src/HOL/Parity.thy
author wenzelm
Mon Aug 31 21:28:08 2015 +0200 (2015-08-31)
changeset 61070 b72a990adfe2
parent 60867 86e7560e07d0
child 61531 ab2e862263e7
permissions -rw-r--r--
prefer symbols;
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(*  Title:      HOL/Parity.thy
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    Author:     Jeremy Avigad
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    Author:     Jacques D. Fleuriot
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*)
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section \<open>Parity in rings and semirings\<close>
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theory Parity
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imports Nat_Transfer
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begin
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subsection \<open>Ring structures with parity and @{text even}/@{text odd} predicates\<close>
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class semiring_parity = comm_semiring_1_cancel + numeral +
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  assumes odd_one [simp]: "\<not> 2 dvd 1"
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  assumes odd_even_add: "\<not> 2 dvd a \<Longrightarrow> \<not> 2 dvd b \<Longrightarrow> 2 dvd a + b"
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  assumes even_multD: "2 dvd a * b \<Longrightarrow> 2 dvd a \<or> 2 dvd b"
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  assumes odd_ex_decrement: "\<not> 2 dvd a \<Longrightarrow> \<exists>b. a = b + 1"
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begin
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subclass semiring_numeral ..
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abbreviation even :: "'a \<Rightarrow> bool"
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where
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  "even a \<equiv> 2 dvd a"
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abbreviation odd :: "'a \<Rightarrow> bool"
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where
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  "odd a \<equiv> \<not> 2 dvd a"
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lemma even_zero [simp]:
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  "even 0"
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  by (fact dvd_0_right)
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lemma even_plus_one_iff [simp]:
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  "even (a + 1) \<longleftrightarrow> odd a"
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  by (auto simp add: dvd_add_right_iff intro: odd_even_add)
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lemma evenE [elim?]:
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  assumes "even a"
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  obtains b where "a = 2 * b"
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  using assms by (rule dvdE)
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lemma oddE [elim?]:
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  assumes "odd a"
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  obtains b where "a = 2 * b + 1"
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proof -
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  from assms obtain b where *: "a = b + 1"
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    by (blast dest: odd_ex_decrement)
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  with assms have "even (b + 2)" by simp
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  then have "even b" by simp
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  then obtain c where "b = 2 * c" ..
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  with * have "a = 2 * c + 1" by simp
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  with that show thesis .
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qed
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lemma even_times_iff [simp]:
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  "even (a * b) \<longleftrightarrow> even a \<or> even b"
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  by (auto dest: even_multD)
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lemma even_numeral [simp]:
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  "even (numeral (Num.Bit0 n))"
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proof -
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  have "even (2 * numeral n)"
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    unfolding even_times_iff by simp
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  then have "even (numeral n + numeral n)"
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    unfolding mult_2 .
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  then show ?thesis
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    unfolding numeral.simps .
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qed
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lemma odd_numeral [simp]:
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  "odd (numeral (Num.Bit1 n))"
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proof
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  assume "even (numeral (num.Bit1 n))"
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  then have "even (numeral n + numeral n + 1)"
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    unfolding numeral.simps .
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  then have "even (2 * numeral n + 1)"
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    unfolding mult_2 .
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  then have "2 dvd numeral n * 2 + 1"
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    by (simp add: ac_simps)
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  with dvd_add_times_triv_left_iff [of 2 "numeral n" 1]
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    have "2 dvd 1"
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    by simp
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  then show False by simp
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qed
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lemma even_add [simp]:
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  "even (a + b) \<longleftrightarrow> (even a \<longleftrightarrow> even b)"
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  by (auto simp add: dvd_add_right_iff dvd_add_left_iff odd_even_add)
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lemma odd_add [simp]:
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  "odd (a + b) \<longleftrightarrow> (\<not> (odd a \<longleftrightarrow> odd b))"
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  by simp
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lemma even_power [simp]:
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  "even (a ^ n) \<longleftrightarrow> even a \<and> n > 0"
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  by (induct n) auto
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end
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class ring_parity = ring + semiring_parity
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begin
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subclass comm_ring_1 ..
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lemma even_minus [simp]:
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  "even (- a) \<longleftrightarrow> even a"
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  by (fact dvd_minus_iff)
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lemma even_diff [simp]:
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  "even (a - b) \<longleftrightarrow> even (a + b)"
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  using even_add [of a "- b"] by simp
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end
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subsection \<open>Instances for @{typ nat} and @{typ int}\<close>
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lemma even_Suc_Suc_iff [simp]:
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  "2 dvd Suc (Suc n) \<longleftrightarrow> 2 dvd n"
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  using dvd_add_triv_right_iff [of 2 n] by simp
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lemma even_Suc [simp]:
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  "2 dvd Suc n \<longleftrightarrow> \<not> 2 dvd n"
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  by (induct n) auto
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lemma even_diff_nat [simp]:
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  fixes m n :: nat
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  shows "2 dvd (m - n) \<longleftrightarrow> m < n \<or> 2 dvd (m + n)"
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proof (cases "n \<le> m")
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  case True
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  then have "m - n + n * 2 = m + n" by (simp add: mult_2_right)
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  moreover have "2 dvd (m - n) \<longleftrightarrow> 2 dvd (m - n + n * 2)" by simp
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  ultimately have "2 dvd (m - n) \<longleftrightarrow> 2 dvd (m + n)" by (simp only:)
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  then show ?thesis by auto
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next
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  case False
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  then show ?thesis by simp
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qed 
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instance nat :: semiring_parity
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proof
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  show "\<not> 2 dvd (1 :: nat)"
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    by (rule notI, erule dvdE) simp
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next
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  fix m n :: nat
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  assume "\<not> 2 dvd m"
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  moreover assume "\<not> 2 dvd n"
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  ultimately have *: "2 dvd Suc m \<and> 2 dvd Suc n"
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    by simp
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  then have "2 dvd (Suc m + Suc n)"
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    by (blast intro: dvd_add)
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  also have "Suc m + Suc n = m + n + 2"
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    by simp
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  finally show "2 dvd (m + n)"
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    using dvd_add_triv_right_iff [of 2 "m + n"] by simp
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next
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  fix m n :: nat
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  assume *: "2 dvd (m * n)"
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  show "2 dvd m \<or> 2 dvd n"
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  proof (rule disjCI)
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    assume "\<not> 2 dvd n"
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    then have "2 dvd (Suc n)" by simp
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    then obtain r where "Suc n = 2 * r" ..
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    moreover from * obtain s where "m * n = 2 * s" ..
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    then have "2 * s + m = m * Suc n" by simp
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    ultimately have " 2 * s + m = 2 * (m * r)" by (simp add: algebra_simps)
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    then have "m = 2 * (m * r - s)" by simp
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    then show "2 dvd m" ..
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  qed
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next
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  fix n :: nat
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  assume "\<not> 2 dvd n"
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  then show "\<exists>m. n = m + 1"
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    by (cases n) simp_all
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qed
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lemma odd_pos: 
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  "odd (n :: nat) \<Longrightarrow> 0 < n"
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  by (auto elim: oddE)
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lemma even_diff_iff [simp]:
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  fixes k l :: int
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  shows "2 dvd (k - l) \<longleftrightarrow> 2 dvd (k + l)"
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  using dvd_add_times_triv_right_iff [of 2 "k - l" l] by (simp add: mult_2_right)
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lemma even_abs_add_iff [simp]:
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  fixes k l :: int
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  shows "2 dvd (\<bar>k\<bar> + l) \<longleftrightarrow> 2 dvd (k + l)"
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  by (cases "k \<ge> 0") (simp_all add: ac_simps)
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lemma even_add_abs_iff [simp]:
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  fixes k l :: int
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  shows "2 dvd (k + \<bar>l\<bar>) \<longleftrightarrow> 2 dvd (k + l)"
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  using even_abs_add_iff [of l k] by (simp add: ac_simps)
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lemma odd_Suc_minus_one [simp]:
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  "odd n \<Longrightarrow> Suc (n - Suc 0) = n"
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  by (auto elim: oddE)
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instance int :: ring_parity
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proof
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  show "\<not> 2 dvd (1 :: int)" by (simp add: dvd_int_unfold_dvd_nat)
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  fix k l :: int
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  assume "\<not> 2 dvd k"
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  moreover assume "\<not> 2 dvd l"
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  ultimately have "2 dvd (nat \<bar>k\<bar> + nat \<bar>l\<bar>)" 
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    by (auto simp add: dvd_int_unfold_dvd_nat intro: odd_even_add)
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  then have "2 dvd (\<bar>k\<bar> + \<bar>l\<bar>)"
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    by (simp add: dvd_int_unfold_dvd_nat nat_add_distrib)
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  then show "2 dvd (k + l)"
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    by simp
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next
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  fix k l :: int
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  assume "2 dvd (k * l)"
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  then show "2 dvd k \<or> 2 dvd l"
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    by (simp add: dvd_int_unfold_dvd_nat even_multD nat_abs_mult_distrib)
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next
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  fix k :: int
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  have "k = (k - 1) + 1" by simp
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  then show "\<exists>l. k = l + 1" ..
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qed
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lemma even_int_iff [simp]:
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  "even (int n) \<longleftrightarrow> even n"
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  by (simp add: dvd_int_iff)
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lemma even_nat_iff:
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  "0 \<le> k \<Longrightarrow> even (nat k) \<longleftrightarrow> even k"
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  by (simp add: even_int_iff [symmetric])
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subsection \<open>Parity and powers\<close>
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context comm_ring_1
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begin
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lemma power_minus_even [simp]:
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  "even n \<Longrightarrow> (- a) ^ n = a ^ n"
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  by (auto elim: evenE)
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lemma power_minus_odd [simp]:
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  "odd n \<Longrightarrow> (- a) ^ n = - (a ^ n)"
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  by (auto elim: oddE)
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lemma neg_one_even_power [simp]:
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  "even n \<Longrightarrow> (- 1) ^ n = 1"
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  by simp
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lemma neg_one_odd_power [simp]:
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  "odd n \<Longrightarrow> (- 1) ^ n = - 1"
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  by simp
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end  
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context linordered_idom
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begin
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lemma zero_le_even_power:
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  "even n \<Longrightarrow> 0 \<le> a ^ n"
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  by (auto elim: evenE)
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lemma zero_le_odd_power:
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  "odd n \<Longrightarrow> 0 \<le> a ^ n \<longleftrightarrow> 0 \<le> a"
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  by (auto simp add: power_even_eq zero_le_mult_iff elim: oddE)
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lemma zero_le_power_eq:
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  "0 \<le> a ^ n \<longleftrightarrow> even n \<or> odd n \<and> 0 \<le> a"
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  by (auto simp add: zero_le_even_power zero_le_odd_power)
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lemma zero_less_power_eq:
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  "0 < a ^ n \<longleftrightarrow> n = 0 \<or> even n \<and> a \<noteq> 0 \<or> odd n \<and> 0 < a"
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proof -
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  have [simp]: "0 = a ^ n \<longleftrightarrow> a = 0 \<and> n > 0"
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    unfolding power_eq_0_iff [of a n, symmetric] by blast
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  show ?thesis
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  unfolding less_le zero_le_power_eq by auto
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qed
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lemma power_less_zero_eq [simp]:
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  "a ^ n < 0 \<longleftrightarrow> odd n \<and> a < 0"
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  unfolding not_le [symmetric] zero_le_power_eq by auto
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lemma power_le_zero_eq:
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  "a ^ n \<le> 0 \<longleftrightarrow> n > 0 \<and> (odd n \<and> a \<le> 0 \<or> even n \<and> a = 0)"
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  unfolding not_less [symmetric] zero_less_power_eq by auto 
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lemma power_even_abs:
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  "even n \<Longrightarrow> \<bar>a\<bar> ^ n = a ^ n"
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  using power_abs [of a n] by (simp add: zero_le_even_power)
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lemma power_mono_even:
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  assumes "even n" and "\<bar>a\<bar> \<le> \<bar>b\<bar>"
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  shows "a ^ n \<le> b ^ n"
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proof -
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  have "0 \<le> \<bar>a\<bar>" by auto
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  with \<open>\<bar>a\<bar> \<le> \<bar>b\<bar>\<close>
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  have "\<bar>a\<bar> ^ n \<le> \<bar>b\<bar> ^ n" by (rule power_mono)
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  with \<open>even n\<close> show ?thesis by (simp add: power_even_abs)  
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qed
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lemma power_mono_odd:
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  assumes "odd n" and "a \<le> b"
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  shows "a ^ n \<le> b ^ n"
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proof (cases "b < 0")
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  case True with \<open>a \<le> b\<close> have "- b \<le> - a" and "0 \<le> - b" by auto
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  hence "(- b) ^ n \<le> (- a) ^ n" by (rule power_mono)
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  with \<open>odd n\<close> show ?thesis by simp
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next
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  case False then have "0 \<le> b" by auto
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  show ?thesis
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  proof (cases "a < 0")
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    case True then have "n \<noteq> 0" and "a \<le> 0" using \<open>odd n\<close> [THEN odd_pos] by auto
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    then have "a ^ n \<le> 0" unfolding power_le_zero_eq using \<open>odd n\<close> by auto
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    moreover
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    from \<open>0 \<le> b\<close> have "0 \<le> b ^ n" by auto
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    ultimately show ?thesis by auto
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  next
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    case False then have "0 \<le> a" by auto
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    with \<open>a \<le> b\<close> show ?thesis using power_mono by auto
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  qed
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qed
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text \<open>Simplify, when the exponent is a numeral\<close>
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lemma zero_le_power_eq_numeral [simp]:
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  "0 \<le> a ^ numeral w \<longleftrightarrow> even (numeral w :: nat) \<or> odd (numeral w :: nat) \<and> 0 \<le> a"
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  by (fact zero_le_power_eq)
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lemma zero_less_power_eq_numeral [simp]:
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  "0 < a ^ numeral w \<longleftrightarrow> numeral w = (0 :: nat)
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    \<or> even (numeral w :: nat) \<and> a \<noteq> 0 \<or> odd (numeral w :: nat) \<and> 0 < a"
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  by (fact zero_less_power_eq)
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lemma power_le_zero_eq_numeral [simp]:
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  "a ^ numeral w \<le> 0 \<longleftrightarrow> (0 :: nat) < numeral w
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    \<and> (odd (numeral w :: nat) \<and> a \<le> 0 \<or> even (numeral w :: nat) \<and> a = 0)"
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  by (fact power_le_zero_eq)
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lemma power_less_zero_eq_numeral [simp]:
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  "a ^ numeral w < 0 \<longleftrightarrow> odd (numeral w :: nat) \<and> a < 0"
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  by (fact power_less_zero_eq)
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lemma power_even_abs_numeral [simp]:
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  "even (numeral w :: nat) \<Longrightarrow> \<bar>a\<bar> ^ numeral w = a ^ numeral w"
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  by (fact power_even_abs)
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end
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subsubsection \<open>Tools setup\<close>
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declare transfer_morphism_int_nat [transfer add return:
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  even_int_iff
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]
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end