prems have pxy: "p dvd x*y" by auto
  from prime_dvd_multD [OF p pxy] have pxyc: "p dvd x \<or> p dvd y" .
  from p have p0: "p \<noteq> 0" by - (rule ccontr, simp)
  {assume px: "p dvd x"
    then obtain d where d: "x = p*d" unfolding dvd_def by blast
    from Suc.prems d  have "p*d*y = p^Suc k" by simp
    hence th: "d*y = p^k" using p0 by simp
    from Suc.hyps[OF th] obtain i j where ij: "d = p^i" "y = p^j" by blast
    with d have "x = p^Suc i" by simp
    with ij(2) have ?case by blast}
  moreover
  {assume px: "p dvd y"
    then obtain d where d: "y = p*d" unfolding dvd_def by blast
    from Suc.prems d  have "p*d*x = p^Suc k" by (simp add: mult.commute)
    hence th: "d*x = p^k" using p0 by simp
    from Suc.hyps[OF th] obtain i j where ij: "d = p^i" "x = p^j" by blast
    with d have "y = p^Suc i" by simp
    with ij(2) have ?case by blast}
  ultimately show ?case  using pxyc by blast
qed

lemma prime_power_exp_nat:
  fixes p::nat
  assumes p: "prime p" and n: "n \<noteq> 0"
    and xn: "x^n = p^k" shows "\<exists>i. x = p^i"
  using n xn
proof(induct n arbitrary: k)
  case 0 thus ?case by simp
next
  case (Suc n k) hence th: "x*x^n = p^k" by simp
  {assume "n = 0" with Suc have ?case by simp (rule exI[where x="k"], simp)}
  moreover
  {assume n: "n \<noteq> 0"
    from prime_power_mult_nat[OF p th]
    obtain i j where ij: "x = p^i" "x^n = p^j"by blast
    from Suc.hyps[OF n ij(2)] have ?case .}
  ultimately show ?case by blast
qed

lemma divides_primepow_nat:
  fixes p :: nat
  assumes p: "prime p"
  shows "d dvd p ^ k \<longleftrightarrow> (\<exists>i\<le>k. d = p ^ i)"
  using assms divides_primepow [of p d k] by (auto intro: le_imp_power_dvd)


subsection \<open>Chinese Remainder Theorem Variants\<close>

lemma bezout_gcd_nat:
  fixes a::nat shows "\<exists>x y. a * x - b * y = gcd a b \<or> b * x - a * y = gcd a b"
  using bezout_nat[of a b]
by (metis bezout_nat diff_add_inverse gcd_add_mult gcd.commute
  gcd_nat.right_neutral mult_0)

lemma gcd_bezout_sum_nat:
  fixes a::nat
  assumes "a * x + b * y = d"
  shows "gcd a b dvd d"
proof-
  let ?g = "gcd a b"
    have dv: "?g dvd a*x" "?g dvd b * y"
      by simp_all
    from dvd_add[OF dv] assms
    show ?thesis by auto
qed


text \<open>A binary form of the Chinese Remainder Theorem.\<close>

(* TODO: Generalise? *)
lemma chinese_remainder:
  fixes a::nat  assumes ab: "coprime a b" and a: "a \<noteq> 0" and b: "b \<noteq> 0"
  shows "\<exists>x q1 q2. x = u + q1 * a \<and> x = v + q2 * b"
proof-
  from bezout_add_strong_nat[OF a, of b] bezout_add_strong_nat[OF b, of a]
  obtain d1 x1 y1 d2 x2 y2 where dxy1: "d1 dvd a" "d1 dvd b" "a * x1 = b * y1 + d1"
    and dxy2: "d2 dvd b" "d2 dvd a" "b * x2 = a * y2 + d2" by blast
  then have d12: "d1 = 1" "d2 = 1"
    using ab coprime_common_divisor_nat [of a b] by blast+
  let ?x = "v * a * x1 + u * b * x2"
  let ?q1 = "v * x1 + u * y2"
  let ?q2 = "v * y1 + u * x2"
  from dxy2(3)[simplified d12] dxy1(3)[simplified d12]
  have "?x = u + ?q1 * a" "?x = v + ?q2 * b"
    by algebra+
  thus ?thesis by blast
qed

text \<open>Primality\<close>

lemma coprime_bezout_strong:
  fixes a::nat assumes "coprime a b"  "b \<noteq> 1"
  shows "\<exists>x y. a * x = b * y + 1"
  by (metis add.commute add.right_neutral assms(1) assms(2) chinese_remainder coprime_1_left coprime_1_right coprime_crossproduct_nat mult.commute mult.right_neutral mult_cancel_left)

lemma bezout_prime:
  assumes p: "prime p" and pa: "\<not> p dvd a"
  shows "\<exists>x y. a*x = Suc (p*y)"
proof -
  have ap: "coprime a p"
    using coprime_commute p pa prime_imp_coprime by auto
  moreover from p have "p \<noteq> 1" by auto
  ultimately have "\<exists>x y. a * x = p * y + 1"
    by (rule coprime_bezout_strong)
  then show ?thesis by simp    
qed
(* END TODO *)



subsection \<open>Multiplicity and primality for natural numbers and integers\<close>

lemma prime_factors_gt_0_nat:
  "p \<in> prime_factors x \<Longrightarrow> p > (0::nat)"
  by (simp add: in_prime_factors_imp_prime prime_gt_0_nat)

lemma prime_factors_gt_0_int:
  "p \<in> prime_factors x \<Longrightarrow> p > (0::int)"
  by (simp add: in_prime_factors_imp_prime prime_gt_0_int)

lemma prime_factors_ge_0_int [elim]: (* FIXME !? *)
  fixes n :: int
  shows "p \<in> prime_factors n \<Longrightarrow> p \<ge> 0"
  by (drule prime_factors_gt_0_int) simp
  
lemma prod_mset_prime_factorization_int:
  fixes n :: int
  assumes "n > 0"
  shows   "prod_mset (prime_factorization n) = n"
  using assms by (simp add: prod_mset_prime_factorization)

lemma prime_factorization_exists_nat:
  "n > 0 \<Longrightarrow> (\<exists>M. (\<forall>p::nat \<in> set_mset M. prime p) \<and> n = (\<Prod>i \<in># M. i))"
  using prime_factorization_exists[of n] by auto

lemma prod_mset_prime_factorization_nat [simp]: 
  "(n::nat) > 0 \<Longrightarrow> prod_mset (prime_factorization n) = n"
  by (subst prod_mset_prime_factorization) simp_all

lemma prime_factorization_nat:
    "n > (0::nat) \<Longrightarrow> n = (\<Prod>p \<in> prime_factors n. p ^ multiplicity p n)"
  by (simp add: prod_prime_factors)

lemma prime_factorization_int:
    "n > (0::int) \<Longrightarrow> n = (\<Prod>p \<in> prime_factors n. p ^ multiplicity p n)"
  by (simp add: prod_prime_factors)

lemma prime_factorization_unique_nat:
  fixes f :: "nat \<Rightarrow> _"
  assumes S_eq: "S = {p. 0 < f p}"
    and "finite S"
    and S: "\<forall>p\<in>S. prime p" "n = (\<Prod>p\<in>S. p ^ f p)"
  shows "S = prime_factors n \<and> (\<forall>p. prime p \<longrightarrow> f p = multiplicity p n)"
  using assms by (intro prime_factorization_unique'') auto

lemma prime_factorization_unique_int:
  fixes f :: "int \<Rightarrow> _"
  assumes S_eq: "S = {p. 0 < f p}"
    and "finite S"
    and S: "\<forall>p\<in>S. prime p" "abs n = (\<Prod>p\<in>S. p ^ f p)"
  shows "S = prime_factors n \<and> (\<forall>p. prime p \<longrightarrow> f p = multiplicity p n)"
  using assms by (intro prime_factorization_unique'') auto

lemma prime_factors_characterization_nat:
  "S = {p. 0 < f (p::nat)} \<Longrightarrow>
    finite S \<Longrightarrow> \<forall>p\<in>S. prime p \<Longrightarrow> n = (\<Prod>p\<in>S. p ^ f p) \<Longrightarrow> prime_factors n = S"
  by (rule prime_factorization_unique_nat [THEN conjunct1, symmetric])

lemma prime_factors_characterization'_nat:
  "finite {p. 0 < f (p::nat)} \<Longrightarrow>
    (\<forall>p. 0 < f p \<longrightarrow> prime p) \<Longrightarrow>
      prime_factors (\<Prod>p | 0 < f p. p ^ f p) = {p. 0 < f p}"
  by (rule prime_factors_characterization_nat) auto

lemma prime_factors_characterization_int:
  "S = {p. 0 < f (p::int)} \<Longrightarrow> finite S \<Longrightarrow>
    \<forall>p\<in>S. prime p \<Longrightarrow> abs n = (\<Prod>p\<in>S. p ^ f p) \<Longrightarrow> prime_factors n = S"
  by (rule prime_factorization_unique_int [THEN conjunct1, symmetric])

(* TODO Move *)
lemma abs_prod: "abs (prod f A :: 'a :: linordered_idom) = prod (\<lambda>x. abs (f x)) A"
  by (cases "finite A", induction A rule: finite_induct) (simp_all add: abs_mult)

lemma primes_characterization'_int [rule_format]:
  "finite {p. p \<ge> 0 \<and> 0 < f (p::int)} \<Longrightarrow> \<forall>p. 0 < f p \<longrightarrow> prime p \<Longrightarrow>
      prime_factors (\<Prod>p | p \<ge> 0 \<and> 0 < f p. p ^ f p) = {p. p \<ge> 0 \<and> 0 < f p}"
  by (rule prime_factors_characterization_int) (auto simp: abs_prod prime_ge_0_int)

lemma multiplicity_characterization_nat:
  "S = {p. 0 < f (p::nat)} \<Longrightarrow> finite S \<Longrightarrow> \<forall>p\<in>S. prime p \<Longrightarrow> prime p \<Longrightarrow>
    n = (\<Prod>p\<in>S. p ^ f p) \<Longrightarrow> multiplicity p n = f p"
  by (frule prime_factorization_unique_nat [of S f n, THEN conjunct2, rule_format, symmetric]) auto

lemma multiplicity_characterization'_nat: "finite {p. 0 < f (p::nat)} \<longrightarrow>
    (\<forall>p. 0 < f p \<longrightarrow> prime p) \<longrightarrow> prime p \<longrightarrow>
      multiplicity p (\<Prod>p | 0 < f p. p ^ f p) = f p"
  by (intro impI, rule multiplicity_characterization_nat) auto

lemma multiplicity_characterization_int: "S = {p. 0 < f (p::int)} \<Longrightarrow>
    finite S \<Longrightarrow> \<forall>p\<in>S. prime p \<Longrightarrow> prime p \<Longrightarrow> n = (\<Prod>p\<in>S. p ^ f p) \<Longrightarrow> multiplicity p n = f p"
  by (frule prime_factorization_unique_int [of S f n, THEN conjunct2, rule_format, symmetric]) 
     (auto simp: abs_prod power_abs prime_ge_0_int intro!: prod.cong)

lemma multiplicity_characterization'_int [rule_format]:
  "finite {p. p \<ge> 0 \<and> 0 < f (p::int)} \<Longrightarrow>
    (\<forall>p. 0 < f p \<longrightarrow> prime p) \<Longrightarrow> prime p \<Longrightarrow>
      multiplicity p (\<Prod>p | p \<ge> 0 \<and> 0 < f p. p ^ f p) = f p"
  by (rule multiplicity_characterization_int) (auto simp: prime_ge_0_int)

lemma multiplicity_one_nat [simp]: "multiplicity p (Suc 0) = 0"
  unfolding One_nat_def [symmetric] by (rule multiplicity_one)

lemma multiplicity_eq_nat:
  fixes x and y::nat
  assumes "x > 0" "y > 0" "\<And>p. prime p \<Longrightarrow> multiplicity p x = multiplicity p y"
  shows "x = y"
  using multiplicity_eq_imp_eq[of x y] assms by simp

lemma multiplicity_eq_int:
  fixes x y :: int
  assumes "x > 0" "y > 0" "\<And>p. prime p \<Longrightarrow> multiplicity p x = multiplicity p y"
  shows "x = y"
  using multiplicity_eq_imp_eq[of x y] assms by simp

lemma multiplicity_prod_prime_powers:
  assumes "finite S" "\<And>x. x \<in> S \<Longrightarrow> prime x" "prime p"
  shows   "multiplicity p (\<Prod>p \<in> S. p ^ f p) = (if p \<in> S then f p else 0)"
proof -
  define g where "g = (\<lambda>x. if x \<in> S then f x else 0)"
  define A where "A = Abs_multiset g"
  have "{x. g x > 0} \<subseteq> S" by (auto simp: g_def)
  from finite_subset[OF this assms(1)] have [simp]: "g \<in> multiset"
    by (simp add: multiset_def)
  from assms have count_A: "count A x = g x" for x unfolding A_def
    by simp
  have set_mset_A: "set_mset A = {x\<in>S. f x > 0}"
    unfolding set_mset_def count_A by (auto simp: g_def)
  with assms have prime: "prime x" if "x \<in># A" for x using that by auto
  from set_mset_A assms have "(\<Prod>p \<in> S. p ^ f p) = (\<Prod>p \<in> S. p ^ g p) "
    by (intro prod.cong) (auto simp: g_def)
  also from set_mset_A assms have "\<dots> = (\<Prod>p \<in> set_mset A. p ^ g p)"
    by (intro prod.mono_neutral_right) (auto simp: g_def set_mset_A)
  also have "\<dots> = prod_mset A"
    by (auto simp: prod_mset_multiplicity count_A set_mset_A intro!: prod.cong)
  also from assms have "multiplicity p \<dots> = sum_mset (image_mset (multiplicity p) A)"
    by (subst prime_elem_multiplicity_prod_mset_distrib) (auto dest: prime)
  also from assms have "image_mset (multiplicity p) A = image_mset (\<lambda>x. if x = p then 1 else 0) A"
    by (intro image_mset_cong) (auto simp: prime_multiplicity_other dest: prime)
  also have "sum_mset \<dots> = (if p \<in> S then f p else 0)" by (simp add: sum_mset_delta count_A g_def)
  finally show ?thesis .
qed

lemma prime_factorization_prod_mset:
  assumes "0 \<notin># A"
  shows "prime_factorization (prod_mset A) = \<Sum>\<^sub>#(image_mset prime_factorization A)"
  using assms by (induct A) (auto simp add: prime_factorization_mult)

lemma prime_factors_prod:
  assumes "finite A" and "0 \<notin> f ` A"
  shows "prime_factors (prod f A) = \<Union>((prime_factors \<circ> f) ` A)"
  using assms by (simp add: prod_unfold_prod_mset prime_factorization_prod_mset)

lemma prime_factors_fact:
  "prime_factors (fact n) = {p \<in> {2..n}. prime p}" (is "?M = ?N")
proof (rule set_eqI)
  fix p
  { fix m :: nat
    assume "p \<in> prime_factors m"
    then have "prime p" and "p dvd m" by auto
    moreover assume "m > 0" 
    ultimately have "2 \<le> p" and "p \<le> m"
      by (auto intro: prime_ge_2_nat dest: dvd_imp_le)
    moreover assume "m \<le> n"
    ultimately have "2 \<le> p" and "p \<le> n"
      by (auto intro: order_trans)
  } note * = this
  show "p \<in> ?M \<longleftrightarrow> p \<in> ?N"
    by (auto simp add: fact_prod prime_factors_prod Suc_le_eq dest!: prime_prime_factors intro: *)
qed

lemma prime_dvd_fact_iff:
  assumes "prime p"
  shows "p dvd fact n \<longleftrightarrow> p \<le> n"
  using assms
  by (auto simp add: prime_factorization_subset_iff_dvd [symmetric]
    prime_factorization_prime prime_factors_fact prime_ge_2_nat)

(* TODO Legacy names *)
lemmas prime_imp_coprime_nat = prime_imp_coprime[where ?'a = nat]
lemmas prime_imp_coprime_int = prime_imp_coprime[where ?'a = int]
lemmas prime_dvd_mult_nat = prime_dvd_mult_iff[where ?'a = nat]
lemmas prime_dvd_mult_int = prime_dvd_mult_iff[where ?'a = int]
lemmas prime_dvd_mult_eq_nat = prime_dvd_mult_iff[where ?'a = nat]
lemmas prime_dvd_mult_eq_int = prime_dvd_mult_iff[where ?'a = int]
lemmas prime_dvd_power_nat = prime_dvd_power[where ?'a = nat]
lemmas prime_dvd_power_int = prime_dvd_power[where ?'a = int]
lemmas prime_dvd_power_nat_iff = prime_dvd_power_iff[where ?'a = nat]
lemmas prime_dvd_power_int_iff = prime_dvd_power_iff[where ?'a = int]
lemmas prime_imp_power_coprime_nat = prime_imp_power_coprime[where ?'a = nat]
lemmas prime_imp_power_coprime_int = prime_imp_power_coprime[where ?'a = int]
lemmas primes_coprime_nat = primes_coprime[where ?'a = nat]
lemmas primes_coprime_int = primes_coprime[where ?'a = nat]
lemmas prime_divprod_pow_nat = prime_elem_divprod_pow[where ?'a = nat]
lemmas prime_exp = prime_elem_power_iff[where ?'a = nat]

text \<open>Code generation\<close>
  
context
begin

qualified definition prime_nat :: "nat \<Rightarrow> bool"
  where [simp, code_abbrev]: "prime_nat = prime"

lemma prime_nat_naive [code]:
  "prime_nat p \<longleftrightarrow> p > 1 \<and> (\<forall>n \<in>{1<..<p}. \<not> n dvd p)"
  by (auto simp add: prime_nat_iff')

qualified definition prime_int :: "int \<Rightarrow> bool"
  where [simp, code_abbrev]: "prime_int = prime"

lemma prime_int_naive [code]:
  "prime_int p \<longleftrightarrow> p > 1 \<and> (\<forall>n \<in>{1<..<p}. \<not> n dvd p)"
  by (auto simp add: prime_int_iff')

lemma "prime(997::nat)" by eval

lemma "prime(997::int)" by eval
  
end

end