src/HOL/GCD.thy

prefer abbreviations for compound operators INFIMUM and SUPREMUM

(*  Authors:    Christophe Tabacznyj, Lawrence C. Paulson, Amine Chaieb,
                Thomas M. Rasmussen, Jeremy Avigad, Tobias Nipkow


This file deals with the functions gcd and lcm.  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.
*)


section \<open>Greatest common divisor and least common multiple\<close>

theory GCD
imports Main
begin

subsection \<open>GCD and LCM definitions\<close>

class gcd = zero + one + dvd +
  fixes gcd :: "'a \<Rightarrow> 'a \<Rightarrow> 'a"
    and lcm :: "'a \<Rightarrow> 'a \<Rightarrow> 'a"
begin

abbreviation coprime :: "'a \<Rightarrow> 'a \<Rightarrow> bool"
  where "coprime x y \<equiv> gcd x y = 1"

end

class Gcd = gcd +
  fixes Gcd :: "'a set \<Rightarrow> 'a"
    and Lcm :: "'a set \<Rightarrow> 'a"

class semiring_gcd = normalization_semidom + gcd +
  assumes gcd_dvd1 [iff]: "gcd a b dvd a"
    and gcd_dvd2 [iff]: "gcd a b dvd b"
    and gcd_greatest: "c dvd a \<Longrightarrow> c dvd b \<Longrightarrow> c dvd gcd a b"
    and normalize_gcd [simp]: "normalize (gcd a b) = gcd a b"
    and lcm_gcd: "lcm a b = normalize (a * b) div gcd a b"
begin    

lemma gcd_greatest_iff [simp]:
  "a dvd gcd b c \<longleftrightarrow> a dvd b \<and> a dvd c"
  by (blast intro!: gcd_greatest intro: dvd_trans)

lemma gcd_dvdI1:
  "a dvd c \<Longrightarrow> gcd a b dvd c"
  by (rule dvd_trans) (rule gcd_dvd1)

lemma gcd_dvdI2:
  "b dvd c \<Longrightarrow> gcd a b dvd c"
  by (rule dvd_trans) (rule gcd_dvd2)

lemma gcd_0_left [simp]:
  "gcd 0 a = normalize a"
  by (rule associated_eqI) simp_all

lemma gcd_0_right [simp]:
  "gcd a 0 = normalize a"
  by (rule associated_eqI) simp_all
  
lemma gcd_eq_0_iff [simp]:
  "gcd a b = 0 \<longleftrightarrow> a = 0 \<and> b = 0" (is "?P \<longleftrightarrow> ?Q")
proof
  assume ?P then have "0 dvd gcd a b" by simp
  then have "0 dvd a" and "0 dvd b" by (blast intro: dvd_trans)+
  then show ?Q by simp
next
  assume ?Q then show ?P by simp
qed

lemma unit_factor_gcd:
  "unit_factor (gcd a b) = (if a = 0 \<and> b = 0 then 0 else 1)"
proof (cases "gcd a b = 0")
  case True then show ?thesis by simp
next
  case False
  have "unit_factor (gcd a b) * normalize (gcd a b) = gcd a b"
    by (rule unit_factor_mult_normalize)
  then have "unit_factor (gcd a b) * gcd a b = gcd a b"
    by simp
  then have "unit_factor (gcd a b) * gcd a b div gcd a b = gcd a b div gcd a b"
    by simp
  with False show ?thesis by simp
qed

lemma is_unit_gcd [simp]:
  "is_unit (gcd a b) \<longleftrightarrow> coprime a b"
  by (cases "a = 0 \<and> b = 0") (auto simp add: unit_factor_gcd dest: is_unit_unit_factor)

sublocale gcd: abel_semigroup gcd
proof
  fix a b c
  show "gcd a b = gcd b a"
    by (rule associated_eqI) simp_all
  from gcd_dvd1 have "gcd (gcd a b) c dvd a"
    by (rule dvd_trans) simp
  moreover from gcd_dvd1 have "gcd (gcd a b) c dvd b"
    by (rule dvd_trans) simp
  ultimately have P1: "gcd (gcd a b) c dvd gcd a (gcd b c)"
    by (auto intro!: gcd_greatest)
  from gcd_dvd2 have "gcd a (gcd b c) dvd b"
    by (rule dvd_trans) simp
  moreover from gcd_dvd2 have "gcd a (gcd b c) dvd c"
    by (rule dvd_trans) simp
  ultimately have P2: "gcd a (gcd b c) dvd gcd (gcd a b) c"
    by (auto intro!: gcd_greatest)
  from P1 P2 show "gcd (gcd a b) c = gcd a (gcd b c)"
    by (rule associated_eqI) simp_all
qed

lemma gcd_self [simp]:
  "gcd a a = normalize a"
proof -
  have "a dvd gcd a a"
    by (rule gcd_greatest) simp_all
  then show ?thesis
    by (auto intro: associated_eqI)
qed

lemma gcd_left_idem [simp]:
  "gcd a (gcd a b) = gcd a b"
  by (auto intro: associated_eqI)

lemma gcd_right_idem [simp]:
  "gcd (gcd a b) b = gcd a b"
  unfolding gcd.commute [of a] gcd.commute [of "gcd b a"] by simp

lemma coprime_1_left [simp]:
  "coprime 1 a"
  by (rule associated_eqI) simp_all

lemma coprime_1_right [simp]:
  "coprime a 1"
  using coprime_1_left [of a] by (simp add: ac_simps)

lemma gcd_mult_left:
  "gcd (c * a) (c * b) = normalize c * gcd a b"
proof (cases "c = 0")
  case True then show ?thesis by simp
next
  case False
  then have "c * gcd a b dvd gcd (c * a) (c * b)"
    by (auto intro: gcd_greatest)
  moreover from calculation False have "gcd (c * a) (c * b) dvd c * gcd a b"
    by (metis div_dvd_iff_mult dvd_mult_left gcd_dvd1 gcd_dvd2 gcd_greatest mult_commute)
  ultimately have "normalize (gcd (c * a) (c * b)) = normalize (c * gcd a b)"
    by (auto intro: associated_eqI)
  then show ?thesis by (simp add: normalize_mult)
qed

lemma gcd_mult_right:
  "gcd (a * c) (b * c) = gcd b a * normalize c"
  using gcd_mult_left [of c a b] by (simp add: ac_simps)

lemma mult_gcd_left:
  "c * gcd a b = unit_factor c * gcd (c * a) (c * b)"
  by (simp add: gcd_mult_left mult.assoc [symmetric])

lemma mult_gcd_right:
  "gcd a b * c = gcd (a * c) (b * c) * unit_factor c"
  using mult_gcd_left [of c a b] by (simp add: ac_simps)

lemma dvd_lcm1 [iff]:
  "a dvd lcm a b"
proof -
  have "normalize (a * b) div gcd a b = normalize a * (normalize b div gcd a b)"
    by (simp add: lcm_gcd normalize_mult div_mult_swap)
  then show ?thesis
    by (simp add: lcm_gcd)
qed
  
lemma dvd_lcm2 [iff]:
  "b dvd lcm a b"
proof -
  have "normalize (a * b) div gcd a b = normalize b * (normalize a div gcd a b)"
    by (simp add: lcm_gcd normalize_mult div_mult_swap ac_simps)
  then show ?thesis
    by (simp add: lcm_gcd)
qed

lemma dvd_lcmI1:
  "a dvd b \<Longrightarrow> a dvd lcm b c"
  by (rule dvd_trans) (assumption, blast) 

lemma dvd_lcmI2:
  "a dvd c \<Longrightarrow> a dvd lcm b c"
  by (rule dvd_trans) (assumption, blast)

lemma lcm_least:
  assumes "a dvd c" and "b dvd c"
  shows "lcm a b dvd c"
proof (cases "c = 0")
  case True then show ?thesis by simp
next
  case False then have U: "is_unit (unit_factor c)" by simp
  show ?thesis
  proof (cases "gcd a b = 0")
    case True with assms show ?thesis by simp
  next
    case False then have "a \<noteq> 0 \<or> b \<noteq> 0" by simp
    with \<open>c \<noteq> 0\<close> assms have "a * b dvd a * c" "a * b dvd c * b"
      by (simp_all add: mult_dvd_mono)
    then have "normalize (a * b) dvd gcd (a * c) (b * c)"
      by (auto intro: gcd_greatest simp add: ac_simps)
    then have "normalize (a * b) dvd gcd (a * c) (b * c) * unit_factor c"
      using U by (simp add: dvd_mult_unit_iff)
    then have "normalize (a * b) dvd gcd a b * c"
      by (simp add: mult_gcd_right [of a b c])
    then have "normalize (a * b) div gcd a b dvd c"
      using False by (simp add: div_dvd_iff_mult ac_simps)
    then show ?thesis by (simp add: lcm_gcd)
  qed
qed

lemma lcm_least_iff [simp]:
  "lcm a b dvd c \<longleftrightarrow> a dvd c \<and> b dvd c"
  by (blast intro!: lcm_least intro: dvd_trans)

lemma normalize_lcm [simp]:
  "normalize (lcm a b) = lcm a b"
  by (simp add: lcm_gcd dvd_normalize_div)

lemma lcm_0_left [simp]:
  "lcm 0 a = 0"
  by (simp add: lcm_gcd)
  
lemma lcm_0_right [simp]:
  "lcm a 0 = 0"
  by (simp add: lcm_gcd)

lemma lcm_eq_0_iff:
  "lcm a b = 0 \<longleftrightarrow> a = 0 \<or> b = 0" (is "?P \<longleftrightarrow> ?Q")
proof
  assume ?P then have "0 dvd lcm a b" by simp
  then have "0 dvd normalize (a * b) div gcd a b"
    by (simp add: lcm_gcd)
  then have "0 * gcd a b dvd normalize (a * b)"
    using dvd_div_iff_mult [of "gcd a b" _ 0] by (cases "gcd a b = 0") simp_all
  then have "normalize (a * b) = 0"
    by simp
  then show ?Q by simp
next
  assume ?Q then show ?P by auto
qed

lemma lcm_eq_1_iff [simp]:
  "lcm a b = 1 \<longleftrightarrow> is_unit a \<and> is_unit b"
  by (auto intro: associated_eqI)

lemma unit_factor_lcm :
  "unit_factor (lcm a b) = (if a = 0 \<or> b = 0 then 0 else 1)"
  by (simp add: unit_factor_gcd dvd_unit_factor_div lcm_gcd)

sublocale lcm: abel_semigroup lcm
proof
  fix a b c
  show "lcm a b = lcm b a"
    by (simp add: lcm_gcd ac_simps normalize_mult dvd_normalize_div)
  have "lcm (lcm a b) c dvd lcm a (lcm b c)"
    and "lcm a (lcm b c) dvd lcm (lcm a b) c"
    by (auto intro: lcm_least
      dvd_trans [of b "lcm b c" "lcm a (lcm b c)"]
      dvd_trans [of c "lcm b c" "lcm a (lcm b c)"]
      dvd_trans [of a "lcm a b" "lcm (lcm a b) c"]
      dvd_trans [of b "lcm a b" "lcm (lcm a b) c"])
  then show "lcm (lcm a b) c = lcm a (lcm b c)"
    by (rule associated_eqI) simp_all
qed

lemma lcm_self [simp]:
  "lcm a a = normalize a"
proof -
  have "lcm a a dvd a"
    by (rule lcm_least) simp_all
  then show ?thesis
    by (auto intro: associated_eqI)
qed

lemma lcm_left_idem [simp]:
  "lcm a (lcm a b) = lcm a b"
  by (auto intro: associated_eqI)

lemma lcm_right_idem [simp]:
  "lcm (lcm a b) b = lcm a b"
  unfolding lcm.commute [of a] lcm.commute [of "lcm b a"] by simp

lemma gcd_mult_lcm [simp]:
  "gcd a b * lcm a b = normalize a * normalize b"
  by (simp add: lcm_gcd normalize_mult)

lemma lcm_mult_gcd [simp]:
  "lcm a b * gcd a b = normalize a * normalize b"
  using gcd_mult_lcm [of a b] by (simp add: ac_simps) 

lemma gcd_lcm:
  assumes "a \<noteq> 0" and "b \<noteq> 0"
  shows "gcd a b = normalize (a * b) div lcm a b"
proof -
  from assms have "lcm a b \<noteq> 0"
    by (simp add: lcm_eq_0_iff)
  have "gcd a b * lcm a b = normalize a * normalize b" by simp
  then have "gcd a b * lcm a b div lcm a b = normalize (a * b) div lcm a b"
    by (simp_all add: normalize_mult)
  with \<open>lcm a b \<noteq> 0\<close> show ?thesis
    using nonzero_mult_divide_cancel_right [of "lcm a b" "gcd a b"] by simp
qed

lemma lcm_1_left [simp]:
  "lcm 1 a = normalize a"
  by (simp add: lcm_gcd)

lemma lcm_1_right [simp]:
  "lcm a 1 = normalize a"
  by (simp add: lcm_gcd)
  
lemma lcm_mult_left:
  "lcm (c * a) (c * b) = normalize c * lcm a b"
  by (cases "c = 0")
    (simp_all add: gcd_mult_right lcm_gcd div_mult_swap normalize_mult ac_simps,
      simp add: dvd_div_mult2_eq mult.left_commute [of "normalize c", symmetric])

lemma lcm_mult_right:
  "lcm (a * c) (b * c) = lcm b a * normalize c"
  using lcm_mult_left [of c a b] by (simp add: ac_simps)

lemma mult_lcm_left:
  "c * lcm a b = unit_factor c * lcm (c * a) (c * b)"
  by (simp add: lcm_mult_left mult.assoc [symmetric])

lemma mult_lcm_right:
  "lcm a b * c = lcm (a * c) (b * c) * unit_factor c"
  using mult_lcm_left [of c a b] by (simp add: ac_simps)
  
end

class semiring_Gcd = semiring_gcd + Gcd +
  assumes Gcd_dvd: "a \<in> A \<Longrightarrow> Gcd A dvd a"
    and Gcd_greatest: "(\<And>b. b \<in> A \<Longrightarrow> a dvd b) \<Longrightarrow> a dvd Gcd A"
    and normalize_Gcd [simp]: "normalize (Gcd A) = Gcd A"
begin

lemma Gcd_empty [simp]:
  "Gcd {} = 0"
  by (rule dvd_0_left, rule Gcd_greatest) simp

lemma Gcd_0_iff [simp]:
  "Gcd A = 0 \<longleftrightarrow> A \<subseteq> {0}" (is "?P \<longleftrightarrow> ?Q")
proof
  assume ?P
  show ?Q
  proof
    fix a
    assume "a \<in> A"
    then have "Gcd A dvd a" by (rule Gcd_dvd)
    with \<open>?P\<close> have "a = 0" by simp
    then show "a \<in> {0}" by simp
  qed
next
  assume ?Q
  have "0 dvd Gcd A"
  proof (rule Gcd_greatest)
    fix a
    assume "a \<in> A"
    with \<open>?Q\<close> have "a = 0" by auto
    then show "0 dvd a" by simp
  qed
  then show ?P by simp
qed

lemma unit_factor_Gcd:
  "unit_factor (Gcd A) = (if \<forall>a\<in>A. a = 0 then 0 else 1)"
proof (cases "Gcd A = 0")
  case True then show ?thesis by auto
next
  case False
  from unit_factor_mult_normalize
  have "unit_factor (Gcd A) * normalize (Gcd A) = Gcd A" .
  then have "unit_factor (Gcd A) * Gcd A = Gcd A" by simp
  then have "unit_factor (Gcd A) * Gcd A div Gcd A = Gcd A div Gcd A" by simp
  with False have "unit_factor (Gcd A) = 1" by simp
  with False show ?thesis by auto
qed

lemma Gcd_UNIV [simp]:
  "Gcd UNIV = 1"
  by (rule associated_eqI) (auto intro: Gcd_dvd simp add: unit_factor_Gcd)

lemma Gcd_eq_1_I:
  assumes "is_unit a" and "a \<in> A"
  shows "Gcd A = 1"
proof -
  from assms have "is_unit (Gcd A)"
    by (blast intro: Gcd_dvd dvd_unit_imp_unit)
  then have "normalize (Gcd A) = 1"
    by (rule is_unit_normalize)
  then show ?thesis
    by simp
qed

lemma Gcd_insert [simp]:
  "Gcd (insert a A) = gcd a (Gcd A)"
proof -
  have "Gcd (insert a A) dvd gcd a (Gcd A)"
    by (auto intro: Gcd_dvd Gcd_greatest)
  moreover have "gcd a (Gcd A) dvd Gcd (insert a A)"
  proof (rule Gcd_greatest)
    fix b
    assume "b \<in> insert a A"
    then show "gcd a (Gcd A) dvd b"
    proof
      assume "b = a" then show ?thesis by simp
    next
      assume "b \<in> A"
      then have "Gcd A dvd b" by (rule Gcd_dvd)
      moreover have "gcd a (Gcd A) dvd Gcd A" by simp
      ultimately show ?thesis by (blast intro: dvd_trans)
    qed
  qed
  ultimately show ?thesis
    by (auto intro: associated_eqI)
qed

lemma dvd_Gcd: \<comment> \<open>FIXME remove\<close>
  "\<forall>b\<in>A. a dvd b \<Longrightarrow> a dvd Gcd A"
  by (blast intro: Gcd_greatest)

lemma Gcd_set [code_unfold]:
  "Gcd (set as) = foldr gcd as 0"
  by (induct as) simp_all

lemma Gcd_image_normalize [simp]:
  "Gcd (normalize ` A) = Gcd A"
proof -
  have "Gcd (normalize ` A) dvd a" if "a \<in> A" for a
  proof -
    from that obtain B where "A = insert a B" by blast
    moreover have " gcd (normalize a) (Gcd (normalize ` B)) dvd normalize a"
      by (rule gcd_dvd1)
    ultimately show "Gcd (normalize ` A) dvd a"
      by simp
  qed
  then have "Gcd (normalize ` A) dvd Gcd A" and "Gcd A dvd Gcd (normalize ` A)"
    by (auto intro!: Gcd_greatest intro: Gcd_dvd)
  then show ?thesis
    by (auto intro: associated_eqI)
qed

end  

class semiring_Lcm = semiring_Gcd +
  assumes Lcm_Gcd: "Lcm A = Gcd {b. \<forall>a\<in>A. a dvd b}"
begin

lemma dvd_Lcm:
  assumes "a \<in> A"
  shows "a dvd Lcm A"
  using assms by (auto intro: Gcd_greatest simp add: Lcm_Gcd)

lemma Lcm_least:
  assumes "\<And>b. b \<in> A \<Longrightarrow> b dvd a"
  shows "Lcm A dvd a"
  using assms by (auto intro: Gcd_dvd simp add: Lcm_Gcd)

lemma normalize_Lcm [simp]:
  "normalize (Lcm A) = Lcm A"
  by (simp add: Lcm_Gcd)

lemma unit_factor_Lcm:
  "unit_factor (Lcm A) = (if Lcm A = 0 then 0 else 1)"
proof (cases "Lcm A = 0")
  case True then show ?thesis by simp
next
  case False
  with unit_factor_normalize have "unit_factor (normalize (Lcm A)) = 1"
    by blast
  with False show ?thesis
    by simp
qed

lemma Gcd_Lcm:
  "Gcd A = Lcm {b. \<forall>a\<in>A. b dvd a}"
  by (rule associated_eqI) (auto intro: Gcd_dvd dvd_Lcm Gcd_greatest Lcm_least)
 
lemma Lcm_empty [simp]:
  "Lcm {} = 1"
  by (simp add: Lcm_Gcd)

lemma Lcm_insert [simp]:
  "Lcm (insert a A) = lcm a (Lcm A)"
proof (rule sym)
  have "lcm a (Lcm A) dvd Lcm (insert a A)"
    by (auto intro: dvd_Lcm Lcm_least)
  moreover have "Lcm (insert a A) dvd lcm a (Lcm A)"
  proof (rule Lcm_least)
    fix b
    assume "b \<in> insert a A"
    then show "b dvd lcm a (Lcm A)"
    proof
      assume "b = a" then show ?thesis by simp
    next
      assume "b \<in> A"
      then have "b dvd Lcm A" by (rule dvd_Lcm)
      moreover have "Lcm A dvd lcm a (Lcm A)" by simp
      ultimately show ?thesis by (blast intro: dvd_trans)
    qed
  qed
  ultimately show "lcm a (Lcm A) = Lcm (insert a A)"
    by (rule associated_eqI) (simp_all add: lcm_eq_0_iff)
qed

lemma Lcm_1_iff [simp]:
  "Lcm A = 1 \<longleftrightarrow> (\<forall>a\<in>A. is_unit a)" (is "?P \<longleftrightarrow> ?Q")
proof
  assume ?P
  show ?Q
  proof
    fix a
    assume "a \<in> A"
    then have "a dvd Lcm A"
      by (rule dvd_Lcm)
    with \<open>?P\<close> show "is_unit a"
      by simp
  qed
next
  assume ?Q
  then have "is_unit (Lcm A)"
    by (blast intro: Lcm_least)
  then have "normalize (Lcm A) = 1"
    by (rule is_unit_normalize)
  then show ?P
    by simp
qed

lemma Lcm_eq_0_I:
  assumes "0 \<in> A"
  shows "Lcm A = 0"
proof -
  from assms have "0 dvd Lcm A"
    by (rule dvd_Lcm)
  then show ?thesis
    by simp
qed

lemma Lcm_UNIV [simp]:
  "Lcm UNIV = 0"
  by (rule Lcm_eq_0_I) simp

lemma Lcm_0_iff:
  assumes "finite A"
  shows "Lcm A = 0 \<longleftrightarrow> 0 \<in> A"
proof (cases "A = {}")
  case True then show ?thesis by simp
next
  case False with assms show ?thesis
    by (induct A rule: finite_ne_induct)
      (auto simp add: lcm_eq_0_iff)
qed

lemma Lcm_set [code_unfold]:
  "Lcm (set as) = foldr lcm as 1"
  by (induct as) simp_all
  
end

class ring_gcd = comm_ring_1 + semiring_gcd

instantiation nat :: gcd
begin

fun
  gcd_nat  :: "nat \<Rightarrow> nat \<Rightarrow> nat"
where
  "gcd_nat x y =
   (if y = 0 then x else gcd y (x mod y))"

definition
  lcm_nat :: "nat \<Rightarrow> nat \<Rightarrow> nat"
where
  "lcm_nat x y = x * y div (gcd x y)"

instance proof qed

end

instantiation int :: gcd
begin

definition
  gcd_int  :: "int \<Rightarrow> int \<Rightarrow> int"
where
  "gcd_int x y = int (gcd (nat \<bar>x\<bar>) (nat \<bar>y\<bar>))"

definition
  lcm_int :: "int \<Rightarrow> int \<Rightarrow> int"
where
  "lcm_int x y = int (lcm (nat \<bar>x\<bar>) (nat \<bar>y\<bar>))"

instance ..

end


subsection \<open>Transfer setup\<close>

lemma transfer_nat_int_gcd:
  "(x::int) >= 0 \<Longrightarrow> y >= 0 \<Longrightarrow> gcd (nat x) (nat y) = nat (gcd x y)"
  "(x::int) >= 0 \<Longrightarrow> y >= 0 \<Longrightarrow> lcm (nat x) (nat y) = nat (lcm x y)"
  unfolding gcd_int_def lcm_int_def
  by auto

lemma transfer_nat_int_gcd_closures:
  "x >= (0::int) \<Longrightarrow> y >= 0 \<Longrightarrow> gcd x y >= 0"
  "x >= (0::int) \<Longrightarrow> y >= 0 \<Longrightarrow> lcm x y >= 0"
  by (auto simp add: gcd_int_def lcm_int_def)

declare transfer_morphism_nat_int[transfer add return:
    transfer_nat_int_gcd transfer_nat_int_gcd_closures]

lemma transfer_int_nat_gcd:
  "gcd (int x) (int y) = int (gcd x y)"
  "lcm (int x) (int y) = int (lcm x y)"
  by (unfold gcd_int_def lcm_int_def, auto)

lemma transfer_int_nat_gcd_closures:
  "is_nat x \<Longrightarrow> is_nat y \<Longrightarrow> gcd x y >= 0"
  "is_nat x \<Longrightarrow> is_nat y \<Longrightarrow> lcm x y >= 0"
  by (auto simp add: gcd_int_def lcm_int_def)

declare transfer_morphism_int_nat[transfer add return:
    transfer_int_nat_gcd transfer_int_nat_gcd_closures]


subsection \<open>GCD properties\<close>

(* was gcd_induct *)
lemma gcd_nat_induct:
  fixes m n :: nat
  assumes "\<And>m. P m 0"
    and "\<And>m n. 0 < n \<Longrightarrow> P n (m mod n) \<Longrightarrow> P m n"
  shows "P m n"
  apply (rule gcd_nat.induct)
  apply (case_tac "y = 0")
  using assms apply simp_all
done

(* specific to int *)

lemma gcd_neg1_int [simp]: "gcd (-x::int) y = gcd x y"
  by (simp add: gcd_int_def)

lemma gcd_neg2_int [simp]: "gcd (x::int) (-y) = gcd x y"
  by (simp add: gcd_int_def)

lemma gcd_neg_numeral_1_int [simp]:
  "gcd (- numeral n :: int) x = gcd (numeral n) x"
  by (fact gcd_neg1_int)

lemma gcd_neg_numeral_2_int [simp]:
  "gcd x (- numeral n :: int) = gcd x (numeral n)"
  by (fact gcd_neg2_int)

lemma abs_gcd_int[simp]: "\<bar>gcd (x::int) y\<bar> = gcd x y"
by(simp add: gcd_int_def)

lemma gcd_abs_int: "gcd (x::int) y = gcd \<bar>x\<bar> \<bar>y\<bar>"
by (simp add: gcd_int_def)

lemma gcd_abs1_int[simp]: "gcd \<bar>x\<bar> (y::int) = gcd x y"
by (metis abs_idempotent gcd_abs_int)

lemma gcd_abs2_int[simp]: "gcd x \<bar>y::int\<bar> = gcd x y"
by (metis abs_idempotent gcd_abs_int)

lemma gcd_cases_int:
  fixes x :: int and y
  assumes "x >= 0 \<Longrightarrow> y >= 0 \<Longrightarrow> P (gcd x y)"
      and "x >= 0 \<Longrightarrow> y <= 0 \<Longrightarrow> P (gcd x (-y))"
      and "x <= 0 \<Longrightarrow> y >= 0 \<Longrightarrow> P (gcd (-x) y)"
      and "x <= 0 \<Longrightarrow> y <= 0 \<Longrightarrow> P (gcd (-x) (-y))"
  shows "P (gcd x y)"
  by (insert assms, auto, arith)

lemma gcd_ge_0_int [simp]: "gcd (x::int) y >= 0"
  by (simp add: gcd_int_def)

lemma lcm_neg1_int: "lcm (-x::int) y = lcm x y"
  by (simp add: lcm_int_def)

lemma lcm_neg2_int: "lcm (x::int) (-y) = lcm x y"
  by (simp add: lcm_int_def)

lemma lcm_abs_int: "lcm (x::int) y = lcm \<bar>x\<bar> \<bar>y\<bar>"
  by (simp add: lcm_int_def)

lemma abs_lcm_int [simp]: "\<bar>lcm i j::int\<bar> = lcm i j"
  by (simp add:lcm_int_def)

lemma lcm_abs1_int[simp]: "lcm \<bar>x\<bar> (y::int) = lcm x y"
  by (metis abs_idempotent lcm_int_def)

lemma lcm_abs2_int[simp]: "lcm x \<bar>y::int\<bar> = lcm x y"
  by (metis abs_idempotent lcm_int_def)

lemma lcm_cases_int:
  fixes x :: int and y
  assumes "x >= 0 \<Longrightarrow> y >= 0 \<Longrightarrow> P (lcm x y)"
      and "x >= 0 \<Longrightarrow> y <= 0 \<Longrightarrow> P (lcm x (-y))"
      and "x <= 0 \<Longrightarrow> y >= 0 \<Longrightarrow> P (lcm (-x) y)"
      and "x <= 0 \<Longrightarrow> y <= 0 \<Longrightarrow> P (lcm (-x) (-y))"
  shows "P (lcm x y)"
  using assms by (auto simp add: lcm_neg1_int lcm_neg2_int) arith

lemma lcm_ge_0_int [simp]: "lcm (x::int) y >= 0"
  by (simp add: lcm_int_def)

(* was gcd_0, etc. *)
lemma gcd_0_nat: "gcd (x::nat) 0 = x"
  by simp

(* was igcd_0, etc. *)
lemma gcd_0_int [simp]: "gcd (x::int) 0 = \<bar>x\<bar>"
  by (unfold gcd_int_def, auto)

lemma gcd_0_left_nat: "gcd 0 (x::nat) = x"
  by simp

lemma gcd_0_left_int [simp]: "gcd 0 (x::int) = \<bar>x\<bar>"
  by (unfold gcd_int_def, auto)

lemma gcd_red_nat: "gcd (x::nat) y = gcd y (x mod y)"
  by (case_tac "y = 0", auto)

(* weaker, but useful for the simplifier *)

lemma gcd_non_0_nat: "y ~= (0::nat) \<Longrightarrow> gcd (x::nat) y = gcd y (x mod y)"
  by simp

lemma gcd_1_nat [simp]: "gcd (m::nat) 1 = 1"
  by simp

lemma gcd_Suc_0 [simp]: "gcd (m::nat) (Suc 0) = Suc 0"
  by simp

lemma gcd_1_int [simp]: "gcd (m::int) 1 = 1"
  by (simp add: gcd_int_def)

lemma gcd_idem_nat: "gcd (x::nat) x = x"
by simp

lemma gcd_idem_int: "gcd (x::int) x = \<bar>x\<bar>"
by (auto simp add: gcd_int_def)

declare gcd_nat.simps [simp del]

text \<open>
  \medskip @{term "gcd m n"} divides \<open>m\<close> and \<open>n\<close>.  The
  conjunctions don't seem provable separately.
\<close>

instance nat :: semiring_gcd
proof
  fix m n :: nat
  show "gcd m n dvd m" and "gcd m n dvd n"
  proof (induct m n rule: gcd_nat_induct)
    fix m n :: nat
    assume "gcd n (m mod n) dvd m mod n" and "gcd n (m mod n) dvd n"
    then have "gcd n (m mod n) dvd m"
      by (rule dvd_mod_imp_dvd)
    moreover assume "0 < n"
    ultimately show "gcd m n dvd m"
      by (simp add: gcd_non_0_nat)
  qed (simp_all add: gcd_0_nat gcd_non_0_nat)
next
  fix m n k :: nat
  assume "k dvd m" and "k dvd n"
  then show "k dvd gcd m n"
    by (induct m n rule: gcd_nat_induct) (simp_all add: gcd_non_0_nat dvd_mod gcd_0_nat)
qed (simp_all add: lcm_nat_def)

instance int :: ring_gcd
  by standard
    (simp_all add: dvd_int_unfold_dvd_nat gcd_int_def lcm_int_def zdiv_int nat_abs_mult_distrib [symmetric] lcm_gcd gcd_greatest)

lemma dvd_gcd_D1_nat: "k dvd gcd m n \<Longrightarrow> (k::nat) dvd m"
  by (metis gcd_dvd1 dvd_trans)

lemma dvd_gcd_D2_nat: "k dvd gcd m n \<Longrightarrow> (k::nat) dvd n"
  by (metis gcd_dvd2 dvd_trans)

lemma dvd_gcd_D1_int: "i dvd gcd m n \<Longrightarrow> (i::int) dvd m"
  by (metis gcd_dvd1 dvd_trans)

lemma dvd_gcd_D2_int: "i dvd gcd m n \<Longrightarrow> (i::int) dvd n"
  by (metis gcd_dvd2 dvd_trans)

lemma gcd_le1_nat [simp]: "a \<noteq> 0 \<Longrightarrow> gcd (a::nat) b \<le> a"
  by (rule dvd_imp_le, auto)

lemma gcd_le2_nat [simp]: "b \<noteq> 0 \<Longrightarrow> gcd (a::nat) b \<le> b"
  by (rule dvd_imp_le, auto)

lemma gcd_le1_int [simp]: "a > 0 \<Longrightarrow> gcd (a::int) b \<le> a"
  by (rule zdvd_imp_le, auto)

lemma gcd_le2_int [simp]: "b > 0 \<Longrightarrow> gcd (a::int) b \<le> b"
  by (rule zdvd_imp_le, auto)

lemma gcd_greatest_iff_nat:
  "(k dvd gcd (m::nat) n) = (k dvd m & k dvd n)"
  by (fact gcd_greatest_iff)

lemma gcd_greatest_iff_int:
  "((k::int) dvd gcd m n) = (k dvd m & k dvd n)"
  by (fact gcd_greatest_iff)

lemma gcd_zero_nat: 
  "(gcd (m::nat) n = 0) = (m = 0 & n = 0)"
  by (fact gcd_eq_0_iff)

lemma gcd_zero_int [simp]:
  "(gcd (m::int) n = 0) = (m = 0 & n = 0)"
  by (fact gcd_eq_0_iff)

lemma gcd_pos_nat [simp]: "(gcd (m::nat) n > 0) = (m ~= 0 | n ~= 0)"
  by (insert gcd_zero_nat [of m n], arith)

lemma gcd_pos_int [simp]: "(gcd (m::int) n > 0) = (m ~= 0 | n ~= 0)"
  by (insert gcd_zero_int [of m n], insert gcd_ge_0_int [of m n], arith)

lemma gcd_unique_nat: "(d::nat) dvd a \<and> d dvd b \<and>
    (\<forall>e. e dvd a \<and> e dvd b \<longrightarrow> e dvd d) \<longleftrightarrow> d = gcd a b"
  apply auto
  apply (rule dvd_antisym)
  apply (erule (1) gcd_greatest)
  apply auto
done

lemma gcd_unique_int: "d >= 0 & (d::int) dvd a \<and> d dvd b \<and>
    (\<forall>e. e dvd a \<and> e dvd b \<longrightarrow> e dvd d) \<longleftrightarrow> d = gcd a b"
apply (case_tac "d = 0")
 apply simp
apply (rule iffI)
 apply (rule zdvd_antisym_nonneg)
 apply (auto intro: gcd_greatest)
done

interpretation gcd_nat:
  semilattice_neutr_order gcd "0::nat" Rings.dvd "(\<lambda>m n. m dvd n \<and> \<not> n dvd m)"
  by standard (auto simp add: gcd_unique_nat [symmetric] intro: dvd.antisym dvd_trans)

lemmas gcd_assoc_nat = gcd.assoc [where ?'a = nat]
lemmas gcd_commute_nat = gcd.commute [where ?'a = nat]
lemmas gcd_left_commute_nat = gcd.left_commute [where ?'a = nat]
lemmas gcd_assoc_int = gcd.assoc [where ?'a = int]
lemmas gcd_commute_int = gcd.commute [where ?'a = int]
lemmas gcd_left_commute_int = gcd.left_commute [where ?'a = int]

lemmas gcd_ac_nat = gcd_assoc_nat gcd_commute_nat gcd_left_commute_nat

lemmas gcd_ac_int = gcd_assoc_int gcd_commute_int gcd_left_commute_int

lemma gcd_proj1_if_dvd_nat [simp]: "(x::nat) dvd y \<Longrightarrow> gcd x y = x"
  by (fact gcd_nat.absorb1)

lemma gcd_proj2_if_dvd_nat [simp]: "(y::nat) dvd x \<Longrightarrow> gcd x y = y"
  by (fact gcd_nat.absorb2)

lemma gcd_proj1_if_dvd_int [simp]: "x dvd y \<Longrightarrow> gcd (x::int) y = \<bar>x\<bar>"
  by (metis abs_dvd_iff gcd_0_left_int gcd_abs_int gcd_unique_int)

lemma gcd_proj2_if_dvd_int [simp]: "y dvd x \<Longrightarrow> gcd (x::int) y = \<bar>y\<bar>"
  by (metis gcd_proj1_if_dvd_int gcd_commute_int)

text \<open>
  \medskip Multiplication laws
\<close>

lemma gcd_mult_distrib_nat: "(k::nat) * gcd m n = gcd (k * m) (k * n)"
    \<comment> \<open>@{cite \<open>page 27\<close> davenport92}\<close>
  apply (induct m n rule: gcd_nat_induct)
  apply simp
  apply (case_tac "k = 0")
  apply (simp_all add: gcd_non_0_nat)
done

lemma gcd_mult_distrib_int: "\<bar>k::int\<bar> * gcd m n = gcd (k * m) (k * n)"
  apply (subst (1 2) gcd_abs_int)
  apply (subst (1 2) abs_mult)
  apply (rule gcd_mult_distrib_nat [transferred])
  apply auto
done

context semiring_gcd
begin

lemma coprime_dvd_mult:
  assumes "coprime a b" and "a dvd c * b"
  shows "a dvd c"
proof (cases "c = 0")
  case True then show ?thesis by simp
next
  case False
  then have unit: "is_unit (unit_factor c)" by simp
  from \<open>coprime a b\<close> mult_gcd_left [of c a b]
  have "gcd (c * a) (c * b) * unit_factor c = c"
    by (simp add: ac_simps)
  moreover from \<open>a dvd c * b\<close> have "a dvd gcd (c * a) (c * b) * unit_factor c"
    by (simp add: dvd_mult_unit_iff unit)
  ultimately show ?thesis by simp
qed

end

lemmas coprime_dvd_mult_nat = coprime_dvd_mult [where ?'a = nat]
lemmas coprime_dvd_mult_int = coprime_dvd_mult [where ?'a = int]

lemma coprime_dvd_mult_iff_nat: "coprime (k::nat) n \<Longrightarrow>
    (k dvd m * n) = (k dvd m)"
  by (auto intro: coprime_dvd_mult_nat)

lemma coprime_dvd_mult_iff_int: "coprime (k::int) n \<Longrightarrow>
    (k dvd m * n) = (k dvd m)"
  by (auto intro: coprime_dvd_mult_int)

context semiring_gcd
begin

lemma gcd_mult_cancel:
  "coprime c b \<Longrightarrow> gcd (c * a) b = gcd a b"
  apply (rule associated_eqI)
  apply (rule gcd_greatest)
  apply (rule_tac b = c in coprime_dvd_mult)
  apply (simp add: gcd.assoc)
  apply (simp_all add: ac_simps)
  done

end  

lemmas gcd_mult_cancel_nat = gcd_mult_cancel [where ?'a = nat] 
lemmas gcd_mult_cancel_int = gcd_mult_cancel [where ?'a = int] 

lemma coprime_crossproduct_nat:
  fixes a b c d :: nat
  assumes "coprime a d" and "coprime b c"
  shows "a * c = b * d \<longleftrightarrow> a = b \<and> c = d" (is "?lhs \<longleftrightarrow> ?rhs")
proof
  assume ?rhs then show ?lhs by simp
next
  assume ?lhs
  from \<open>?lhs\<close> have "a dvd b * d" by (auto intro: dvdI dest: sym)
  with \<open>coprime a d\<close> have "a dvd b" by (simp add: coprime_dvd_mult_iff_nat)
  from \<open>?lhs\<close> have "b dvd a * c" by (auto intro: dvdI dest: sym)
  with \<open>coprime b c\<close> have "b dvd a" by (simp add: coprime_dvd_mult_iff_nat)
  from \<open>?lhs\<close> have "c dvd d * b" by (auto intro: dvdI dest: sym simp add: mult.commute)
  with \<open>coprime b c\<close> have "c dvd d" by (simp add: coprime_dvd_mult_iff_nat gcd_commute_nat)
  from \<open>?lhs\<close> have "d dvd c * a" by (auto intro: dvdI dest: sym simp add: mult.commute)
  with \<open>coprime a d\<close> have "d dvd c" by (simp add: coprime_dvd_mult_iff_nat gcd_commute_nat)
  from \<open>a dvd b\<close> \<open>b dvd a\<close> have "a = b" by (rule Nat.dvd.antisym)
  moreover from \<open>c dvd d\<close> \<open>d dvd c\<close> have "c = d" by (rule Nat.dvd.antisym)
  ultimately show ?rhs ..
qed

lemma coprime_crossproduct_int:
  fixes a b c d :: int
  assumes "coprime a d" and "coprime b c"
  shows "\<bar>a\<bar> * \<bar>c\<bar> = \<bar>b\<bar> * \<bar>d\<bar> \<longleftrightarrow> \<bar>a\<bar> = \<bar>b\<bar> \<and> \<bar>c\<bar> = \<bar>d\<bar>"
  using assms by (intro coprime_crossproduct_nat [transferred]) auto

text \<open>\medskip Addition laws\<close>

lemma gcd_add1_nat [simp]: "gcd ((m::nat) + n) n = gcd m n"
  apply (case_tac "n = 0")
  apply (simp_all add: gcd_non_0_nat)
done

lemma gcd_add2_nat [simp]: "gcd (m::nat) (m + n) = gcd m n"
  apply (subst (1 2) gcd_commute_nat)
  apply (subst add.commute)
  apply simp
done

(* to do: add the other variations? *)

lemma gcd_diff1_nat: "(m::nat) >= n \<Longrightarrow> gcd (m - n) n = gcd m n"
  by (subst gcd_add1_nat [symmetric], auto)

lemma gcd_diff2_nat: "(n::nat) >= m \<Longrightarrow> gcd (n - m) n = gcd m n"
  apply (subst gcd_commute_nat)
  apply (subst gcd_diff1_nat [symmetric])
  apply auto
  apply (subst gcd_commute_nat)
  apply (subst gcd_diff1_nat)
  apply assumption
  apply (rule gcd_commute_nat)
done

lemma gcd_non_0_int: "(y::int) > 0 \<Longrightarrow> gcd x y = gcd y (x mod y)"
  apply (frule_tac b = y and a = x in pos_mod_sign)
  apply (simp del: pos_mod_sign add: gcd_int_def abs_if nat_mod_distrib)
  apply (auto simp add: gcd_non_0_nat nat_mod_distrib [symmetric]
    zmod_zminus1_eq_if)
  apply (frule_tac a = x in pos_mod_bound)
  apply (subst (1 2) gcd_commute_nat)
  apply (simp del: pos_mod_bound add: nat_diff_distrib gcd_diff2_nat
    nat_le_eq_zle)
done

lemma gcd_red_int: "gcd (x::int) y = gcd y (x mod y)"
  apply (case_tac "y = 0")
  apply force
  apply (case_tac "y > 0")
  apply (subst gcd_non_0_int, auto)
  apply (insert gcd_non_0_int [of "-y" "-x"])
  apply auto
done

lemma gcd_add1_int [simp]: "gcd ((m::int) + n) n = gcd m n"
by (metis gcd_red_int mod_add_self1 add.commute)

lemma gcd_add2_int [simp]: "gcd m ((m::int) + n) = gcd m n"
by (metis gcd_add1_int gcd_commute_int add.commute)

lemma gcd_add_mult_nat: "gcd (m::nat) (k * m + n) = gcd m n"
by (metis mod_mult_self3 gcd_commute_nat gcd_red_nat)

lemma gcd_add_mult_int: "gcd (m::int) (k * m + n) = gcd m n"
by (metis gcd_commute_int gcd_red_int mod_mult_self1 add.commute)


(* to do: differences, and all variations of addition rules
    as simplification rules for nat and int *)

lemma gcd_dvd_prod_nat: "gcd (m::nat) n dvd k * n"
  using mult_dvd_mono [of 1] by auto

(* to do: add the three variations of these, and for ints? *)

lemma finite_divisors_nat[simp]:
  assumes "(m::nat) ~= 0" shows "finite{d. d dvd m}"
proof-
  have "finite{d. d <= m}"
    by (blast intro: bounded_nat_set_is_finite)
  from finite_subset[OF _ this] show ?thesis using assms
    by (metis Collect_mono dvd_imp_le neq0_conv)
qed

lemma finite_divisors_int[simp]:
  assumes "(i::int) ~= 0" shows "finite{d. d dvd i}"
proof-
  have "{d. \<bar>d\<bar> <= \<bar>i\<bar>} = {- \<bar>i\<bar> .. \<bar>i\<bar>}" by(auto simp:abs_if)
  hence "finite {d. \<bar>d\<bar> <= \<bar>i\<bar>}" by simp
  from finite_subset[OF _ this] show ?thesis using assms
    by (simp add: dvd_imp_le_int subset_iff)
qed

lemma Max_divisors_self_nat[simp]: "n\<noteq>0 \<Longrightarrow> Max{d::nat. d dvd n} = n"
apply(rule antisym)
 apply (fastforce intro: Max_le_iff[THEN iffD2] simp: dvd_imp_le)
apply simp
done

lemma Max_divisors_self_int[simp]: "n\<noteq>0 \<Longrightarrow> Max{d::int. d dvd n} = \<bar>n\<bar>"
apply(rule antisym)
 apply(rule Max_le_iff [THEN iffD2])
  apply (auto intro: abs_le_D1 dvd_imp_le_int)
done

lemma gcd_is_Max_divisors_nat:
  "m ~= 0 \<Longrightarrow> n ~= 0 \<Longrightarrow> gcd (m::nat) n = (Max {d. d dvd m & d dvd n})"
apply(rule Max_eqI[THEN sym])
  apply (metis finite_Collect_conjI finite_divisors_nat)
 apply simp
 apply(metis Suc_diff_1 Suc_neq_Zero dvd_imp_le gcd_greatest_iff_nat gcd_pos_nat)
apply simp
done

lemma gcd_is_Max_divisors_int:
  "m ~= 0 ==> n ~= 0 ==> gcd (m::int) n = (Max {d. d dvd m & d dvd n})"
apply(rule Max_eqI[THEN sym])
  apply (metis finite_Collect_conjI finite_divisors_int)
 apply simp
 apply (metis gcd_greatest_iff_int gcd_pos_int zdvd_imp_le)
apply simp
done

lemma gcd_code_int [code]:
  "gcd k l = \<bar>if l = (0::int) then k else gcd l (\<bar>k\<bar> mod \<bar>l\<bar>)\<bar>"
  by (simp add: gcd_int_def nat_mod_distrib gcd_non_0_nat)


subsection \<open>Coprimality\<close>

context semiring_gcd
begin

lemma div_gcd_coprime:
  assumes nz: "a \<noteq> 0 \<or> b \<noteq> 0"
  shows "coprime (a div gcd a b) (b div gcd a b)"
proof -
  let ?g = "gcd a b"
  let ?a' = "a div ?g"
  let ?b' = "b div ?g"
  let ?g' = "gcd ?a' ?b'"
  have dvdg: "?g dvd a" "?g dvd b" by simp_all
  have dvdg': "?g' dvd ?a'" "?g' dvd ?b'" by simp_all
  from dvdg dvdg' obtain ka kb ka' kb' where
      kab: "a = ?g * ka" "b = ?g * kb" "?a' = ?g' * ka'" "?b' = ?g' * kb'"
    unfolding dvd_def by blast
  from this [symmetric] have "?g * ?a' = (?g * ?g') * ka'" "?g * ?b' = (?g * ?g') * kb'"
    by (simp_all add: mult.assoc mult.left_commute [of "gcd a b"])
  then have dvdgg':"?g * ?g' dvd a" "?g* ?g' dvd b"
    by (auto simp add: dvd_mult_div_cancel [OF dvdg(1)]
      dvd_mult_div_cancel [OF dvdg(2)] dvd_def)
  have "?g \<noteq> 0" using nz by simp
  moreover from gcd_greatest [OF dvdgg'] have "?g * ?g' dvd ?g" .
  thm dvd_mult_cancel_left
  ultimately show ?thesis using dvd_times_left_cancel_iff [of "gcd a b" _ 1] by simp
qed

end

lemmas div_gcd_coprime_nat = div_gcd_coprime [where ?'a = nat]
lemmas div_gcd_coprime_int = div_gcd_coprime [where ?'a = int]

lemma coprime_nat: "coprime (a::nat) b \<longleftrightarrow> (\<forall>d. d dvd a \<and> d dvd b \<longleftrightarrow> d = 1)"
  using gcd_unique_nat[of 1 a b, simplified] by auto

lemma coprime_Suc_0_nat:
    "coprime (a::nat) b \<longleftrightarrow> (\<forall>d. d dvd a \<and> d dvd b \<longleftrightarrow> d = Suc 0)"
  using coprime_nat by simp

lemma coprime_int: "coprime (a::int) b \<longleftrightarrow>
    (\<forall>d. d >= 0 \<and> d dvd a \<and> d dvd b \<longleftrightarrow> d = 1)"
  using gcd_unique_int [of 1 a b]
  apply clarsimp
  apply (erule subst)
  apply (rule iffI)
  apply force
  using abs_dvd_iff abs_ge_zero apply blast
  done

lemma gcd_coprime_nat:
  assumes z: "gcd (a::nat) b \<noteq> 0" and a: "a = a' * gcd a b" and
    b: "b = b' * gcd a b"
  shows    "coprime a' b'"

  apply (subgoal_tac "a' = a div gcd a b")
  apply (erule ssubst)
  apply (subgoal_tac "b' = b div gcd a b")
  apply (erule ssubst)
  apply (rule div_gcd_coprime_nat)
  using z apply force
  apply (subst (1) b)
  using z apply force
  apply (subst (1) a)
  using z apply force
  done

lemma gcd_coprime_int:
  assumes z: "gcd (a::int) b \<noteq> 0" and a: "a = a' * gcd a b" and
    b: "b = b' * gcd a b"
  shows    "coprime a' b'"

  apply (subgoal_tac "a' = a div gcd a b")
  apply (erule ssubst)
  apply (subgoal_tac "b' = b div gcd a b")
  apply (erule ssubst)
  apply (rule div_gcd_coprime_int)
  using z apply force
  apply (subst (1) b)
  using z apply force
  apply (subst (1) a)
  using z apply force
  done

context semiring_gcd
begin

lemma coprime_mult:
  assumes da: "coprime d a" and db: "coprime d b"
  shows "coprime d (a * b)"
  apply (subst gcd.commute)
  using da apply (subst gcd_mult_cancel)
  apply (subst gcd.commute, assumption)
  apply (subst gcd.commute, rule db)
  done

end

lemmas coprime_mult_nat = coprime_mult [where ?'a = nat]
lemmas coprime_mult_int = coprime_mult [where ?'a = int]
  
lemma coprime_lmult_nat:
  assumes dab: "coprime (d::nat) (a * b)" shows "coprime d a"
proof -
  have "gcd d a dvd gcd d (a * b)"
    by (rule gcd_greatest, auto)
  with dab show ?thesis
    by auto
qed

lemma coprime_lmult_int:
  assumes "coprime (d::int) (a * b)" shows "coprime d a"
proof -
  have "gcd d a dvd gcd d (a * b)"
    by (rule gcd_greatest, auto)
  with assms show ?thesis
    by auto
qed

lemma coprime_rmult_nat:
  assumes "coprime (d::nat) (a * b)" shows "coprime d b"
proof -
  have "gcd d b dvd gcd d (a * b)"
    by (rule gcd_greatest, auto intro: dvd_mult)
  with assms show ?thesis
    by auto
qed

lemma coprime_rmult_int:
  assumes dab: "coprime (d::int) (a * b)" shows "coprime d b"
proof -
  have "gcd d b dvd gcd d (a * b)"
    by (rule gcd_greatest, auto intro: dvd_mult)
  with dab show ?thesis
    by auto
qed

lemma coprime_mul_eq_nat: "coprime (d::nat) (a * b) \<longleftrightarrow>
    coprime d a \<and>  coprime d b"
  using coprime_rmult_nat[of d a b] coprime_lmult_nat[of d a b]
    coprime_mult_nat[of d a b]
  by blast

lemma coprime_mul_eq_int: "coprime (d::int) (a * b) \<longleftrightarrow>
    coprime d a \<and>  coprime d b"
  using coprime_rmult_int[of d a b] coprime_lmult_int[of d a b]
    coprime_mult_int[of d a b]
  by blast

lemma coprime_power_int:
  assumes "0 < n" shows "coprime (a :: int) (b ^ n) \<longleftrightarrow> coprime a b"
  using assms
proof (induct n)
  case (Suc n) then show ?case
    by (cases n) (simp_all add: coprime_mul_eq_int)
qed simp

lemma gcd_coprime_exists_nat:
    assumes nz: "gcd (a::nat) b \<noteq> 0"
    shows "\<exists>a' b'. a = a' * gcd a b \<and> b = b' * gcd a b \<and> coprime a' b'"
  apply (rule_tac x = "a div gcd a b" in exI)
  apply (rule_tac x = "b div gcd a b" in exI)
  using nz apply (auto simp add: div_gcd_coprime_nat dvd_div_mult)
done

lemma gcd_coprime_exists_int:
    assumes nz: "gcd (a::int) b \<noteq> 0"
    shows "\<exists>a' b'. a = a' * gcd a b \<and> b = b' * gcd a b \<and> coprime a' b'"
  apply (rule_tac x = "a div gcd a b" in exI)
  apply (rule_tac x = "b div gcd a b" in exI)
  using nz apply (auto simp add: div_gcd_coprime_int)
done

lemma coprime_exp_nat: "coprime (d::nat) a \<Longrightarrow> coprime d (a^n)"
  by (induct n) (simp_all add: coprime_mult_nat)

lemma coprime_exp_int: "coprime (d::int) a \<Longrightarrow> coprime d (a^n)"
  by (induct n) (simp_all add: coprime_mult_int)

context semiring_gcd
begin

lemma coprime_exp_left:
  assumes "coprime a b"
  shows "coprime (a ^ n) b"
  using assms by (induct n) (simp_all add: gcd_mult_cancel)

lemma coprime_exp2:
  assumes "coprime a b"
  shows "coprime (a ^ n) (b ^ m)"
proof (rule coprime_exp_left)
  from assms show "coprime a (b ^ m)"
    by (induct m) (simp_all add: gcd_mult_cancel gcd.commute [of a])
qed

end

lemma coprime_exp2_nat [intro]: "coprime (a::nat) b \<Longrightarrow> coprime (a^n) (b^m)"
  by (fact coprime_exp2)

lemma coprime_exp2_int [intro]: "coprime (a::int) b \<Longrightarrow> coprime (a^n) (b^m)"
  by (fact coprime_exp2)

lemma gcd_exp_nat:
  "gcd ((a :: nat) ^ n) (b ^ n) = gcd a b ^ n"
proof (cases "a = 0 \<and> b = 0")
  case True then show ?thesis by (cases "n > 0") (simp_all add: zero_power)
next
  case False
  then have "coprime (a div gcd a b) (b div gcd a b)"
    by (auto simp: div_gcd_coprime)
  then have "coprime ((a div gcd a b) ^ n) ((b div gcd a b) ^ n)"
    by (simp add: coprime_exp2)
  then have "gcd ((a div gcd a b)^n * (gcd a b)^n)
      ((b div gcd a b)^n * (gcd a b)^n) = (gcd a b)^n"
    by (metis gcd_mult_distrib_nat mult.commute mult.right_neutral)
  also have "(a div gcd a b)^n * (gcd a b)^n = a^n"
    by (metis dvd_div_mult_self gcd_unique_nat power_mult_distrib)
  also have "(b div gcd a b)^n * (gcd a b)^n = b^n"
    by (metis dvd_div_mult_self gcd_unique_nat power_mult_distrib)
  finally show ?thesis .
qed

lemma gcd_exp_int: "gcd ((a::int)^n) (b^n) = (gcd a b)^n"
  apply (subst (1 2) gcd_abs_int)
  apply (subst (1 2) power_abs)
  apply (rule gcd_exp_nat [where n = n, transferred])
  apply auto
done

lemma division_decomp_nat: assumes dc: "(a::nat) dvd b * c"
  shows "\<exists>b' c'. a = b' * c' \<and> b' dvd b \<and> c' dvd c"
proof-
  let ?g = "gcd a b"
  {assume "?g = 0" with dc have ?thesis by auto}
  moreover
  {assume z: "?g \<noteq> 0"
    from gcd_coprime_exists_nat[OF z]
    obtain a' b' where ab': "a = a' * ?g" "b = b' * ?g" "coprime a' b'"
      by blast
    have thb: "?g dvd b" by auto
    from ab'(1) have "a' dvd a"  unfolding dvd_def by blast
    with dc have th0: "a' dvd b*c" using dvd_trans[of a' a "b*c"] by simp
    from dc ab'(1,2) have "a'*?g dvd (b'*?g) *c" by auto
    hence "?g*a' dvd ?g * (b' * c)" by (simp add: mult.assoc)
    with z have th_1: "a' dvd b' * c" by auto
    from coprime_dvd_mult_nat[OF ab'(3)] th_1
    have thc: "a' dvd c" by (subst (asm) mult.commute, blast)
    from ab' have "a = ?g*a'" by algebra
    with thb thc have ?thesis by blast }
  ultimately show ?thesis by blast
qed

lemma division_decomp_int: assumes dc: "(a::int) dvd b * c"
  shows "\<exists>b' c'. a = b' * c' \<and> b' dvd b \<and> c' dvd c"
proof-
  let ?g = "gcd a b"
  {assume "?g = 0" with dc have ?thesis by auto}
  moreover
  {assume z: "?g \<noteq> 0"
    from gcd_coprime_exists_int[OF z]
    obtain a' b' where ab': "a = a' * ?g" "b = b' * ?g" "coprime a' b'"
      by blast
    have thb: "?g dvd b" by auto
    from ab'(1) have "a' dvd a"  unfolding dvd_def by blast
    with dc have th0: "a' dvd b*c"
      using dvd_trans[of a' a "b*c"] by simp
    from dc ab'(1,2) have "a'*?g dvd (b'*?g) *c" by auto
    hence "?g*a' dvd ?g * (b' * c)" by (simp add: ac_simps)
    with z have th_1: "a' dvd b' * c" by auto
    from coprime_dvd_mult_int[OF ab'(3)] th_1
    have thc: "a' dvd c" by (subst (asm) mult.commute, blast)
    from ab' have "a = ?g*a'" by algebra
    with thb thc have ?thesis by blast }
  ultimately show ?thesis by blast
qed

lemma pow_divides_pow_nat:
  assumes ab: "(a::nat) ^ n dvd b ^n" and n:"n \<noteq> 0"
  shows "a dvd b"
proof-
  let ?g = "gcd a b"
  from n obtain m where m: "n = Suc m" by (cases n, simp_all)
  {assume "?g = 0" with ab n have ?thesis by auto }
  moreover
  {assume z: "?g \<noteq> 0"
    hence zn: "?g ^ n \<noteq> 0" using n by simp
    from gcd_coprime_exists_nat[OF z]
    obtain a' b' where ab': "a = a' * ?g" "b = b' * ?g" "coprime a' b'"
      by blast
    from ab have "(a' * ?g) ^ n dvd (b' * ?g)^n"
      by (simp add: ab'(1,2)[symmetric])
    hence "?g^n*a'^n dvd ?g^n *b'^n"
      by (simp only: power_mult_distrib mult.commute)
    then have th0: "a'^n dvd b'^n"
      using zn by auto
    have "a' dvd a'^n" by (simp add: m)
    with th0 have "a' dvd b'^n" using dvd_trans[of a' "a'^n" "b'^n"] by simp
    hence th1: "a' dvd b'^m * b'" by (simp add: m mult.commute)
    from coprime_dvd_mult_nat[OF coprime_exp_nat [OF ab'(3), of m]] th1
    have "a' dvd b'" by (subst (asm) mult.commute, blast)
    hence "a'*?g dvd b'*?g" by simp
    with ab'(1,2)  have ?thesis by simp }
  ultimately show ?thesis by blast
qed

lemma pow_divides_pow_int:
  assumes ab: "(a::int) ^ n dvd b ^n" and n:"n \<noteq> 0"
  shows "a dvd b"
proof-
  let ?g = "gcd a b"
  from n obtain m where m: "n = Suc m" by (cases n, simp_all)
  {assume "?g = 0" with ab n have ?thesis by auto }
  moreover
  {assume z: "?g \<noteq> 0"
    hence zn: "?g ^ n \<noteq> 0" using n by simp
    from gcd_coprime_exists_int[OF z]
    obtain a' b' where ab': "a = a' * ?g" "b = b' * ?g" "coprime a' b'"
      by blast
    from ab have "(a' * ?g) ^ n dvd (b' * ?g)^n"
      by (simp add: ab'(1,2)[symmetric])
    hence "?g^n*a'^n dvd ?g^n *b'^n"
      by (simp only: power_mult_distrib mult.commute)
    with zn z n have th0:"a'^n dvd b'^n" by auto
    have "a' dvd a'^n" by (simp add: m)
    with th0 have "a' dvd b'^n"
      using dvd_trans[of a' "a'^n" "b'^n"] by simp
    hence th1: "a' dvd b'^m * b'" by (simp add: m mult.commute)
    from coprime_dvd_mult_int[OF coprime_exp_int [OF ab'(3), of m]] th1
    have "a' dvd b'" by (subst (asm) mult.commute, blast)
    hence "a'*?g dvd b'*?g" by simp
    with ab'(1,2)  have ?thesis by simp }
  ultimately show ?thesis by blast
qed

lemma pow_divides_eq_nat [simp]: "n ~= 0 \<Longrightarrow> ((a::nat)^n dvd b^n) = (a dvd b)"
  by (auto intro: pow_divides_pow_nat dvd_power_same)

lemma pow_divides_eq_int [simp]: "n ~= 0 \<Longrightarrow> ((a::int)^n dvd b^n) = (a dvd b)"
  by (auto intro: pow_divides_pow_int dvd_power_same)

lemma divides_mult_nat:
  assumes mr: "(m::nat) dvd r" and nr: "n dvd r" and mn:"coprime m n"
  shows "m * n dvd r"
proof-
  from mr nr obtain m' n' where m': "r = m*m'" and n': "r = n*n'"
    unfolding dvd_def by blast
  from mr n' have "m dvd n'*n" by (simp add: mult.commute)
  hence "m dvd n'" using coprime_dvd_mult_iff_nat[OF mn] by simp
  then obtain k where k: "n' = m*k" unfolding dvd_def by blast
  from n' k show ?thesis unfolding dvd_def by auto
qed

lemma divides_mult_int:
  assumes mr: "(m::int) dvd r" and nr: "n dvd r" and mn:"coprime m n"
  shows "m * n dvd r"
proof-
  from mr nr obtain m' n' where m': "r = m*m'" and n': "r = n*n'"
    unfolding dvd_def by blast
  from mr n' have "m dvd n'*n" by (simp add: mult.commute)
  hence "m dvd n'" using coprime_dvd_mult_iff_int[OF mn] by simp
  then obtain k where k: "n' = m*k" unfolding dvd_def by blast
  from n' k show ?thesis unfolding dvd_def by auto
qed

lemma coprime_plus_one_nat [simp]: "coprime ((n::nat) + 1) n"
  by (simp add: gcd.commute del: One_nat_def)

lemma coprime_Suc_nat [simp]: "coprime (Suc n) n"
  using coprime_plus_one_nat by simp

lemma coprime_plus_one_int [simp]: "coprime ((n::int) + 1) n"
  by (simp add: gcd.commute)

lemma coprime_minus_one_nat: "(n::nat) \<noteq> 0 \<Longrightarrow> coprime (n - 1) n"
  using coprime_plus_one_nat [of "n - 1"]
    gcd_commute_nat [of "n - 1" n] by auto

lemma coprime_minus_one_int: "coprime ((n::int) - 1) n"
  using coprime_plus_one_int [of "n - 1"]
    gcd_commute_int [of "n - 1" n] by auto

lemma setprod_coprime_nat [rule_format]:
    "(ALL i: A. coprime (f i) (x::nat)) --> coprime (\<Prod>i\<in>A. f i) x"
  apply (case_tac "finite A")
  apply (induct set: finite)
  apply (auto simp add: gcd_mult_cancel_nat)
done

lemma setprod_coprime_int [rule_format]:
    "(ALL i: A. coprime (f i) (x::int)) --> coprime (\<Prod>i\<in>A. f i) x"
  apply (case_tac "finite A")
  apply (induct set: finite)
  apply (auto simp add: gcd_mult_cancel_int)
done

lemma coprime_common_divisor_nat: 
  "coprime (a::nat) b \<Longrightarrow> x dvd a \<Longrightarrow> x dvd b \<Longrightarrow> x = 1"
  by (metis gcd_greatest_iff_nat nat_dvd_1_iff_1)

lemma coprime_common_divisor_int:
  "coprime (a::int) b \<Longrightarrow> x dvd a \<Longrightarrow> x dvd b \<Longrightarrow> \<bar>x\<bar> = 1"
  using gcd_greatest_iff [of x a b] by auto

lemma coprime_divisors_nat:
    "(d::int) dvd a \<Longrightarrow> e dvd b \<Longrightarrow> coprime a b \<Longrightarrow> coprime d e"
  by (meson coprime_int dvd_trans gcd_dvd1 gcd_dvd2 gcd_ge_0_int)

lemma invertible_coprime_nat: "(x::nat) * y mod m = 1 \<Longrightarrow> coprime x m"
by (metis coprime_lmult_nat gcd_1_nat gcd_commute_nat gcd_red_nat)

lemma invertible_coprime_int: "(x::int) * y mod m = 1 \<Longrightarrow> coprime x m"
by (metis coprime_lmult_int gcd_1_int gcd_commute_int gcd_red_int)


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

(* Function bezw returns a pair of witnesses to Bezout's theorem --
   see the theorems that follow the definition. *)
fun
  bezw  :: "nat \<Rightarrow> nat \<Rightarrow> int * int"
where
  "bezw x y =
  (if y = 0 then (1, 0) else
      (snd (bezw y (x mod y)),
       fst (bezw y (x mod y)) - snd (bezw y (x mod y)) * int(x div y)))"

lemma bezw_0 [simp]: "bezw x 0 = (1, 0)" by simp

lemma bezw_non_0: "y > 0 \<Longrightarrow> bezw x y = (snd (bezw y (x mod y)),
       fst (bezw y (x mod y)) - snd (bezw y (x mod y)) * int(x div y))"
  by simp

declare bezw.simps [simp del]

lemma bezw_aux [rule_format]:
    "fst (bezw x y) * int x + snd (bezw x y) * int y = int (gcd x y)"
proof (induct x y rule: gcd_nat_induct)
  fix m :: nat
  show "fst (bezw m 0) * int m + snd (bezw m 0) * int 0 = int (gcd m 0)"
    by auto
  next fix m :: nat and n
    assume ngt0: "n > 0" and
      ih: "fst (bezw n (m mod n)) * int n +
        snd (bezw n (m mod n)) * int (m mod n) =
        int (gcd n (m mod n))"
    thus "fst (bezw m n) * int m + snd (bezw m n) * int n = int (gcd m n)"
      apply (simp add: bezw_non_0 gcd_non_0_nat)
      apply (erule subst)
      apply (simp add: field_simps)
      apply (subst mod_div_equality [of m n, symmetric])
      (* applying simp here undoes the last substitution!
         what is procedure cancel_div_mod? *)
      apply (simp only: NO_MATCH_def field_simps of_nat_add of_nat_mult)
      done
qed

lemma bezout_int:
  fixes x y
  shows "EX u v. u * (x::int) + v * y = gcd x y"
proof -
  have bezout_aux: "!!x y. x \<ge> (0::int) \<Longrightarrow> y \<ge> 0 \<Longrightarrow>
      EX u v. u * x + v * y = gcd x y"
    apply (rule_tac x = "fst (bezw (nat x) (nat y))" in exI)
    apply (rule_tac x = "snd (bezw (nat x) (nat y))" in exI)
    apply (unfold gcd_int_def)
    apply simp
    apply (subst bezw_aux [symmetric])
    apply auto
    done
  have "(x \<ge> 0 \<and> y \<ge> 0) | (x \<ge> 0 \<and> y \<le> 0) | (x \<le> 0 \<and> y \<ge> 0) |
      (x \<le> 0 \<and> y \<le> 0)"
    by auto
  moreover have "x \<ge> 0 \<Longrightarrow> y \<ge> 0 \<Longrightarrow> ?thesis"
    by (erule (1) bezout_aux)
  moreover have "x >= 0 \<Longrightarrow> y <= 0 \<Longrightarrow> ?thesis"
    apply (insert bezout_aux [of x "-y"])
    apply auto
    apply (rule_tac x = u in exI)
    apply (rule_tac x = "-v" in exI)
    apply (subst gcd_neg2_int [symmetric])
    apply auto
    done
  moreover have "x <= 0 \<Longrightarrow> y >= 0 \<Longrightarrow> ?thesis"
    apply (insert bezout_aux [of "-x" y])
    apply auto
    apply (rule_tac x = "-u" in exI)
    apply (rule_tac x = v in exI)
    apply (subst gcd_neg1_int [symmetric])
    apply auto
    done
  moreover have "x <= 0 \<Longrightarrow> y <= 0 \<Longrightarrow> ?thesis"
    apply (insert bezout_aux [of "-x" "-y"])
    apply auto
    apply (rule_tac x = "-u" in exI)
    apply (rule_tac x = "-v" in exI)
    apply (subst gcd_neg1_int [symmetric])
    apply (subst gcd_neg2_int [symmetric])
    apply auto
    done
  ultimately show ?thesis by blast
qed

text \<open>versions of Bezout for nat, by Amine Chaieb\<close>

lemma ind_euclid:
  assumes c: " \<forall>a b. P (a::nat) b \<longleftrightarrow> P b a" and z: "\<forall>a. P a 0"
  and add: "\<forall>a b. P a b \<longrightarrow> P a (a + b)"
  shows "P a b"
proof(induct "a + b" arbitrary: a b rule: less_induct)
  case less
  have "a = b \<or> a < b \<or> b < a" by arith
  moreover {assume eq: "a= b"
    from add[rule_format, OF z[rule_format, of a]] have "P a b" using eq
    by simp}
  moreover
  {assume lt: "a < b"
    hence "a + b - a < a + b \<or> a = 0" by arith
    moreover
    {assume "a =0" with z c have "P a b" by blast }
    moreover
    {assume "a + b - a < a + b"
      also have th0: "a + b - a = a + (b - a)" using lt by arith
      finally have "a + (b - a) < a + b" .
      then have "P a (a + (b - a))" by (rule add[rule_format, OF less])
      then have "P a b" by (simp add: th0[symmetric])}
    ultimately have "P a b" by blast}
  moreover
  {assume lt: "a > b"
    hence "b + a - b < a + b \<or> b = 0" by arith
    moreover
    {assume "b =0" with z c have "P a b" by blast }
    moreover
    {assume "b + a - b < a + b"
      also have th0: "b + a - b = b + (a - b)" using lt by arith
      finally have "b + (a - b) < a + b" .
      then have "P b (b + (a - b))" by (rule add[rule_format, OF less])
      then have "P b a" by (simp add: th0[symmetric])
      hence "P a b" using c by blast }
    ultimately have "P a b" by blast}
ultimately  show "P a b" by blast
qed

lemma bezout_lemma_nat:
  assumes ex: "\<exists>(d::nat) x y. d dvd a \<and> d dvd b \<and>
    (a * x = b * y + d \<or> b * x = a * y + d)"
  shows "\<exists>d x y. d dvd a \<and> d dvd a + b \<and>
    (a * x = (a + b) * y + d \<or> (a + b) * x = a * y + d)"
  using ex
  apply clarsimp
  apply (rule_tac x="d" in exI, simp)
  apply (case_tac "a * x = b * y + d" , simp_all)
  apply (rule_tac x="x + y" in exI)
  apply (rule_tac x="y" in exI)
  apply algebra
  apply (rule_tac x="x" in exI)
  apply (rule_tac x="x + y" in exI)
  apply algebra
done

lemma bezout_add_nat: "\<exists>(d::nat) x y. d dvd a \<and> d dvd b \<and>
    (a * x = b * y + d \<or> b * x = a * y + d)"
  apply(induct a b rule: ind_euclid)
  apply blast
  apply clarify
  apply (rule_tac x="a" in exI, simp)
  apply clarsimp
  apply (rule_tac x="d" in exI)
  apply (case_tac "a * x = b * y + d", simp_all)
  apply (rule_tac x="x+y" in exI)
  apply (rule_tac x="y" in exI)
  apply algebra
  apply (rule_tac x="x" in exI)
  apply (rule_tac x="x+y" in exI)
  apply algebra
done

lemma bezout1_nat: "\<exists>(d::nat) x y. d dvd a \<and> d dvd b \<and>
    (a * x - b * y = d \<or> b * x - a * y = d)"
  using bezout_add_nat[of a b]
  apply clarsimp
  apply (rule_tac x="d" in exI, simp)
  apply (rule_tac x="x" in exI)
  apply (rule_tac x="y" in exI)
  apply auto
done

lemma bezout_add_strong_nat: assumes nz: "a \<noteq> (0::nat)"
  shows "\<exists>d x y. d dvd a \<and> d dvd b \<and> a * x = b * y + d"
proof-
 from nz have ap: "a > 0" by simp
 from bezout_add_nat[of a b]
 have "(\<exists>d x y. d dvd a \<and> d dvd b \<and> a * x = b * y + d) \<or>
   (\<exists>d x y. d dvd a \<and> d dvd b \<and> b * x = a * y + d)" by blast
 moreover
    {fix d x y assume H: "d dvd a" "d dvd b" "a * x = b * y + d"
     from H have ?thesis by blast }
 moreover
 {fix d x y assume H: "d dvd a" "d dvd b" "b * x = a * y + d"
   {assume b0: "b = 0" with H  have ?thesis by simp}
   moreover
   {assume b: "b \<noteq> 0" hence bp: "b > 0" by simp
     from b dvd_imp_le [OF H(2)] have "d < b \<or> d = b"
       by auto
     moreover
     {assume db: "d=b"
       with nz H have ?thesis apply simp
         apply (rule exI[where x = b], simp)
         apply (rule exI[where x = b])
        by (rule exI[where x = "a - 1"], simp add: diff_mult_distrib2)}
    moreover
    {assume db: "d < b"
        {assume "x=0" hence ?thesis using nz H by simp }
        moreover
        {assume x0: "x \<noteq> 0" hence xp: "x > 0" by simp
          from db have "d \<le> b - 1" by simp
          hence "d*b \<le> b*(b - 1)" by simp
          with xp mult_mono[of "1" "x" "d*b" "b*(b - 1)"]
          have dble: "d*b \<le> x*b*(b - 1)" using bp by simp
          from H (3) have "d + (b - 1) * (b*x) = d + (b - 1) * (a*y + d)"
            by simp
          hence "d + (b - 1) * a * y + (b - 1) * d = d + (b - 1) * b * x"
            by (simp only: mult.assoc distrib_left)
          hence "a * ((b - 1) * y) + d * (b - 1 + 1) = d + x*b*(b - 1)"
            by algebra
          hence "a * ((b - 1) * y) = d + x*b*(b - 1) - d*b" using bp by simp
          hence "a * ((b - 1) * y) = d + (x*b*(b - 1) - d*b)"
            by (simp only: diff_add_assoc[OF dble, of d, symmetric])
          hence "a * ((b - 1) * y) = b*(x*(b - 1) - d) + d"
            by (simp only: diff_mult_distrib2 ac_simps)
          hence ?thesis using H(1,2)
            apply -
            apply (rule exI[where x=d], simp)
            apply (rule exI[where x="(b - 1) * y"])
            by (rule exI[where x="x*(b - 1) - d"], simp)}
        ultimately have ?thesis by blast}
    ultimately have ?thesis by blast}
  ultimately have ?thesis by blast}
 ultimately show ?thesis by blast
qed

lemma bezout_nat: assumes a: "(a::nat) \<noteq> 0"
  shows "\<exists>x y. a * x = b * y + gcd a b"
proof-
  let ?g = "gcd a b"
  from bezout_add_strong_nat[OF a, of b]
  obtain d x y where d: "d dvd a" "d dvd b" "a * x = b * y + d" by blast
  from d(1,2) have "d dvd ?g" by simp
  then obtain k where k: "?g = d*k" unfolding dvd_def by blast
  from d(3) have "a * x * k = (b * y + d) *k " by auto
  hence "a * (x * k) = b * (y*k) + ?g" by (algebra add: k)
  thus ?thesis by blast
qed


subsection \<open>LCM properties\<close>

lemma lcm_altdef_int [code]: "lcm (a::int) b = \<bar>a\<bar> * \<bar>b\<bar> div gcd a b"
  by (simp add: lcm_int_def lcm_nat_def zdiv_int
    of_nat_mult gcd_int_def)

lemma prod_gcd_lcm_nat: "(m::nat) * n = gcd m n * lcm m n"
  unfolding lcm_nat_def
  by (simp add: dvd_mult_div_cancel [OF gcd_dvd_prod_nat])

lemma prod_gcd_lcm_int: "\<bar>m::int\<bar> * \<bar>n\<bar> = gcd m n * lcm m n"
  unfolding lcm_int_def gcd_int_def
  apply (subst int_mult [symmetric])
  apply (subst prod_gcd_lcm_nat [symmetric])
  apply (subst nat_abs_mult_distrib [symmetric])
  apply (simp, simp add: abs_mult)
done

lemma lcm_0_nat [simp]: "lcm (m::nat) 0 = 0"
  unfolding lcm_nat_def by simp

lemma lcm_0_int [simp]: "lcm (m::int) 0 = 0"
  unfolding lcm_int_def by simp

lemma lcm_0_left_nat [simp]: "lcm (0::nat) n = 0"
  unfolding lcm_nat_def by simp

lemma lcm_0_left_int [simp]: "lcm (0::int) n = 0"
  unfolding lcm_int_def by simp

lemma lcm_pos_nat:
  "(m::nat) > 0 \<Longrightarrow> n>0 \<Longrightarrow> lcm m n > 0"
by (metis gr0I mult_is_0 prod_gcd_lcm_nat)

lemma lcm_pos_int:
  "(m::int) ~= 0 \<Longrightarrow> n ~= 0 \<Longrightarrow> lcm m n > 0"
  apply (subst lcm_abs_int)
  apply (rule lcm_pos_nat [transferred])
  apply auto
done

lemma dvd_pos_nat:
  fixes n m :: nat
  assumes "n > 0" and "m dvd n"
  shows "m > 0"
using assms by (cases m) auto

lemma lcm_least_nat:
  assumes "(m::nat) dvd k" and "n dvd k"
  shows "lcm m n dvd k"
  using assms by (rule lcm_least)

lemma lcm_least_int:
  "(m::int) dvd k \<Longrightarrow> n dvd k \<Longrightarrow> lcm m n dvd k"
  by (rule lcm_least)

lemma lcm_dvd1_nat: "(m::nat) dvd lcm m n"
  by (fact dvd_lcm1)

lemma lcm_dvd1_int: "(m::int) dvd lcm m n"
  by (fact dvd_lcm1)

lemma lcm_dvd2_nat: "(n::nat) dvd lcm m n"
  by (fact dvd_lcm2)

lemma lcm_dvd2_int: "(n::int) dvd lcm m n"
  by (fact dvd_lcm2)

lemma dvd_lcm_I1_nat[simp]: "(k::nat) dvd m \<Longrightarrow> k dvd lcm m n"
by(metis lcm_dvd1_nat dvd_trans)

lemma dvd_lcm_I2_nat[simp]: "(k::nat) dvd n \<Longrightarrow> k dvd lcm m n"
by(metis lcm_dvd2_nat dvd_trans)

lemma dvd_lcm_I1_int[simp]: "(i::int) dvd m \<Longrightarrow> i dvd lcm m n"
by(metis lcm_dvd1_int dvd_trans)

lemma dvd_lcm_I2_int[simp]: "(i::int) dvd n \<Longrightarrow> i dvd lcm m n"
by(metis lcm_dvd2_int dvd_trans)

lemma lcm_unique_nat: "(a::nat) dvd d \<and> b dvd d \<and>
    (\<forall>e. a dvd e \<and> b dvd e \<longrightarrow> d dvd e) \<longleftrightarrow> d = lcm a b"
  by (auto intro: dvd_antisym lcm_least_nat lcm_dvd1_nat lcm_dvd2_nat)

lemma lcm_unique_int: "d >= 0 \<and> (a::int) dvd d \<and> b dvd d \<and>
    (\<forall>e. a dvd e \<and> b dvd e \<longrightarrow> d dvd e) \<longleftrightarrow> d = lcm a b"
  using lcm_least_int zdvd_antisym_nonneg by auto

interpretation lcm_nat: abel_semigroup "lcm :: nat \<Rightarrow> nat \<Rightarrow> nat"
  + lcm_nat: semilattice_neutr "lcm :: nat \<Rightarrow> nat \<Rightarrow> nat" 1
  by standard (simp_all del: One_nat_def)

interpretation lcm_int: abel_semigroup "lcm :: int \<Rightarrow> int \<Rightarrow> int" ..

lemmas lcm_assoc_nat = lcm.assoc [where ?'a = nat]
lemmas lcm_commute_nat = lcm.commute [where ?'a = nat]
lemmas lcm_left_commute_nat = lcm.left_commute [where ?'a = nat]
lemmas lcm_assoc_int = lcm.assoc [where ?'a = int]
lemmas lcm_commute_int = lcm.commute [where ?'a = int]
lemmas lcm_left_commute_int = lcm.left_commute [where ?'a = int]

lemmas lcm_ac_nat = lcm_assoc_nat lcm_commute_nat lcm_left_commute_nat
lemmas lcm_ac_int = lcm_assoc_int lcm_commute_int lcm_left_commute_int

lemma lcm_proj2_if_dvd_nat [simp]: "(x::nat) dvd y \<Longrightarrow> lcm x y = y"
  apply (rule sym)
  apply (subst lcm_unique_nat [symmetric])
  apply auto
done

lemma lcm_proj2_if_dvd_int [simp]: "(x::int) dvd y \<Longrightarrow> lcm x y = \<bar>y\<bar>"
  apply (rule sym)
  apply (subst lcm_unique_int [symmetric])
  apply auto
done

lemma lcm_proj1_if_dvd_nat [simp]: "(x::nat) dvd y \<Longrightarrow> lcm y x = y"
by (subst lcm_commute_nat, erule lcm_proj2_if_dvd_nat)

lemma lcm_proj1_if_dvd_int [simp]: "(x::int) dvd y \<Longrightarrow> lcm y x = \<bar>y\<bar>"
by (subst lcm_commute_int, erule lcm_proj2_if_dvd_int)

lemma lcm_proj1_iff_nat[simp]: "lcm m n = (m::nat) \<longleftrightarrow> n dvd m"
by (metis lcm_proj1_if_dvd_nat lcm_unique_nat)

lemma lcm_proj2_iff_nat[simp]: "lcm m n = (n::nat) \<longleftrightarrow> m dvd n"
by (metis lcm_proj2_if_dvd_nat lcm_unique_nat)

lemma lcm_proj1_iff_int[simp]: "lcm m n = \<bar>m::int\<bar> \<longleftrightarrow> n dvd m"
by (metis dvd_abs_iff lcm_proj1_if_dvd_int lcm_unique_int)

lemma lcm_proj2_iff_int[simp]: "lcm m n = \<bar>n::int\<bar> \<longleftrightarrow> m dvd n"
by (metis dvd_abs_iff lcm_proj2_if_dvd_int lcm_unique_int)

lemma (in semiring_gcd) comp_fun_idem_gcd:
  "comp_fun_idem gcd"
  by standard (simp_all add: fun_eq_iff ac_simps)

lemma (in semiring_gcd) comp_fun_idem_lcm:
  "comp_fun_idem lcm"
  by standard (simp_all add: fun_eq_iff ac_simps)

lemma comp_fun_idem_gcd_nat: "comp_fun_idem (gcd :: nat\<Rightarrow>nat\<Rightarrow>nat)"
  by (fact comp_fun_idem_gcd)

lemma comp_fun_idem_gcd_int: "comp_fun_idem (gcd :: int\<Rightarrow>int\<Rightarrow>int)"
  by (fact comp_fun_idem_gcd)

lemma comp_fun_idem_lcm_nat: "comp_fun_idem (lcm :: nat\<Rightarrow>nat\<Rightarrow>nat)"
  by (fact comp_fun_idem_lcm)

lemma comp_fun_idem_lcm_int: "comp_fun_idem (lcm :: int\<Rightarrow>int\<Rightarrow>int)"
  by (fact comp_fun_idem_lcm)

lemma lcm_0_iff_nat [simp]: "lcm (m::nat) n = 0 \<longleftrightarrow> m=0 \<or> n=0"
  by (fact lcm_eq_0_iff)

lemma lcm_0_iff_int [simp]: "lcm (m::int) n = 0 \<longleftrightarrow> m=0 \<or> n=0"
  by (fact lcm_eq_0_iff)

lemma lcm_1_iff_nat [simp]: "lcm (m::nat) n = 1 \<longleftrightarrow> m=1 \<and> n=1"
  by (simp only: lcm_eq_1_iff) simp
  
lemma lcm_1_iff_int [simp]: "lcm (m::int) n = 1 \<longleftrightarrow> (m=1 \<or> m = -1) \<and> (n=1 \<or> n = -1)"
  by auto


subsection \<open>The complete divisibility lattice\<close>

interpretation gcd_semilattice_nat: semilattice_inf gcd Rings.dvd "(\<lambda>m n::nat. m dvd n \<and> \<not> n dvd m)"
  by standard simp_all

interpretation lcm_semilattice_nat: semilattice_sup lcm Rings.dvd "(\<lambda>m n::nat. m dvd n \<and> \<not> n dvd m)"
  by standard simp_all

interpretation gcd_lcm_lattice_nat: lattice gcd Rings.dvd "(\<lambda>m n::nat. m dvd n & ~ n dvd m)" lcm ..

text\<open>Lifting gcd and lcm to sets (Gcd/Lcm).
Gcd is defined via Lcm to facilitate the proof that we have a complete lattice.
\<close>

instantiation nat :: Gcd
begin

interpretation semilattice_neutr_set lcm "1::nat" ..

definition
  "Lcm (M::nat set) = (if finite M then F M else 0)"

lemma Lcm_nat_empty:
  "Lcm {} = (1::nat)"
  by (simp add: Lcm_nat_def del: One_nat_def)

lemma Lcm_nat_insert:
  "Lcm (insert n M) = lcm (n::nat) (Lcm M)"
  by (cases "finite M") (auto simp add: Lcm_nat_def simp del: One_nat_def)

lemma Lcm_nat_infinite:
  "infinite M \<Longrightarrow> Lcm M = (0::nat)"
  by (simp add: Lcm_nat_def)

lemma dvd_Lcm_nat [simp]:
  fixes M :: "nat set"
  assumes "m \<in> M"
  shows "m dvd Lcm M"
proof -
  from assms have "insert m M = M" by auto
  moreover have "m dvd Lcm (insert m M)"
    by (simp add: Lcm_nat_insert)
  ultimately show ?thesis by simp
qed

lemma Lcm_dvd_nat [simp]:
  fixes M :: "nat set"
  assumes "\<forall>m\<in>M. m dvd n"
  shows "Lcm M dvd n"
proof (cases "n = 0")
  case True then show ?thesis by simp
next
  case False
  then have "finite {d. d dvd n}" by (rule finite_divisors_nat)
  moreover have "M \<subseteq> {d. d dvd n}" using assms by fast
  ultimately have "finite M" by (rule rev_finite_subset)
  then show ?thesis using assms by (induct M) (simp_all add: Lcm_nat_empty Lcm_nat_insert)
qed

definition
  "Gcd (M::nat set) = Lcm {d. \<forall>m\<in>M. d dvd m}"

interpretation bla: semilattice_neutr_set gcd "0::nat" ..

instance ..

end

instance nat :: semiring_Gcd
proof
  show "Gcd N dvd n" if "n \<in> N" for N and n :: nat
  using that by (induct N rule: infinite_finite_induct)
    (auto simp add: Gcd_nat_def)
  show "n dvd Gcd N" if "\<And>m. m \<in> N \<Longrightarrow> n dvd m" for N and n :: nat
  using that by (induct N rule: infinite_finite_induct)
    (auto simp add: Gcd_nat_def)
qed simp

instance nat :: semiring_Lcm
proof
  show "Lcm N = Gcd {m. \<forall>n\<in>N. n dvd m}" for N :: "nat set"
    by (rule associated_eqI) (auto intro!: Gcd_dvd Gcd_greatest)
qed

interpretation gcd_lcm_complete_lattice_nat:
  complete_lattice Gcd Lcm gcd Rings.dvd "\<lambda>m n. m dvd n \<and> \<not> n dvd m" lcm 1 "0::nat"
  by standard (auto simp add: Gcd_nat_def Lcm_nat_empty Lcm_nat_infinite)

lemma Lcm_empty_nat:
  "Lcm {} = (1::nat)"
  by (fact Lcm_empty)

lemma Lcm_insert_nat [simp]:
  "Lcm (insert (n::nat) N) = lcm n (Lcm N)"
  by (fact Lcm_insert)

lemma Lcm_eq_0 [simp]:
  "finite (M::nat set) \<Longrightarrow> 0 \<in> M \<Longrightarrow> Lcm M = 0"
  by (rule Lcm_eq_0_I)

lemma Lcm0_iff [simp]:
  fixes M :: "nat set"
  assumes "finite M" and "M \<noteq> {}"
  shows "Lcm M = 0 \<longleftrightarrow> 0 \<in> M"
  using assms by (simp add: Lcm_0_iff)

text\<open>Alternative characterizations of Gcd:\<close>

lemma Gcd_eq_Max: "finite(M::nat set) \<Longrightarrow> M \<noteq> {} \<Longrightarrow> 0 \<notin> M \<Longrightarrow> Gcd M = Max(\<Inter>m\<in>M. {d. d dvd m})"
apply(rule antisym)
 apply(rule Max_ge)
  apply (metis all_not_in_conv finite_divisors_nat finite_INT)
 apply (simp add: Gcd_dvd)
apply (rule Max_le_iff[THEN iffD2])
  apply (metis all_not_in_conv finite_divisors_nat finite_INT)
 apply fastforce
apply clarsimp
apply (metis Gcd_dvd Max_in dvd_0_left dvd_Gcd dvd_imp_le linorder_antisym_conv3 not_less0)
done

lemma Gcd_remove0_nat: "finite M \<Longrightarrow> Gcd M = Gcd (M - {0::nat})"
apply(induct pred:finite)
 apply simp
apply(case_tac "x=0")
 apply simp
apply(subgoal_tac "insert x F - {0} = insert x (F - {0})")
 apply simp
apply blast
done

lemma Lcm_in_lcm_closed_set_nat:
  "finite M \<Longrightarrow> M \<noteq> {} \<Longrightarrow> ALL m n :: nat. m:M \<longrightarrow> n:M \<longrightarrow> lcm m n : M \<Longrightarrow> Lcm M : M"
apply(induct rule:finite_linorder_min_induct)
 apply simp
apply simp
apply(subgoal_tac "ALL m n :: nat. m:A \<longrightarrow> n:A \<longrightarrow> lcm m n : A")
 apply simp
 apply(case_tac "A={}")
  apply simp
 apply simp
apply (metis lcm_pos_nat lcm_unique_nat linorder_neq_iff nat_dvd_not_less not_less0)
done

lemma Lcm_eq_Max_nat:
  "finite M \<Longrightarrow> M \<noteq> {} \<Longrightarrow> 0 \<notin> M \<Longrightarrow> ALL m n :: nat. m:M \<longrightarrow> n:M \<longrightarrow> lcm m n : M \<Longrightarrow> Lcm M = Max M"
apply(rule antisym)
 apply(rule Max_ge, assumption)
 apply(erule (2) Lcm_in_lcm_closed_set_nat)
apply clarsimp
apply (metis Lcm0_iff dvd_Lcm_nat dvd_imp_le neq0_conv)
done

lemma Lcm_set_nat [code, code_unfold]:
  "Lcm (set ns) = fold lcm ns (1::nat)"
  by (fact gcd_lcm_complete_lattice_nat.Sup_set_fold)

lemma Gcd_set_nat [code]:
  "Gcd (set ns) = fold gcd ns (0::nat)"
  by (fact gcd_lcm_complete_lattice_nat.Inf_set_fold)

lemma mult_inj_if_coprime_nat:
  "inj_on f A \<Longrightarrow> inj_on g B \<Longrightarrow> ALL a:A. ALL b:B. coprime (f a) (g b)
   \<Longrightarrow> inj_on (%(a,b). f a * g b::nat) (A \<times> B)"
  by (auto simp add: inj_on_def coprime_crossproduct_nat simp del: One_nat_def)

text\<open>Nitpick:\<close>

lemma gcd_eq_nitpick_gcd [nitpick_unfold]: "gcd x y = Nitpick.nat_gcd x y"
by (induct x y rule: nat_gcd.induct)
   (simp add: gcd_nat.simps Nitpick.nat_gcd.simps)

lemma lcm_eq_nitpick_lcm [nitpick_unfold]: "lcm x y = Nitpick.nat_lcm x y"
by (simp only: lcm_nat_def Nitpick.nat_lcm_def gcd_eq_nitpick_gcd)


subsubsection \<open>Setwise gcd and lcm for integers\<close>

instantiation int :: Gcd
begin

definition
  "Lcm M = int (Lcm (nat ` abs ` M))"

definition
  "Gcd M = int (Gcd (nat ` abs ` M))"

instance ..

end

instance int :: semiring_Gcd
  by standard (auto intro!: Gcd_dvd Gcd_greatest simp add: Gcd_int_def Lcm_int_def int_dvd_iff dvd_int_iff
    dvd_int_unfold_dvd_nat [symmetric])

instance int :: semiring_Lcm
proof
  fix K :: "int set"
  have "{n. \<forall>k\<in>K. nat \<bar>k\<bar> dvd n} = ((\<lambda>k. nat \<bar>k\<bar>) ` {l. \<forall>k\<in>K. k dvd l})"
  proof (rule set_eqI)
    fix n
    have "(\<forall>k\<in>K. nat \<bar>k\<bar> dvd n) \<longleftrightarrow> (\<exists>l. (\<forall>k\<in>K. k dvd l) \<and> n = nat \<bar>l\<bar>)" (is "?P \<longleftrightarrow> ?Q")
    proof
      assume ?P
      then have "(\<forall>k\<in>K. k dvd int n) \<and> n = nat \<bar>int n\<bar>"
        by (auto simp add: dvd_int_unfold_dvd_nat)
      then show ?Q by blast
    next
      assume ?Q then show ?P
        by (auto simp add: dvd_int_unfold_dvd_nat)
    qed
    then show "n \<in> {n. \<forall>k\<in>K. nat \<bar>k\<bar> dvd n} \<longleftrightarrow> n \<in> (\<lambda>k. nat \<bar>k\<bar>) ` {l. \<forall>k\<in>K. k dvd l}"
      by auto
  qed
  then show "Lcm K = Gcd {l. \<forall>k\<in>K. k dvd l}"
    by (simp add: Gcd_int_def Lcm_int_def Lcm_Gcd)
qed

lemma Lcm_empty_int [simp]: "Lcm {} = (1::int)"
  by (fact Lcm_empty)

lemma Lcm_insert_int [simp]:
  "Lcm (insert (n::int) N) = lcm n (Lcm N)"
  by (fact Lcm_insert)

lemma dvd_int_iff: "x dvd y \<longleftrightarrow> nat \<bar>x\<bar> dvd nat \<bar>y\<bar>"
  by (fact dvd_int_unfold_dvd_nat)

lemma dvd_Lcm_int [simp]:
  fixes M :: "int set" assumes "m \<in> M" shows "m dvd Lcm M"
  using assms by (fact dvd_Lcm)

lemma Lcm_dvd_int [simp]:
  fixes M :: "int set"
  assumes "\<forall>m\<in>M. m dvd n" shows "Lcm M dvd n"
  using assms by (simp add: Lcm_int_def dvd_int_iff)

lemma Lcm_set_int [code, code_unfold]:
  "Lcm (set xs) = fold lcm xs (1::int)"
  by (induct xs rule: rev_induct) (simp_all add: lcm_commute_int)

lemma Gcd_set_int [code]:
  "Gcd (set xs) = fold gcd xs (0::int)"
  by (induct xs rule: rev_induct) (simp_all add: gcd_commute_int)


text \<open>Fact aliasses\<close>

lemmas gcd_dvd1_nat = gcd_dvd1 [where ?'a = nat]
  and gcd_dvd2_nat = gcd_dvd2 [where ?'a = nat]
  and gcd_greatest_nat = gcd_greatest [where ?'a = nat]

lemmas gcd_dvd1_int = gcd_dvd1 [where ?'a = int]
  and gcd_dvd2_int = gcd_dvd2 [where ?'a = int]
  and gcd_greatest_int = gcd_greatest [where ?'a = int]

lemmas Gcd_dvd_nat [simp] = Gcd_dvd [where ?'a = nat]
  and dvd_Gcd_nat [simp] = dvd_Gcd [where ?'a = nat]

lemmas Gcd_dvd_int [simp] = Gcd_dvd [where ?'a = int]
  and dvd_Gcd_int [simp] = dvd_Gcd [where ?'a = int]

lemmas Gcd_empty_nat = Gcd_empty [where ?'a = nat]
  and Gcd_insert_nat = Gcd_insert [where ?'a = nat]

lemmas Gcd_empty_int = Gcd_empty [where ?'a = int]
  and Gcd_insert_int = Gcd_insert [where ?'a = int]

subsection \<open>gcd and lcm instances for @{typ integer}\<close>

instantiation integer :: gcd begin
context includes integer.lifting begin
lift_definition gcd_integer :: "integer \<Rightarrow> integer \<Rightarrow> integer" is gcd .
lift_definition lcm_integer :: "integer \<Rightarrow> integer \<Rightarrow> integer" is lcm .
end
instance ..
end
lifting_update integer.lifting
lifting_forget integer.lifting

context includes integer.lifting begin

lemma gcd_code_integer [code]:
  "gcd k l = \<bar>if l = (0::integer) then k else gcd l (\<bar>k\<bar> mod \<bar>l\<bar>)\<bar>"
by transfer(fact gcd_code_int)

lemma lcm_code_integer [code]: "lcm (a::integer) b = \<bar>a\<bar> * \<bar>b\<bar> div gcd a b"
by transfer(fact lcm_altdef_int)

end

code_printing constant "gcd :: integer \<Rightarrow> _"
  \<rightharpoonup> (OCaml) "Big'_int.gcd'_big'_int"
  and (Haskell) "Prelude.gcd"
  and (Scala) "_.gcd'((_)')"
  \<comment> \<open>There is no gcd operation in the SML standard library, so no code setup for SML\<close>

end