src/HOL/Number_Theory/Residues.thy
author wenzelm
Mon, 26 Oct 2020 21:35:39 +0100
changeset 72510 a471730347e0
parent 71252 c5914bdd896b
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NEWS;

(*  Title:      HOL/Number_Theory/Residues.thy
    Author:     Jeremy Avigad

An algebraic treatment of residue rings, and resulting proofs of
Euler's theorem and Wilson's theorem.
*)

section \<open>Residue rings\<close>

theory Residues
imports
  Cong
  "HOL-Algebra.Multiplicative_Group"
  Totient
begin

definition QuadRes :: "int \<Rightarrow> int \<Rightarrow> bool"
  where "QuadRes p a = (\<exists>y. ([y^2 = a] (mod p)))"

definition Legendre :: "int \<Rightarrow> int \<Rightarrow> int"
  where "Legendre a p =
    (if ([a = 0] (mod p)) then 0
     else if QuadRes p a then 1
     else -1)"


subsection \<open>A locale for residue rings\<close>

definition residue_ring :: "int \<Rightarrow> int ring"
  where
    "residue_ring m =
      \<lparr>carrier = {0..m - 1},
       monoid.mult = \<lambda>x y. (x * y) mod m,
       one = 1,
       zero = 0,
       add = \<lambda>x y. (x + y) mod m\<rparr>"

locale residues =
  fixes m :: int and R (structure)
  assumes m_gt_one: "m > 1"
  defines "R \<equiv> residue_ring m"
begin

lemma abelian_group: "abelian_group R"
proof -
  have "\<exists>y\<in>{0..m - 1}. (x + y) mod m = 0" if "0 \<le> x" "x < m" for x
  proof (cases "x = 0")
    case True
    with m_gt_one show ?thesis by simp
  next
    case False
    then have "(x + (m - x)) mod m = 0"
      by simp
    with m_gt_one that show ?thesis
      by (metis False atLeastAtMost_iff diff_ge_0_iff_ge diff_left_mono int_one_le_iff_zero_less less_le)
  qed
  with m_gt_one show ?thesis
    by (fastforce simp add: R_def residue_ring_def mod_add_right_eq ac_simps  intro!: abelian_groupI)
qed

lemma comm_monoid: "comm_monoid R"
  unfolding R_def residue_ring_def
  apply (rule comm_monoidI)
    using m_gt_one  apply auto
  apply (metis mod_mult_right_eq mult.assoc mult.commute)
  apply (metis mult.commute)
  done

lemma cring: "cring R"
  apply (intro cringI abelian_group comm_monoid)
  unfolding R_def residue_ring_def
  apply (auto simp add: comm_semiring_class.distrib mod_add_eq mod_mult_left_eq)
  done

end

sublocale residues < cring
  by (rule cring)


context residues
begin

text \<open>
  These lemmas translate back and forth between internal and
  external concepts.
\<close>

lemma res_carrier_eq: "carrier R = {0..m - 1}"
  by (auto simp: R_def residue_ring_def)

lemma res_add_eq: "x \<oplus> y = (x + y) mod m"
  by (auto simp: R_def residue_ring_def)

lemma res_mult_eq: "x \<otimes> y = (x * y) mod m"
  by (auto simp: R_def residue_ring_def)

lemma res_zero_eq: "\<zero> = 0"
  by (auto simp: R_def residue_ring_def)

lemma res_one_eq: "\<one> = 1"
  by (auto simp: R_def residue_ring_def units_of_def)

lemma res_units_eq: "Units R = {x. 0 < x \<and> x < m \<and> coprime x m}"
  using m_gt_one
  apply (auto simp add: Units_def R_def residue_ring_def ac_simps invertible_coprime intro: ccontr)
  apply (subst (asm) coprime_iff_invertible'_int)
   apply (auto simp add: cong_def)
  done

lemma res_neg_eq: "\<ominus> x = (- x) mod m"
  using m_gt_one unfolding R_def a_inv_def m_inv_def residue_ring_def
  apply simp
  apply (rule the_equality)
   apply (simp add: mod_add_right_eq)
   apply (simp add: add.commute mod_add_right_eq)
  apply (metis add.right_neutral minus_add_cancel mod_add_right_eq mod_pos_pos_trivial)
  done

lemma finite [iff]: "finite (carrier R)"
  by (simp add: res_carrier_eq)

lemma finite_Units [iff]: "finite (Units R)"
  by (simp add: finite_ring_finite_units)

text \<open>
  The function \<open>a \<mapsto> a mod m\<close> maps the integers to the
  residue classes. The following lemmas show that this mapping
  respects addition and multiplication on the integers.
\<close>

lemma mod_in_carrier [iff]: "a mod m \<in> carrier R"
  unfolding res_carrier_eq
  using insert m_gt_one by auto

lemma add_cong: "(x mod m) \<oplus> (y mod m) = (x + y) mod m"
  by (auto simp: R_def residue_ring_def mod_simps)

lemma mult_cong: "(x mod m) \<otimes> (y mod m) = (x * y) mod m"
  by (auto simp: R_def residue_ring_def mod_simps)

lemma zero_cong: "\<zero> = 0"
  by (auto simp: R_def residue_ring_def)

lemma one_cong: "\<one> = 1 mod m"
  using m_gt_one by (auto simp: R_def residue_ring_def)

(* FIXME revise algebra library to use 1? *)
lemma pow_cong: "(x mod m) [^] n = x^n mod m"
  using m_gt_one
  apply (induct n)
  apply (auto simp add: nat_pow_def one_cong)
  apply (metis mult.commute mult_cong)
  done

lemma neg_cong: "\<ominus> (x mod m) = (- x) mod m"
  by (metis mod_minus_eq res_neg_eq)

lemma (in residues) prod_cong: "finite A \<Longrightarrow> (\<Otimes>i\<in>A. (f i) mod m) = (\<Prod>i\<in>A. f i) mod m"
  by (induct set: finite) (auto simp: one_cong mult_cong)

lemma (in residues) sum_cong: "finite A \<Longrightarrow> (\<Oplus>i\<in>A. (f i) mod m) = (\<Sum>i\<in>A. f i) mod m"
  by (induct set: finite) (auto simp: zero_cong add_cong)

lemma mod_in_res_units [simp]:
  assumes "1 < m" and "coprime a m"
  shows "a mod m \<in> Units R"
proof (cases "a mod m = 0")
  case True
  with assms show ?thesis
    by (auto simp add: res_units_eq gcd_red_int [symmetric])
next
  case False
  from assms have "0 < m" by simp
  then have "0 \<le> a mod m" by (rule pos_mod_sign [of m a])
  with False have "0 < a mod m" by simp
  with assms show ?thesis
    by (auto simp add: res_units_eq gcd_red_int [symmetric] ac_simps)
qed

lemma res_eq_to_cong: "(a mod m) = (b mod m) \<longleftrightarrow> [a = b] (mod m)"
  by (auto simp: cong_def)


text \<open>Simplifying with these will translate a ring equation in R to a congruence.\<close>
lemmas res_to_cong_simps =
  add_cong mult_cong pow_cong one_cong
  prod_cong sum_cong neg_cong res_eq_to_cong

text \<open>Other useful facts about the residue ring.\<close>
lemma one_eq_neg_one: "\<one> = \<ominus> \<one> \<Longrightarrow> m = 2"
  apply (simp add: res_one_eq res_neg_eq)
  apply (metis add.commute add_diff_cancel mod_mod_trivial one_add_one uminus_add_conv_diff
    zero_neq_one zmod_zminus1_eq_if)
  done

end


subsection \<open>Prime residues\<close>

locale residues_prime =
  fixes p :: nat and R (structure)
  assumes p_prime [intro]: "prime p"
  defines "R \<equiv> residue_ring (int p)"

sublocale residues_prime < residues p
  unfolding R_def residues_def
  using p_prime apply auto
  apply (metis (full_types) of_nat_1 of_nat_less_iff prime_gt_1_nat)
  done

context residues_prime
begin

lemma p_coprime_left:
  "coprime p a \<longleftrightarrow> \<not> p dvd a"
  using p_prime by (auto intro: prime_imp_coprime dest: coprime_common_divisor)

lemma p_coprime_right:
  "coprime a p  \<longleftrightarrow> \<not> p dvd a"
  using p_coprime_left [of a] by (simp add: ac_simps)

lemma p_coprime_left_int:
  "coprime (int p) a \<longleftrightarrow> \<not> int p dvd a"
  using p_prime by (auto intro: prime_imp_coprime dest: coprime_common_divisor)

lemma p_coprime_right_int:
  "coprime a (int p) \<longleftrightarrow> \<not> int p dvd a"
  using p_coprime_left_int [of a] by (simp add: ac_simps)

lemma is_field: "field R"
proof -
  have "0 < x \<Longrightarrow> x < int p \<Longrightarrow> coprime (int p) x" for x
    by (rule prime_imp_coprime) (auto simp add: zdvd_not_zless)
  then show ?thesis
    by (intro cring.field_intro2 cring)
      (auto simp add: res_carrier_eq res_one_eq res_zero_eq res_units_eq ac_simps)
qed

lemma res_prime_units_eq: "Units R = {1..p - 1}"
  apply (subst res_units_eq)
  apply (auto simp add: p_coprime_right_int zdvd_not_zless)
  done

end

sublocale residues_prime < field
  by (rule is_field)


section \<open>Test cases: Euler's theorem and Wilson's theorem\<close>

subsection \<open>Euler's theorem\<close>

lemma (in residues) totatives_eq:
  "totatives (nat m) = nat ` Units R"
proof -
  from m_gt_one have "\<bar>m\<bar> > 1"
    by simp
  then have "totatives (nat \<bar>m\<bar>) = nat ` abs ` Units R"
    by (auto simp add: totatives_def res_units_eq image_iff le_less)
      (use m_gt_one zless_nat_eq_int_zless in force)
  moreover have "\<bar>m\<bar> = m" "abs ` Units R = Units R"
    using m_gt_one by (auto simp add: res_units_eq image_iff)
  ultimately show ?thesis
    by simp
qed

lemma (in residues) totient_eq:
  "totient (nat m) = card (Units R)"
proof  -
  have *: "inj_on nat (Units R)"
    by (rule inj_onI) (auto simp add: res_units_eq)
  then show ?thesis
    by (simp add: totient_def totatives_eq card_image)
qed

lemma (in residues_prime) totient_eq: "totient p = p - 1"
  using totient_eq by (simp add: res_prime_units_eq)

lemma (in residues) euler_theorem:
  assumes "coprime a m"
  shows "[a ^ totient (nat m) = 1] (mod m)"
proof -
  have "a ^ totient (nat m) mod m = 1 mod m"
    by (metis assms finite_Units m_gt_one mod_in_res_units one_cong totient_eq pow_cong units_power_order_eq_one)
  then show ?thesis
    using res_eq_to_cong by blast
qed

lemma euler_theorem:
  fixes a m :: nat
  assumes "coprime a m"
  shows "[a ^ totient m = 1] (mod m)"
proof (cases "m = 0 \<or> m = 1")
  case True
  then show ?thesis by auto
next
  case False
  with assms show ?thesis
    using residues.euler_theorem [of "int m" "int a"] cong_int_iff
    by (auto simp add: residues_def gcd_int_def) fastforce
qed

lemma fermat_theorem:
  fixes p a :: nat
  assumes "prime p" and "\<not> p dvd a"
  shows "[a ^ (p - 1) = 1] (mod p)"
proof -
  from assms prime_imp_coprime [of p a] have "coprime a p"
    by (auto simp add: ac_simps)
  then have "[a ^ totient p = 1] (mod p)"
     by (rule euler_theorem)
  also have "totient p = p - 1"
    by (rule totient_prime) (rule assms)
  finally show ?thesis .
qed


subsection \<open>Wilson's theorem\<close>

lemma (in field) inv_pair_lemma: "x \<in> Units R \<Longrightarrow> y \<in> Units R \<Longrightarrow>
    {x, inv x} \<noteq> {y, inv y} \<Longrightarrow> {x, inv x} \<inter> {y, inv y} = {}"
  apply auto
  apply (metis Units_inv_inv)+
  done

lemma (in residues_prime) wilson_theorem1:
  assumes a: "p > 2"
  shows "[fact (p - 1) = (-1::int)] (mod p)"
proof -
  let ?Inverse_Pairs = "{{x, inv x}| x. x \<in> Units R - {\<one>, \<ominus> \<one>}}"
  have UR: "Units R = {\<one>, \<ominus> \<one>} \<union> \<Union>?Inverse_Pairs"
    by auto
  have "(\<Otimes>i\<in>Units R. i) = (\<Otimes>i\<in>{\<one>, \<ominus> \<one>}. i) \<otimes> (\<Otimes>i\<in>\<Union>?Inverse_Pairs. i)"
    apply (subst UR)
    apply (subst finprod_Un_disjoint)
         apply (auto intro: funcsetI)
    using inv_one apply auto[1]
    using inv_eq_neg_one_eq apply auto
    done
  also have "(\<Otimes>i\<in>{\<one>, \<ominus> \<one>}. i) = \<ominus> \<one>"
    apply (subst finprod_insert)
        apply auto
    apply (frule one_eq_neg_one)
    using a apply force
    done
  also have "(\<Otimes>i\<in>(\<Union>?Inverse_Pairs). i) = (\<Otimes>A\<in>?Inverse_Pairs. (\<Otimes>y\<in>A. y))"
    apply (subst finprod_Union_disjoint)
       apply (auto simp: pairwise_def disjnt_def)
     apply (metis Units_inv_inv)+
    done
  also have "\<dots> = \<one>"
    apply (rule finprod_one_eqI)
     apply auto
    apply (subst finprod_insert)
        apply auto
    apply (metis inv_eq_self)
    done
  finally have "(\<Otimes>i\<in>Units R. i) = \<ominus> \<one>"
    by simp
  also have "(\<Otimes>i\<in>Units R. i) = (\<Otimes>i\<in>Units R. i mod p)"
    by (rule finprod_cong') (auto simp: res_units_eq)
  also have "\<dots> = (\<Prod>i\<in>Units R. i) mod p"
    by (rule prod_cong) auto
  also have "\<dots> = fact (p - 1) mod p"
    apply (simp add: fact_prod)
    using assms
    apply (subst res_prime_units_eq)
    apply (simp add: int_prod zmod_int prod_int_eq)
    done
  finally have "fact (p - 1) mod p = \<ominus> \<one>" .
  then show ?thesis
    by (simp add: cong_def res_neg_eq res_one_eq zmod_int)
qed

lemma wilson_theorem:
  assumes "prime p"
  shows "[fact (p - 1) = - 1] (mod p)"
proof (cases "p = 2")
  case True
  then show ?thesis
    by (simp add: cong_def fact_prod)
next
  case False
  then show ?thesis
    using assms prime_ge_2_nat
    by (metis residues_prime.wilson_theorem1 residues_prime.intro le_eq_less_or_eq)
qed

text \<open>
  This result can be transferred to the multiplicative group of
  \<open>\<int>/p\<int>\<close> for \<open>p\<close> prime.\<close>

lemma mod_nat_int_pow_eq:
  fixes n :: nat and p a :: int
  shows "a \<ge> 0 \<Longrightarrow> p \<ge> 0 \<Longrightarrow> (nat a ^ n) mod (nat p) = nat ((a ^ n) mod p)"
  by (simp add: int_one_le_iff_zero_less nat_mod_distrib order_less_imp_le nat_power_eq[symmetric])

theorem residue_prime_mult_group_has_gen:
 fixes p :: nat
 assumes prime_p : "prime p"
 shows "\<exists>a \<in> {1 .. p - 1}. {1 .. p - 1} = {a^i mod p|i . i \<in> UNIV}"
proof -
  have "p \<ge> 2"
    using prime_gt_1_nat[OF prime_p] by simp
  interpret R: residues_prime p "residue_ring p"
    by (simp add: residues_prime_def prime_p)
  have car: "carrier (residue_ring (int p)) - {\<zero>\<^bsub>residue_ring (int p)\<^esub>} = {1 .. int p - 1}"
    by (auto simp add: R.zero_cong R.res_carrier_eq)

  have "x [^]\<^bsub>residue_ring (int p)\<^esub> i = x ^ i mod (int p)"
    if "x \<in> {1 .. int p - 1}" for x and i :: nat
    using that R.pow_cong[of x i] by auto
  moreover
  obtain a where a: "a \<in> {1 .. int p - 1}"
    and a_gen: "{1 .. int p - 1} = {a[^]\<^bsub>residue_ring (int p)\<^esub>i|i::nat . i \<in> UNIV}"
    using field.finite_field_mult_group_has_gen[OF R.is_field]
    by (auto simp add: car[symmetric] carrier_mult_of)
  moreover
  have "nat ` {1 .. int p - 1} = {1 .. p - 1}" (is "?L = ?R")
  proof
    have "n \<in> ?R" if "n \<in> ?L" for n
      using that \<open>p\<ge>2\<close> by force
    then show "?L \<subseteq> ?R" by blast
    have "n \<in> ?L" if "n \<in> ?R" for n
      using that \<open>p\<ge>2\<close> by (auto intro: rev_image_eqI [of "int n"])
    then show "?R \<subseteq> ?L" by blast
  qed
  moreover
  have "nat ` {a^i mod (int p) | i::nat. i \<in> UNIV} = {nat a^i mod p | i . i \<in> UNIV}" (is "?L = ?R")
  proof
    have "x \<in> ?R" if "x \<in> ?L" for x
    proof -
      from that obtain i where i: "x = nat (a^i mod (int p))"
        by blast
      then have "x = nat a ^ i mod p"
        using mod_nat_int_pow_eq[of a "int p" i] a \<open>p\<ge>2\<close> by auto
      with i show ?thesis by blast
    qed
    then show "?L \<subseteq> ?R" by blast
    have "x \<in> ?L" if "x \<in> ?R" for x
    proof -
      from that obtain i where i: "x = nat a^i mod p"
        by blast
      with mod_nat_int_pow_eq[of a "int p" i] a \<open>p\<ge>2\<close> show ?thesis
        by auto
    qed
    then show "?R \<subseteq> ?L" by blast
  qed
  ultimately have "{1 .. p - 1} = {nat a^i mod p | i. i \<in> UNIV}"
    by presburger
  moreover from a have "nat a \<in> {1 .. p - 1}" by force
  ultimately show ?thesis ..
qed


subsection \<open>Upper bound for the number of $n$-th roots\<close>

lemma roots_mod_prime_bound:
  fixes n c p :: nat
  assumes "prime p" "n > 0"
  defines "A \<equiv> {x\<in>{..<p}. [x ^ n = c] (mod p)}"
  shows   "card A \<le> n"
proof -
  define R where "R = residue_ring (int p)"
  from assms(1) interpret residues_prime p R
    by unfold_locales (simp_all add: R_def)
  interpret R: UP_domain R "UP R" by (unfold_locales)

  let ?f = "UnivPoly.monom (UP R) \<one>\<^bsub>R\<^esub> n \<ominus>\<^bsub>(UP R)\<^esub> UnivPoly.monom (UP R) (int (c mod p)) 0"
  have in_carrier: "int (c mod p) \<in> carrier R"
    using prime_gt_1_nat[OF assms(1)] by (simp add: R_def residue_ring_def)
  
  have "deg R ?f = n"
    using assms in_carrier by (simp add: R.deg_minus_eq)
  hence f_not_zero: "?f \<noteq> \<zero>\<^bsub>UP R\<^esub>" using assms by (auto simp add : R.deg_nzero_nzero)
  have roots_bound: "finite {a \<in> carrier R. UnivPoly.eval R R id a ?f = \<zero>\<^bsub>R\<^esub>} \<and>
                     card {a \<in> carrier R. UnivPoly.eval R R id a ?f = \<zero>\<^bsub>R\<^esub>} \<le> deg R ?f"
                    using finite in_carrier by (intro R.roots_bound[OF _ f_not_zero]) simp
  have subs: "{x \<in> carrier R. x [^]\<^bsub>R\<^esub> n = int (c mod p)} \<subseteq>
                {a \<in> carrier R. UnivPoly.eval R R id a ?f = \<zero>\<^bsub>R\<^esub>}"
    using in_carrier by (auto simp: R.evalRR_simps)
  then have "card {x \<in> carrier R. x [^]\<^bsub>R\<^esub> n = int (c mod p)} \<le>
               card {a \<in> carrier R. UnivPoly.eval R R id a ?f = \<zero>\<^bsub>R\<^esub>}"
    using finite by (intro card_mono) auto
  also have "\<dots> \<le> n"
    using \<open>deg R ?f = n\<close> roots_bound by linarith
  also {
    fix x assume "x \<in> carrier R"
    hence "x [^]\<^bsub>R\<^esub> n = (x ^ n) mod (int p)"
      by (subst pow_cong [symmetric]) (auto simp: R_def residue_ring_def)
  }
  hence "{x \<in> carrier R. x [^]\<^bsub>R\<^esub> n = int (c mod p)} = {x \<in> carrier R. [x ^ n = int c] (mod p)}"
    by (fastforce simp: cong_def zmod_int)
  also have "bij_betw int A {x \<in> carrier R. [x ^ n = int c] (mod p)}"
    by (rule bij_betwI[of int _ _ nat])
       (use cong_int_iff in \<open>force simp: R_def residue_ring_def A_def\<close>)+
  from bij_betw_same_card[OF this] have "card {x \<in> carrier R. [x ^ n = int c] (mod p)} = card A" ..
  finally show ?thesis .
qed


end