src/HOL/Library/Nonpos_Ints.thy
author wenzelm
Tue, 05 Jan 2016 21:57:21 +0100
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isabelle update_cartouches -c -t;
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(*  Title:    HOL/Library/Nonpos_Ints.thy
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    Author:   Manuel Eberl, TU M√ľnchen
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*)
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section \<open>Non-positive integers\<close>
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theory Nonpos_Ints
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imports Complex_Main
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begin
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text \<open>
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  The set of non-positive integers on a ring. (in analogy to the set of non-negative
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  integers @{term "\<nat>"}) This is useful e.g. for the Gamma function.
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\<close>
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definition nonpos_Ints ("\<int>\<^sub>\<le>\<^sub>0") where "\<int>\<^sub>\<le>\<^sub>0 = {of_int n |n. n \<le> 0}"
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lemma zero_in_nonpos_Ints [simp,intro]: "0 \<in> \<int>\<^sub>\<le>\<^sub>0"
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  unfolding nonpos_Ints_def by (auto intro!: exI[of _ "0::int"])
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lemma neg_one_in_nonpos_Ints [simp,intro]: "-1 \<in> \<int>\<^sub>\<le>\<^sub>0"
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  unfolding nonpos_Ints_def by (auto intro!: exI[of _ "-1::int"])
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lemma neg_numeral_in_nonpos_Ints [simp,intro]: "-numeral n \<in> \<int>\<^sub>\<le>\<^sub>0"
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  unfolding nonpos_Ints_def by (auto intro!: exI[of _ "-numeral n::int"])
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lemma one_notin_nonpos_Ints [simp]: "(1 :: 'a :: ring_char_0) \<notin> \<int>\<^sub>\<le>\<^sub>0"
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  by (auto simp: nonpos_Ints_def)
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lemma numeral_notin_nonpos_Ints [simp]: "(numeral n :: 'a :: ring_char_0) \<notin> \<int>\<^sub>\<le>\<^sub>0"
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  by (auto simp: nonpos_Ints_def)
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lemma minus_of_nat_in_nonpos_Ints [simp, intro]: "- of_nat n \<in> \<int>\<^sub>\<le>\<^sub>0"
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proof -
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  have "- of_nat n = of_int (-int n)" by simp
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  also have "-int n \<le> 0" by simp
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  hence "of_int (-int n) \<in> \<int>\<^sub>\<le>\<^sub>0" unfolding nonpos_Ints_def by blast
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  finally show ?thesis .
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qed
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lemma of_nat_in_nonpos_Ints_iff: "(of_nat n :: 'a :: {ring_1,ring_char_0}) \<in> \<int>\<^sub>\<le>\<^sub>0 \<longleftrightarrow> n = 0"
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proof
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  assume "(of_nat n :: 'a) \<in> \<int>\<^sub>\<le>\<^sub>0"
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  then obtain m where "of_nat n = (of_int m :: 'a)" "m \<le> 0" by (auto simp: nonpos_Ints_def)
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  hence "(of_int m :: 'a) = of_nat n" by simp
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  also have "... = of_int (int n)" by simp
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  finally have "m = int n" by (subst (asm) of_int_eq_iff)
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  with \<open>m \<le> 0\<close> show "n = 0" by auto
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qed simp
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lemma nonpos_Ints_of_int: "n \<le> 0 \<Longrightarrow> of_int n \<in> \<int>\<^sub>\<le>\<^sub>0"
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  unfolding nonpos_Ints_def by blast
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lemma nonpos_IntsI: 
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  "x \<in> \<int> \<Longrightarrow> x \<le> 0 \<Longrightarrow> (x :: 'a :: linordered_idom) \<in> \<int>\<^sub>\<le>\<^sub>0"
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  using assms unfolding nonpos_Ints_def Ints_def by auto
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lemma nonpos_Ints_subset_Ints: "\<int>\<^sub>\<le>\<^sub>0 \<subseteq> \<int>"
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  unfolding nonpos_Ints_def Ints_def by blast
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lemma nonpos_Ints_nonpos [dest]: "x \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> x \<le> (0 :: 'a :: linordered_idom)"
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  unfolding nonpos_Ints_def by auto
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lemma nonpos_Ints_Int [dest]: "x \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> x \<in> \<int>"
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  unfolding nonpos_Ints_def Ints_def by blast
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lemma nonpos_Ints_cases:
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  assumes "x \<in> \<int>\<^sub>\<le>\<^sub>0"
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  obtains n where "x = of_int n" "n \<le> 0"
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  using assms unfolding nonpos_Ints_def by (auto elim!: Ints_cases)
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lemma nonpos_Ints_cases':
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  assumes "x \<in> \<int>\<^sub>\<le>\<^sub>0"
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  obtains n where "x = -of_nat n"
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proof -
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  from assms obtain m where "x = of_int m" and m: "m \<le> 0" by (auto elim!: nonpos_Ints_cases)
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  hence "x = - of_int (-m)" by auto
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  also from m have "(of_int (-m) :: 'a) = of_nat (nat (-m))" by simp_all
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  finally show ?thesis by (rule that)
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qed
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lemma of_real_in_nonpos_Ints_iff: "(of_real x :: 'a :: real_algebra_1) \<in> \<int>\<^sub>\<le>\<^sub>0 \<longleftrightarrow> x \<in> \<int>\<^sub>\<le>\<^sub>0"
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proof
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  assume "of_real x \<in> (\<int>\<^sub>\<le>\<^sub>0 :: 'a set)"
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  then obtain n where "(of_real x :: 'a) = of_int n" "n \<le> 0" by (erule nonpos_Ints_cases)
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  note \<open>of_real x = of_int n\<close>
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  also have "of_int n = of_real (of_int n)" by (rule of_real_of_int_eq [symmetric])
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  finally have "x = of_int n" by (subst (asm) of_real_eq_iff)
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  with \<open>n \<le> 0\<close> show "x \<in> \<int>\<^sub>\<le>\<^sub>0" by (simp add: nonpos_Ints_of_int)
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qed (auto elim!: nonpos_Ints_cases intro!: nonpos_Ints_of_int)
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lemma nonpos_Ints_altdef: "\<int>\<^sub>\<le>\<^sub>0 = {n \<in> \<int>. (n :: 'a :: linordered_idom) \<le> 0}"
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  by (auto intro!: nonpos_IntsI elim!: nonpos_Ints_cases)
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lemma uminus_in_Nats_iff: "-x \<in> \<nat> \<longleftrightarrow> x \<in> \<int>\<^sub>\<le>\<^sub>0"
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proof
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  assume "-x \<in> \<nat>"
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  then obtain n where "n \<ge> 0" "-x = of_int n" by (auto simp: Nats_altdef1)
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  hence "-n \<le> 0" "x = of_int (-n)" by (simp_all add: eq_commute minus_equation_iff[of x])
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   101
  thus "x \<in> \<int>\<^sub>\<le>\<^sub>0" unfolding nonpos_Ints_def by blast
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   102
next
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  assume "x \<in> \<int>\<^sub>\<le>\<^sub>0"
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   104
  then obtain n where "n \<le> 0" "x = of_int n" by (auto simp: nonpos_Ints_def)
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  hence "-n \<ge> 0" "-x = of_int (-n)" by (simp_all add: eq_commute minus_equation_iff[of x])
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   106
  thus "-x \<in> \<nat>" unfolding Nats_altdef1 by blast
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qed
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   108
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lemma uminus_in_nonpos_Ints_iff: "-x \<in> \<int>\<^sub>\<le>\<^sub>0 \<longleftrightarrow> x \<in> \<nat>"
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  using uminus_in_Nats_iff[of "-x"] by simp
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   111
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lemma nonpos_Ints_mult: "x \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> y \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> x * y \<in> \<nat>"
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   113
  using Nats_mult[of "-x" "-y"] by (simp add: uminus_in_Nats_iff)
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   114
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lemma Nats_mult_nonpos_Ints: "x \<in> \<nat> \<Longrightarrow> y \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> x * y \<in> \<int>\<^sub>\<le>\<^sub>0"
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  using Nats_mult[of x "-y"] by (simp add: uminus_in_Nats_iff)
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   117
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lemma nonpos_Ints_mult_Nats:
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  "x \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> y \<in> \<nat> \<Longrightarrow> x * y \<in> \<int>\<^sub>\<le>\<^sub>0"
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   120
  using Nats_mult[of "-x" y] by (simp add: uminus_in_Nats_iff)
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   121
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lemma nonpos_Ints_add:
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  "x \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> y \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> x + y \<in> \<int>\<^sub>\<le>\<^sub>0"
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  using Nats_add[of "-x" "-y"] uminus_in_Nats_iff[of "y+x", simplified minus_add] 
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   125
  by (simp add: uminus_in_Nats_iff add.commute)
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   126
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   127
lemma nonpos_Ints_diff_Nats:
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   128
  "x \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> y \<in> \<nat> \<Longrightarrow> x - y \<in> \<int>\<^sub>\<le>\<^sub>0"
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   129
  using Nats_add[of "-x" "y"] uminus_in_Nats_iff[of "x-y", simplified minus_add] 
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   130
  by (simp add: uminus_in_Nats_iff add.commute)
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   131
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   132
lemma Nats_diff_nonpos_Ints:
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   133
  "x \<in> \<nat> \<Longrightarrow> y \<in> \<int>\<^sub>\<le>\<^sub>0 \<Longrightarrow> x - y \<in> \<nat>"
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   134
  using Nats_add[of "x" "-y"] by (simp add: uminus_in_Nats_iff add.commute)
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   135
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   136
lemma plus_of_nat_eq_0_imp: "z + of_nat n = 0 \<Longrightarrow> z \<in> \<int>\<^sub>\<le>\<^sub>0"
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   137
proof -
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   138
  assume "z + of_nat n = 0"
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  hence A: "z = - of_nat n" by (simp add: eq_neg_iff_add_eq_0)
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   140
  show "z \<in> \<int>\<^sub>\<le>\<^sub>0" by (subst A) simp
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   141
qed
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   142
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diff changeset
   143
end