src/HOL/GCD.thy

more robust caret painting wrt. surrogate characters;
discontinued glyphvector drawing -- less special cases;

(*  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.
*)


header {* Greatest common divisor and least common multiple *}

theory GCD
imports Fact Parity
begin

declare One_nat_def [simp del]

subsection {* GCD and LCM definitions *}

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 == (gcd x y = 1)"

end

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 (abs x)) (nat (abs y)))"

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

instance proof qed

end


subsection {* Transfer setup *}

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 {* GCD properties *}

(* 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 abs_gcd_int[simp]: "abs(gcd (x::int) y) = gcd x y"
by(simp add: gcd_int_def)

lemma gcd_abs_int: "gcd (x::int) y = gcd (abs x) (abs y)"
by (simp add: gcd_int_def)

lemma gcd_abs1_int[simp]: "gcd (abs x) (y::int) = gcd x y"
by (metis abs_idempotent gcd_abs_int)

lemma gcd_abs2_int[simp]: "gcd x (abs y::int) = 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 (abs x) (abs y)"
  by (simp add: lcm_int_def)

lemma abs_lcm_int [simp]: "abs (lcm i j::int) = lcm i j"
by(simp add:lcm_int_def)

lemma lcm_abs1_int[simp]: "lcm (abs x) (y::int) = lcm x y"
by (metis abs_idempotent lcm_int_def)

lemma lcm_abs2_int[simp]: "lcm x (abs y::int) = 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 [simp]: "gcd (x::nat) 0 = x"
  by simp

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

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

lemma gcd_0_left_int [simp]: "gcd 0 (x::int) = abs x"
  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 add: One_nat_def)

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 = abs x"
by (auto simp add: gcd_int_def)

declare gcd_nat.simps [simp del]

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

lemma gcd_dvd1_nat [iff]: "(gcd (m::nat)) n dvd m"
  and gcd_dvd2_nat [iff]: "(gcd m n) dvd n"
  apply (induct m n rule: gcd_nat_induct)
  apply (simp_all add: gcd_non_0_nat)
  apply (blast dest: dvd_mod_imp_dvd)
done

lemma gcd_dvd1_int [iff]: "gcd (x::int) y dvd x"
by (metis gcd_int_def int_dvd_iff gcd_dvd1_nat)

lemma gcd_dvd2_int [iff]: "gcd (x::int) y dvd y"
by (metis gcd_int_def int_dvd_iff gcd_dvd2_nat)

lemma dvd_gcd_D1_nat: "k dvd gcd m n \<Longrightarrow> (k::nat) dvd m"
by(metis gcd_dvd1_nat dvd_trans)

lemma dvd_gcd_D2_nat: "k dvd gcd m n \<Longrightarrow> (k::nat) dvd n"
by(metis gcd_dvd2_nat dvd_trans)

lemma dvd_gcd_D1_int: "i dvd gcd m n \<Longrightarrow> (i::int) dvd m"
by(metis gcd_dvd1_int dvd_trans)

lemma dvd_gcd_D2_int: "i dvd gcd m n \<Longrightarrow> (i::int) dvd n"
by(metis gcd_dvd2_int 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_nat: "(k::nat) dvd m \<Longrightarrow> k dvd n \<Longrightarrow> k dvd gcd m n"
by (induct m n rule: gcd_nat_induct) (simp_all add: gcd_non_0_nat dvd_mod)

lemma gcd_greatest_int:
  "(k::int) dvd m \<Longrightarrow> k dvd n \<Longrightarrow> k dvd gcd m n"
  apply (subst gcd_abs_int)
  apply (subst abs_dvd_iff [symmetric])
  apply (rule gcd_greatest_nat [transferred])
  apply auto
done

lemma gcd_greatest_iff_nat [iff]: "(k dvd gcd (m::nat) n) =
    (k dvd m & k dvd n)"
  by (blast intro!: gcd_greatest_nat intro: dvd_trans)

lemma gcd_greatest_iff_int: "((k::int) dvd gcd m n) = (k dvd m & k dvd n)"
  by (blast intro!: gcd_greatest_int intro: dvd_trans)

lemma gcd_zero_nat [simp]: "(gcd (m::nat) n = 0) = (m = 0 & n = 0)"
  by (simp only: dvd_0_left_iff [symmetric] gcd_greatest_iff_nat)

lemma gcd_zero_int [simp]: "(gcd (m::int) n = 0) = (m = 0 & n = 0)"
  by (auto simp add: gcd_int_def)

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)

interpretation gcd_nat: abel_semigroup "gcd :: nat \<Rightarrow> nat \<Rightarrow> nat"
proof
qed (auto intro: dvd_antisym dvd_trans)

interpretation gcd_int: abel_semigroup "gcd :: int \<Rightarrow> int \<Rightarrow> int"
proof
qed (simp_all add: gcd_int_def gcd_nat.assoc gcd_nat.commute gcd_nat.left_commute)

lemmas gcd_assoc_nat = gcd_nat.assoc
lemmas gcd_commute_nat = gcd_nat.commute
lemmas gcd_left_commute_nat = gcd_nat.left_commute
lemmas gcd_assoc_int = gcd_int.assoc
lemmas gcd_commute_int = gcd_int.commute
lemmas gcd_left_commute_int = gcd_int.left_commute

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_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_nat)
  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_int)
done

lemma gcd_proj1_if_dvd_nat [simp]: "(x::nat) dvd y \<Longrightarrow> gcd x y = x"
by (metis dvd.eq_iff gcd_unique_nat)

lemma gcd_proj2_if_dvd_nat [simp]: "(y::nat) dvd x \<Longrightarrow> gcd x y = y"
by (metis dvd.eq_iff gcd_unique_nat)

lemma gcd_proj1_if_dvd_int[simp]: "x dvd y \<Longrightarrow> gcd (x::int) y = abs x"
by (metis abs_dvd_iff abs_eq_0 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 = abs y"
by (metis gcd_proj1_if_dvd_int gcd_commute_int)


text {*
  \medskip Multiplication laws
*}

lemma gcd_mult_distrib_nat: "(k::nat) * gcd m n = gcd (k * m) (k * n)"
    -- {* \cite[page 27]{davenport92} *}
  apply (induct m n rule: gcd_nat_induct)
  apply simp
  apply (case_tac "k = 0")
  apply (simp_all add: mod_geq gcd_non_0_nat mod_mult_distrib2)
done

lemma gcd_mult_distrib_int: "abs (k::int) * 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

lemma coprime_dvd_mult_nat: "coprime (k::nat) n \<Longrightarrow> k dvd m * n \<Longrightarrow> k dvd m"
  apply (insert gcd_mult_distrib_nat [of m k n])
  apply simp
  apply (erule_tac t = m in ssubst)
  apply simp
  done

lemma coprime_dvd_mult_int:
  "coprime (k::int) n \<Longrightarrow> k dvd m * n \<Longrightarrow> k dvd m"
apply (subst abs_dvd_iff [symmetric])
apply (subst dvd_abs_iff [symmetric])
apply (subst (asm) gcd_abs_int)
apply (rule coprime_dvd_mult_nat [transferred])
    prefer 4 apply assumption
   apply auto
apply (subst abs_mult [symmetric], auto)
done

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)

lemma gcd_mult_cancel_nat: "coprime k n \<Longrightarrow> gcd ((k::nat) * m) n = gcd m n"
  apply (rule dvd_antisym)
  apply (rule gcd_greatest_nat)
  apply (rule_tac n = k in coprime_dvd_mult_nat)
  apply (simp add: gcd_assoc_nat)
  apply (simp add: gcd_commute_nat)
  apply (simp_all add: mult_commute)
done

lemma gcd_mult_cancel_int:
  "coprime (k::int) n \<Longrightarrow> gcd (k * m) n = gcd m n"
apply (subst (1 2) gcd_abs_int)
apply (subst abs_mult)
apply (rule gcd_mult_cancel_nat [transferred], auto)
done

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 `?lhs` have "a dvd b * d" by (auto intro: dvdI dest: sym)
  with `coprime a d` have "a dvd b" by (simp add: coprime_dvd_mult_iff_nat)
  from `?lhs` have "b dvd a * c" by (auto intro: dvdI dest: sym)
  with `coprime b c` have "b dvd a" by (simp add: coprime_dvd_mult_iff_nat)
  from `?lhs` have "c dvd d * b" by (auto intro: dvdI dest: sym simp add: mult_commute)
  with `coprime b c` have "c dvd d" by (simp add: coprime_dvd_mult_iff_nat gcd_commute_nat)
  from `?lhs` have "d dvd c * a" by (auto intro: dvdI dest: sym simp add: mult_commute)
  with `coprime a d` have "d dvd c" by (simp add: coprime_dvd_mult_iff_nat gcd_commute_nat)
  from `a dvd b` `b dvd a` have "a = b" by (rule Nat.dvd.antisym)
  moreover from `c dvd d` `d dvd c` 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 {* \medskip Addition laws *}

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 zadd_commute)

lemma gcd_add2_int [simp]: "gcd m ((m::int) + n) = gcd m n"
by (metis gcd_add1_int gcd_commute_int zadd_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 zadd_commute)


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

(* FIXME remove iff *)
lemma gcd_dvd_prod_nat [iff]: "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(bestsimp intro!:dvd_imp_le)
qed

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

lemma Max_divisors_self_nat[simp]: "n\<noteq>0 \<Longrightarrow> Max{d::nat. d dvd n} = n"
apply(rule antisym)
 apply (fastsimp 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} = abs n"
apply(rule antisym)
 apply(rule Max_le_iff[THEN iffD2])
   apply simp
  apply fastsimp
 apply (metis Collect_def abs_ge_self dvd_imp_le_int mem_def zle_trans)
apply simp
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 {* Coprimality *}

lemma div_gcd_coprime_nat:
  assumes nz: "(a::nat) \<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
  then have "?g * ?a' = (?g * ?g') * ka'" "?g * ?b' = (?g * ?g') * kb'"
    by simp_all
  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
  then have gp: "?g > 0" by arith
  from gcd_greatest_nat [OF dvdgg'] have "?g * ?g' dvd ?g" .
  with dvd_mult_cancel1 [OF gp] show "?g' = 1" by simp
qed

lemma div_gcd_coprime_int:
  assumes nz: "(a::int) \<noteq> 0 \<or> b \<noteq> 0"
  shows "coprime (a div gcd a b) (b div gcd a b)"
apply (subst (1 2 3) gcd_abs_int)
apply (subst (1 2) abs_div)
  apply simp
 apply simp
apply(subst (1 2) abs_gcd_int)
apply (rule div_gcd_coprime_nat [transferred])
using nz apply (auto simp add: gcd_abs_int [symmetric])
done

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 add: One_nat_def)

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
  apply (drule_tac x = "abs e" in exI)
  apply (case_tac "e >= 0")
  apply force
  apply force
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

lemma coprime_mult_nat: assumes da: "coprime (d::nat) a" and db: "coprime d b"
    shows "coprime d (a * b)"
  apply (subst gcd_commute_nat)
  using da apply (subst gcd_mult_cancel_nat)
  apply (subst gcd_commute_nat, assumption)
  apply (subst gcd_commute_nat, rule db)
done

lemma coprime_mult_int: assumes da: "coprime (d::int) a" and db: "coprime d b"
    shows "coprime d (a * b)"
  apply (subst gcd_commute_int)
  using da apply (subst gcd_mult_cancel_int)
  apply (subst gcd_commute_int, assumption)
  apply (subst gcd_commute_int, rule db)
done

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_nat, 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_int, 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_nat, 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_int, 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 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 dvd_div_mult_self)
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)

lemma coprime_exp2_nat [intro]: "coprime (a::nat) b \<Longrightarrow> coprime (a^n) (b^m)"
  apply (rule coprime_exp_nat)
  apply (subst gcd_commute_nat)
  apply (rule coprime_exp_nat)
  apply (subst gcd_commute_nat, assumption)
done

lemma coprime_exp2_int [intro]: "coprime (a::int) b \<Longrightarrow> coprime (a^n) (b^m)"
  apply (rule coprime_exp_int)
  apply (subst gcd_commute_int)
  apply (rule coprime_exp_int)
  apply (subst gcd_commute_int, assumption)
done

lemma gcd_exp_nat: "gcd ((a::nat)^n) (b^n) = (gcd a b)^n"
proof (cases)
  assume "a = 0 & b = 0"
  thus ?thesis by simp
  next assume "~(a = 0 & b = 0)"
  hence "coprime ((a div gcd a b)^n) ((b div gcd a b)^n)"
    by (auto simp:div_gcd_coprime_nat)
  hence "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"
    apply (subst (1 2) mult_commute)
    apply (subst gcd_mult_distrib_nat [symmetric])
    apply simp
    done
  also have "(a div gcd a b)^n * (gcd a b)^n = a^n"
    apply (subst div_power)
    apply auto
    apply (rule dvd_div_mult_self)
    apply (rule dvd_power_same)
    apply auto
    done
  also have "(b div gcd a b)^n * (gcd a b)^n = b^n"
    apply (subst div_power)
    apply auto
    apply (rule dvd_div_mult_self)
    apply (rule dvd_power_same)
    apply auto
    done
  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: mult_assoc)
    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)
    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_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"
  apply (subgoal_tac "gcd (n + 1) n dvd (n + 1 - n)")
  apply force
  apply (rule dvd_diff_nat)
  apply auto
done

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

lemma coprime_plus_one_int [simp]: "coprime ((n::int) + 1) n"
  apply (subgoal_tac "gcd (n + 1) n dvd (n + 1 - n)")
  apply force
  apply (rule dvd_diff)
  apply auto
done

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: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: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"
  apply (subgoal_tac "x dvd gcd a b")
  apply simp
  apply (erule (1) gcd_greatest_nat)
done

lemma coprime_common_divisor_int: "coprime (a::int) b \<Longrightarrow> x dvd a \<Longrightarrow>
    x dvd b \<Longrightarrow> abs x = 1"
  apply (subgoal_tac "x dvd gcd a b")
  apply simp
  apply (erule (1) gcd_greatest_int)
done

lemma coprime_divisors_nat: "(d::int) dvd a \<Longrightarrow> e dvd b \<Longrightarrow> coprime a b \<Longrightarrow>
    coprime d e"
  apply (auto simp add: dvd_def)
  apply (frule coprime_lmult_int)
  apply (subst gcd_commute_int)
  apply (subst (asm) (2) gcd_commute_int)
  apply (erule coprime_lmult_int)
done

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

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


subsection {* Bezout's theorem *}

(* 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: field_simps zadd_int [symmetric]
        zmult_int [symmetric])
      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 {* versions of Bezout for nat, by Amine Chaieb *}

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 right_distrib)
          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 add_commute mult_ac)
          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 {* LCM properties *}

lemma lcm_altdef_int [code]: "lcm (a::int) b = (abs a) * (abs b) div gcd a b"
  by (simp add: lcm_int_def lcm_nat_def zdiv_int
    zmult_int [symmetric] 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: "abs(m::int) * abs n = 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"
proof (cases k)
  case 0 then show ?thesis by auto
next
  case (Suc _) then have pos_k: "k > 0" by auto
  from assms dvd_pos_nat [OF this] have pos_mn: "m > 0" "n > 0" by auto
  with gcd_zero_nat [of m n] have pos_gcd: "gcd m n > 0" by simp
  from assms obtain p where k_m: "k = m * p" using dvd_def by blast
  from assms obtain q where k_n: "k = n * q" using dvd_def by blast
  from pos_k k_m have pos_p: "p > 0" by auto
  from pos_k k_n have pos_q: "q > 0" by auto
  have "k * k * gcd q p = k * gcd (k * q) (k * p)"
    by (simp add: mult_ac gcd_mult_distrib_nat)
  also have "\<dots> = k * gcd (m * p * q) (n * q * p)"
    by (simp add: k_m [symmetric] k_n [symmetric])
  also have "\<dots> = k * p * q * gcd m n"
    by (simp add: mult_ac gcd_mult_distrib_nat)
  finally have "(m * p) * (n * q) * gcd q p = k * p * q * gcd m n"
    by (simp only: k_m [symmetric] k_n [symmetric])
  then have "p * q * m * n * gcd q p = p * q * k * gcd m n"
    by (simp add: mult_ac)
  with pos_p pos_q have "m * n * gcd q p = k * gcd m n"
    by simp
  with prod_gcd_lcm_nat [of m n]
  have "lcm m n * gcd q p * gcd m n = k * gcd m n"
    by (simp add: mult_ac)
  with pos_gcd have "lcm m n * gcd q p = k" by auto
  then show ?thesis using dvd_def by auto
qed

lemma lcm_least_int:
  "(m::int) dvd k \<Longrightarrow> n dvd k \<Longrightarrow> lcm m n dvd k"
apply (subst lcm_abs_int)
apply (rule dvd_trans)
apply (rule lcm_least_nat [transferred, of _ "abs k" _])
apply auto
done

lemma lcm_dvd1_nat: "(m::nat) dvd lcm m n"
proof (cases m)
  case 0 then show ?thesis by simp
next
  case (Suc _)
  then have mpos: "m > 0" by simp
  show ?thesis
  proof (cases n)
    case 0 then show ?thesis by simp
  next
    case (Suc _)
    then have npos: "n > 0" by simp
    have "gcd m n dvd n" by simp
    then obtain k where "n = gcd m n * k" using dvd_def by auto
    then have "m * n div gcd m n = m * (gcd m n * k) div gcd m n"
      by (simp add: mult_ac)
    also have "\<dots> = m * k" using mpos npos gcd_zero_nat by simp
    finally show ?thesis by (simp add: lcm_nat_def)
  qed
qed

lemma lcm_dvd1_int: "(m::int) dvd lcm m n"
  apply (subst lcm_abs_int)
  apply (rule dvd_trans)
  prefer 2
  apply (rule lcm_dvd1_nat [transferred])
  apply auto
done

lemma lcm_dvd2_nat: "(n::nat) dvd lcm m n"
  using lcm_dvd1_nat [of n m] by (simp only: lcm_nat_def mult.commute gcd_nat.commute)

lemma lcm_dvd2_int: "(n::int) dvd lcm m n"
  using lcm_dvd1_int [of n m] by (simp only: lcm_int_def lcm_nat_def mult.commute gcd_nat.commute)

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"
  by (auto intro: dvd_antisym [transferred] lcm_least_int)

interpretation lcm_nat: abel_semigroup "lcm :: nat \<Rightarrow> nat \<Rightarrow> nat"
proof
  fix n m p :: nat
  show "lcm (lcm n m) p = lcm n (lcm m p)"
    by (rule lcm_unique_nat [THEN iffD1]) (metis dvd.order_trans lcm_unique_nat)
  show "lcm m n = lcm n m"
    by (simp add: lcm_nat_def gcd_commute_nat field_simps)
qed

interpretation lcm_int: abel_semigroup "lcm :: int \<Rightarrow> int \<Rightarrow> int"
proof
  fix n m p :: int
  show "lcm (lcm n m) p = lcm n (lcm m p)"
    by (rule lcm_unique_int [THEN iffD1]) (metis dvd_trans lcm_unique_int)
  show "lcm m n = lcm n m"
    by (simp add: lcm_int_def lcm_nat.commute)
qed

lemmas lcm_assoc_nat = lcm_nat.assoc
lemmas lcm_commute_nat = lcm_nat.commute
lemmas lcm_left_commute_nat = lcm_nat.left_commute
lemmas lcm_assoc_int = lcm_int.assoc
lemmas lcm_commute_int = lcm_int.commute
lemmas lcm_left_commute_int = lcm_int.left_commute

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 = abs y"
  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 = abs y"
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 = abs(m::int) \<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 = abs(n::int) \<longleftrightarrow> m dvd n"
by (metis dvd_abs_iff lcm_proj2_if_dvd_int lcm_unique_int)

lemma comp_fun_idem_gcd_nat: "comp_fun_idem (gcd :: nat\<Rightarrow>nat\<Rightarrow>nat)"
proof qed (auto simp add: gcd_ac_nat)

lemma comp_fun_idem_gcd_int: "comp_fun_idem (gcd :: int\<Rightarrow>int\<Rightarrow>int)"
proof qed (auto simp add: gcd_ac_int)

lemma comp_fun_idem_lcm_nat: "comp_fun_idem (lcm :: nat\<Rightarrow>nat\<Rightarrow>nat)"
proof qed (auto simp add: lcm_ac_nat)

lemma comp_fun_idem_lcm_int: "comp_fun_idem (lcm :: int\<Rightarrow>int\<Rightarrow>int)"
proof qed (auto simp add: lcm_ac_int)


(* FIXME introduce selimattice_bot/top and derive the following lemmas in there: *)

lemma lcm_0_iff_nat[simp]: "lcm (m::nat) n = 0 \<longleftrightarrow> m=0 \<or> n=0"
by (metis lcm_0_left_nat lcm_0_nat mult_is_0 prod_gcd_lcm_nat)

lemma lcm_0_iff_int[simp]: "lcm (m::int) n = 0 \<longleftrightarrow> m=0 \<or> n=0"
by (metis lcm_0_int lcm_0_left_int lcm_pos_int zless_le)

lemma lcm_1_iff_nat[simp]: "lcm (m::nat) n = 1 \<longleftrightarrow> m=1 \<and> n=1"
by (metis gcd_1_nat lcm_unique_nat nat_mult_1 prod_gcd_lcm_nat)

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 simp add: abs_mult_self trans [OF lcm_unique_int eq_commute, symmetric] zmult_eq_1_iff)


subsubsection {* The complete divisibility lattice *}


interpretation gcd_semilattice_nat: semilattice_inf "op dvd" "(%m n::nat. m dvd n & ~ n dvd m)" gcd
proof
  case goal3 thus ?case by(metis gcd_unique_nat)
qed auto

interpretation lcm_semilattice_nat: semilattice_sup "op dvd" "(%m n::nat. m dvd n & ~ n dvd m)" lcm
proof
  case goal3 thus ?case by(metis lcm_unique_nat)
qed auto

interpretation gcd_lcm_lattice_nat: lattice "op dvd" "(%m n::nat. m dvd n & ~ n dvd m)" gcd lcm ..

text{* Lifting gcd and lcm to finite (Gcd/Lcm) and infinite sets (GCD/LCM).
GCD is defined via LCM to facilitate the proof that we have a complete lattice.
Later on we show that GCD and Gcd coincide on finite sets.
*}
context gcd
begin

definition Gcd :: "'a set \<Rightarrow> 'a"
where "Gcd = fold gcd 0"

definition Lcm :: "'a set \<Rightarrow> 'a"
where "Lcm = fold lcm 1"

definition LCM :: "'a set \<Rightarrow> 'a" where
"LCM M = (if finite M then Lcm M else 0)"

definition GCD :: "'a set \<Rightarrow> 'a" where
"GCD M = LCM(INT m:M. {d. d dvd m})"

end

lemma Gcd_empty[simp]: "Gcd {} = 0"
by(simp add:Gcd_def)

lemma Lcm_empty[simp]: "Lcm {} = 1"
by(simp add:Lcm_def)

lemma GCD_empty_nat[simp]: "GCD {} = (0::nat)"
by(simp add:GCD_def LCM_def)

lemma LCM_eq_Lcm[simp]: "finite M \<Longrightarrow> LCM M = Lcm M"
by(simp add:LCM_def)

lemma Lcm_insert_nat [simp]:
  assumes "finite N"
  shows "Lcm (insert (n::nat) N) = lcm n (Lcm N)"
proof -
  interpret comp_fun_idem "lcm::nat\<Rightarrow>nat\<Rightarrow>nat"
    by (rule comp_fun_idem_lcm_nat)
  from assms show ?thesis by(simp add: Lcm_def)
qed

lemma Lcm_insert_int [simp]:
  assumes "finite N"
  shows "Lcm (insert (n::int) N) = lcm n (Lcm N)"
proof -
  interpret comp_fun_idem "lcm::int\<Rightarrow>int\<Rightarrow>int"
    by (rule comp_fun_idem_lcm_int)
  from assms show ?thesis by(simp add: Lcm_def)
qed

lemma Gcd_insert_nat [simp]:
  assumes "finite N"
  shows "Gcd (insert (n::nat) N) = gcd n (Gcd N)"
proof -
  interpret comp_fun_idem "gcd::nat\<Rightarrow>nat\<Rightarrow>nat"
    by (rule comp_fun_idem_gcd_nat)
  from assms show ?thesis by(simp add: Gcd_def)
qed

lemma Gcd_insert_int [simp]:
  assumes "finite N"
  shows "Gcd (insert (n::int) N) = gcd n (Gcd N)"
proof -
  interpret comp_fun_idem "gcd::int\<Rightarrow>int\<Rightarrow>int"
    by (rule comp_fun_idem_gcd_int)
  from assms show ?thesis by(simp add: Gcd_def)
qed

lemma Lcm0_iff[simp]: "finite (M::nat set) \<Longrightarrow> M \<noteq> {} \<Longrightarrow> Lcm M = 0 \<longleftrightarrow> 0 : M"
by(induct rule:finite_ne_induct) auto

lemma Lcm_eq_0[simp]: "finite (M::nat set) \<Longrightarrow> 0 : M \<Longrightarrow> Lcm M = 0"
by (metis Lcm0_iff empty_iff)

lemma Gcd_dvd_nat [simp]:
  assumes "finite M" and "(m::nat) \<in> M"
  shows "Gcd M dvd m"
proof -
  show ?thesis using gcd_semilattice_nat.fold_inf_le_inf[OF assms, of 0] by (simp add: Gcd_def)
qed

lemma dvd_Gcd_nat[simp]:
  assumes "finite M" and "ALL (m::nat) : M. n dvd m"
  shows "n dvd Gcd M"
proof -
  show ?thesis using gcd_semilattice_nat.inf_le_fold_inf[OF assms, of 0] by (simp add: Gcd_def)
qed

lemma dvd_Lcm_nat [simp]:
  assumes "finite M" and "(m::nat) \<in> M"
  shows "m dvd Lcm M"
proof -
  show ?thesis using lcm_semilattice_nat.sup_le_fold_sup[OF assms, of 1] by (simp add: Lcm_def)
qed

lemma Lcm_dvd_nat[simp]:
  assumes "finite M" and "ALL (m::nat) : M. m dvd n"
  shows "Lcm M dvd n"
proof -
  show ?thesis using lcm_semilattice_nat.fold_sup_le_sup[OF assms, of 1] by (simp add: Lcm_def)
qed

interpretation gcd_lcm_complete_lattice_nat:
  complete_lattice GCD LCM "op dvd" "%m n::nat. m dvd n & ~ n dvd m" gcd lcm 1 0
proof
  case goal1 show ?case by simp
next
  case goal2 show ?case by simp
next
  case goal5 thus ?case by (auto simp: LCM_def)
next
  case goal6 thus ?case
    by(auto simp: LCM_def)(metis finite_nat_set_iff_bounded_le gcd_proj2_if_dvd_nat gcd_le1_nat)
next
  case goal3 thus ?case by (auto simp: GCD_def LCM_def)(metis finite_INT finite_divisors_nat)
next
  case goal4 thus ?case by(auto simp: LCM_def GCD_def)
qed

text{* Alternative characterizations of Gcd and GCD: *}

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
apply (rule Max_le_iff[THEN iffD2])
  apply (metis all_not_in_conv finite_divisors_nat finite_INT)
 apply fastsimp
apply clarsimp
apply (metis Gcd_dvd_nat Max_in dvd_0_left dvd_Gcd_nat 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

text{* Finally GCD is Gcd: *}

lemma GCD_eq_Gcd[simp]: assumes "finite(M::nat set)" shows "GCD M = Gcd M"
proof-
  have divisors_remove0_nat: "(\<Inter>m\<in>M. {d::nat. d dvd m}) = (\<Inter>m\<in>M-{0}. {d::nat. d dvd m})" by auto
  show ?thesis
  proof cases
    assume "M={}" thus ?thesis by simp
  next
    assume "M\<noteq>{}"
    show ?thesis
    proof cases
      assume "M={0}" thus ?thesis by(simp add:GCD_def LCM_def)
    next
      assume "M\<noteq>{0}"
      with `M\<noteq>{}` assms show ?thesis
        apply(subst Gcd_remove0_nat[OF assms])
        apply(simp add:GCD_def)
        apply(subst divisors_remove0_nat)
        apply(simp add:LCM_def)
        apply rule
         apply rule
         apply(subst Gcd_eq_Max)
            apply simp
           apply blast
          apply blast
         apply(rule Lcm_eq_Max_nat)
            apply simp
           apply blast
          apply fastsimp
         apply clarsimp
        apply(fastsimp intro: finite_divisors_nat intro!: finite_INT)
        done
    qed
  qed
qed

lemma Lcm_set_nat [code_unfold]:
  "Lcm (set ns) = foldl lcm (1::nat) ns"
proof -
  interpret comp_fun_idem "lcm::nat\<Rightarrow>nat\<Rightarrow>nat" by (rule comp_fun_idem_lcm_nat)
  show ?thesis by(simp add: Lcm_def fold_set lcm_commute_nat)
qed

lemma Lcm_set_int [code_unfold]:
  "Lcm (set is) = foldl lcm (1::int) is"
proof -
  interpret comp_fun_idem "lcm::int\<Rightarrow>int\<Rightarrow>int" by (rule comp_fun_idem_lcm_int)
  show ?thesis by(simp add: Lcm_def fold_set lcm_commute_int)
qed

lemma Gcd_set_nat [code_unfold]:
  "Gcd (set ns) = foldl gcd (0::nat) ns"
proof -
  interpret comp_fun_idem "gcd::nat\<Rightarrow>nat\<Rightarrow>nat" by (rule comp_fun_idem_gcd_nat)
  show ?thesis by(simp add: Gcd_def fold_set gcd_commute_nat)
qed

lemma Gcd_set_int [code_unfold]:
  "Gcd (set ns) = foldl gcd (0::int) ns"
proof -
  interpret comp_fun_idem "gcd::int\<Rightarrow>int\<Rightarrow>int" by (rule comp_fun_idem_gcd_int)
  show ?thesis by(simp add: Gcd_def fold_set gcd_commute_int)
qed


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)"
apply(auto simp add:inj_on_def)
apply (metis coprime_dvd_mult_iff_nat dvd.neq_le_trans dvd_triv_left)
apply (metis gcd_semilattice_nat.inf_commute coprime_dvd_mult_iff_nat
             dvd.neq_le_trans dvd_triv_right mult_commute)
done

text{* Nitpick: *}

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)

end