src/HOL/Power.thy
author huffman
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(*  Title:      HOL/Power.thy
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    ID:         $Id$
0c7625196d95 New theory "Power" of exponentiation (and binomial coefficients)
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   1997  University of Cambridge
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*)
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header{*Exponentiation*}
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theory Power
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imports Nat
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begin
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subsection{*Powers for Arbitrary Monoids*}
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class recpower = monoid_mult + power +
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  assumes power_0 [simp]: "a \<^loc>^ 0       = \<^loc>1"
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  assumes power_Suc:      "a \<^loc>^ Suc n = a \<^loc>* (a \<^loc>^ n)"
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lemma power_0_Suc [simp]: "(0::'a::{recpower,semiring_0}) ^ (Suc n) = 0"
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by (simp add: power_Suc)
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text{*It looks plausible as a simprule, but its effect can be strange.*}
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lemma power_0_left: "0^n = (if n=0 then 1 else (0::'a::{recpower,semiring_0}))"
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by (induct "n", auto)
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lemma power_one [simp]: "1^n = (1::'a::recpower)"
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apply (induct "n")
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apply (auto simp add: power_Suc)
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done
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lemma power_one_right [simp]: "(a::'a::recpower) ^ 1 = a"
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by (simp add: power_Suc)
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lemma power_commutes: "(a::'a::recpower) ^ n * a = a * a ^ n"
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by (induct "n") (simp_all add:power_Suc mult_assoc)
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lemma power_add: "(a::'a::recpower) ^ (m+n) = (a^m) * (a^n)"
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apply (induct "m")
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apply (simp_all add: power_Suc mult_ac)
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done
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lemma power_mult: "(a::'a::recpower) ^ (m*n) = (a^m) ^ n"
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apply (induct "n") 
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apply (simp_all add: power_Suc power_add)
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done
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lemma power_mult_distrib: "((a::'a::{recpower,comm_monoid_mult}) * b) ^ n = (a^n) * (b^n)"
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apply (induct "n")
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apply (auto simp add: power_Suc mult_ac)
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done
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lemma zero_less_power:
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     "0 < (a::'a::{ordered_semidom,recpower}) ==> 0 < a^n"
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apply (induct "n")
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apply (simp_all add: power_Suc zero_less_one mult_pos_pos)
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done
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lemma zero_le_power:
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     "0 \<le> (a::'a::{ordered_semidom,recpower}) ==> 0 \<le> a^n"
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apply (simp add: order_le_less)
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apply (erule disjE)
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apply (simp_all add: zero_less_power zero_less_one power_0_left)
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done
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lemma one_le_power:
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     "1 \<le> (a::'a::{ordered_semidom,recpower}) ==> 1 \<le> a^n"
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apply (induct "n")
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apply (simp_all add: power_Suc)
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apply (rule order_trans [OF _ mult_mono [of 1 _ 1]])
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apply (simp_all add: zero_le_one order_trans [OF zero_le_one])
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done
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lemma gt1_imp_ge0: "1 < a ==> 0 \<le> (a::'a::ordered_semidom)"
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  by (simp add: order_trans [OF zero_le_one order_less_imp_le])
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lemma power_gt1_lemma:
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  assumes gt1: "1 < (a::'a::{ordered_semidom,recpower})"
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  shows "1 < a * a^n"
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proof -
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  have "1*1 < a*1" using gt1 by simp
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  also have "\<dots> \<le> a * a^n" using gt1
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    by (simp only: mult_mono gt1_imp_ge0 one_le_power order_less_imp_le
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        zero_le_one order_refl)
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  finally show ?thesis by simp
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qed
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lemma power_gt1:
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     "1 < (a::'a::{ordered_semidom,recpower}) ==> 1 < a ^ (Suc n)"
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by (simp add: power_gt1_lemma power_Suc)
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lemma power_le_imp_le_exp:
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  assumes gt1: "(1::'a::{recpower,ordered_semidom}) < a"
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  shows "!!n. a^m \<le> a^n ==> m \<le> n"
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proof (induct m)
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  case 0
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  show ?case by simp
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next
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  case (Suc m)
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  show ?case
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  proof (cases n)
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    case 0
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    from prems have "a * a^m \<le> 1" by (simp add: power_Suc)
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    with gt1 show ?thesis
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      by (force simp only: power_gt1_lemma
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          linorder_not_less [symmetric])
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  next
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    case (Suc n)
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    from prems show ?thesis
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      by (force dest: mult_left_le_imp_le
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          simp add: power_Suc order_less_trans [OF zero_less_one gt1])
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  qed
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qed
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text{*Surely we can strengthen this? It holds for @{text "0<a<1"} too.*}
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lemma power_inject_exp [simp]:
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     "1 < (a::'a::{ordered_semidom,recpower}) ==> (a^m = a^n) = (m=n)"
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  by (force simp add: order_antisym power_le_imp_le_exp)
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text{*Can relax the first premise to @{term "0<a"} in the case of the
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natural numbers.*}
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lemma power_less_imp_less_exp:
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     "[| (1::'a::{recpower,ordered_semidom}) < a; a^m < a^n |] ==> m < n"
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by (simp add: order_less_le [of m n] order_less_le [of "a^m" "a^n"]
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              power_le_imp_le_exp)
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lemma power_mono:
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     "[|a \<le> b; (0::'a::{recpower,ordered_semidom}) \<le> a|] ==> a^n \<le> b^n"
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apply (induct "n")
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apply (simp_all add: power_Suc)
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apply (auto intro: mult_mono zero_le_power order_trans [of 0 a b])
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done
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lemma power_strict_mono [rule_format]:
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     "[|a < b; (0::'a::{recpower,ordered_semidom}) \<le> a|]
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      ==> 0 < n --> a^n < b^n"
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apply (induct "n")
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apply (auto simp add: mult_strict_mono zero_le_power power_Suc
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                      order_le_less_trans [of 0 a b])
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done
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lemma power_eq_0_iff [simp]:
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     "(a^n = 0) = (a = (0::'a::{ordered_idom,recpower}) & 0<n)"
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apply (induct "n")
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apply (auto simp add: power_Suc zero_neq_one [THEN not_sym])
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done
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lemma field_power_eq_0_iff [simp]:
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     "(a^n = 0) = (a = (0::'a::{field,recpower}) & 0<n)"
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apply (induct "n")
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apply (auto simp add: power_Suc field_mult_eq_0_iff zero_neq_one[THEN not_sym])
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done
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lemma field_power_not_zero: "a \<noteq> (0::'a::{field,recpower}) ==> a^n \<noteq> 0"
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by force
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lemma nonzero_power_inverse:
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  "a \<noteq> 0 ==> inverse ((a::'a::{field,recpower}) ^ n) = (inverse a) ^ n"
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apply (induct "n")
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apply (auto simp add: power_Suc nonzero_inverse_mult_distrib mult_commute)
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done
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text{*Perhaps these should be simprules.*}
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lemma power_inverse:
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  "inverse ((a::'a::{field,division_by_zero,recpower}) ^ n) = (inverse a) ^ n"
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apply (induct "n")
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apply (auto simp add: power_Suc inverse_mult_distrib)
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done
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lemma power_one_over: "1 / (a::'a::{field,division_by_zero,recpower})^n = 
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    (1 / a)^n"
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apply (simp add: divide_inverse)
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apply (rule power_inverse)
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done
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lemma nonzero_power_divide:
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    "b \<noteq> 0 ==> (a/b) ^ n = ((a::'a::{field,recpower}) ^ n) / (b ^ n)"
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by (simp add: divide_inverse power_mult_distrib nonzero_power_inverse)
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lemma power_divide:
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    "(a/b) ^ n = ((a::'a::{field,division_by_zero,recpower}) ^ n / b ^ n)"
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apply (case_tac "b=0", simp add: power_0_left)
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apply (rule nonzero_power_divide)
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apply assumption
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done
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lemma power_abs: "abs(a ^ n) = abs(a::'a::{ordered_idom,recpower}) ^ n"
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apply (induct "n")
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apply (auto simp add: power_Suc abs_mult)
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done
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lemma zero_less_power_abs_iff [simp]:
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     "(0 < (abs a)^n) = (a \<noteq> (0::'a::{ordered_idom,recpower}) | n=0)"
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proof (induct "n")
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  case 0
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    show ?case by (simp add: zero_less_one)
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next
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  case (Suc n)
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    show ?case by (force simp add: prems power_Suc zero_less_mult_iff)
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qed
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lemma zero_le_power_abs [simp]:
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     "(0::'a::{ordered_idom,recpower}) \<le> (abs a)^n"
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apply (induct "n")
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apply (auto simp add: zero_le_one zero_le_power)
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done
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lemma power_minus: "(-a) ^ n = (- 1)^n * (a::'a::{comm_ring_1,recpower}) ^ n"
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proof -
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  have "-a = (- 1) * a"  by (simp add: minus_mult_left [symmetric])
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  thus ?thesis by (simp only: power_mult_distrib)
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qed
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text{*Lemma for @{text power_strict_decreasing}*}
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lemma power_Suc_less:
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     "[|(0::'a::{ordered_semidom,recpower}) < a; a < 1|]
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      ==> a * a^n < a^n"
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apply (induct n)
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apply (auto simp add: power_Suc mult_strict_left_mono)
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done
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lemma power_strict_decreasing:
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     "[|n < N; 0 < a; a < (1::'a::{ordered_semidom,recpower})|]
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      ==> a^N < a^n"
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apply (erule rev_mp)
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apply (induct "N")
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apply (auto simp add: power_Suc power_Suc_less less_Suc_eq)
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apply (rename_tac m)
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apply (subgoal_tac "a * a^m < 1 * a^n", simp)
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apply (rule mult_strict_mono)
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apply (auto simp add: zero_le_power zero_less_one order_less_imp_le)
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done
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text{*Proof resembles that of @{text power_strict_decreasing}*}
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lemma power_decreasing:
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     "[|n \<le> N; 0 \<le> a; a \<le> (1::'a::{ordered_semidom,recpower})|]
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      ==> a^N \<le> a^n"
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apply (erule rev_mp)
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apply (induct "N")
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apply (auto simp add: power_Suc  le_Suc_eq)
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apply (rename_tac m)
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apply (subgoal_tac "a * a^m \<le> 1 * a^n", simp)
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apply (rule mult_mono)
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apply (auto simp add: zero_le_power zero_le_one)
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done
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lemma power_Suc_less_one:
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     "[| 0 < a; a < (1::'a::{ordered_semidom,recpower}) |] ==> a ^ Suc n < 1"
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apply (insert power_strict_decreasing [of 0 "Suc n" a], simp)
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done
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text{*Proof again resembles that of @{text power_strict_decreasing}*}
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lemma power_increasing:
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     "[|n \<le> N; (1::'a::{ordered_semidom,recpower}) \<le> a|] ==> a^n \<le> a^N"
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apply (erule rev_mp)
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apply (induct "N")
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apply (auto simp add: power_Suc le_Suc_eq)
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apply (rename_tac m)
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apply (subgoal_tac "1 * a^n \<le> a * a^m", simp)
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apply (rule mult_mono)
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apply (auto simp add: order_trans [OF zero_le_one] zero_le_power)
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done
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text{*Lemma for @{text power_strict_increasing}*}
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lemma power_less_power_Suc:
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     "(1::'a::{ordered_semidom,recpower}) < a ==> a^n < a * a^n"
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apply (induct n)
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apply (auto simp add: power_Suc mult_strict_left_mono order_less_trans [OF zero_less_one])
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done
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lemma power_strict_increasing:
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     "[|n < N; (1::'a::{ordered_semidom,recpower}) < a|] ==> a^n < a^N"
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apply (erule rev_mp)
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apply (induct "N")
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apply (auto simp add: power_less_power_Suc power_Suc less_Suc_eq)
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apply (rename_tac m)
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apply (subgoal_tac "1 * a^n < a * a^m", simp)
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apply (rule mult_strict_mono)
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apply (auto simp add: order_less_trans [OF zero_less_one] zero_le_power
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                 order_less_imp_le)
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done
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lemma power_increasing_iff [simp]: 
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     "1 < (b::'a::{ordered_semidom,recpower}) ==> (b ^ x \<le> b ^ y) = (x \<le> y)"
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  by (blast intro: power_le_imp_le_exp power_increasing order_less_imp_le) 
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lemma power_strict_increasing_iff [simp]:
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     "1 < (b::'a::{ordered_semidom,recpower}) ==> (b ^ x < b ^ y) = (x < y)"
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  by (blast intro: power_less_imp_less_exp power_strict_increasing) 
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lemma power_le_imp_le_base:
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  assumes le: "a ^ Suc n \<le> b ^ Suc n"
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      and ynonneg: "(0::'a::{ordered_semidom,recpower}) \<le> b"
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  shows "a \<le> b"
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 proof (rule ccontr)
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   assume "~ a \<le> b"
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   then have "b < a" by (simp only: linorder_not_le)
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   then have "b ^ Suc n < a ^ Suc n"
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     by (simp only: prems power_strict_mono)
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   from le and this show "False"
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      by (simp add: linorder_not_less [symmetric])
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 qed
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lemma power_inject_base:
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     "[| a ^ Suc n = b ^ Suc n; 0 \<le> a; 0 \<le> b |]
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      ==> a = (b::'a::{ordered_semidom,recpower})"
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by (blast intro: power_le_imp_le_base order_antisym order_eq_refl sym)
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subsection{*Exponentiation for the Natural Numbers*}
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instance nat :: power ..
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primrec (power)
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  "p ^ 0 = 1"
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  "p ^ (Suc n) = (p::nat) * (p ^ n)"
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instance nat :: recpower
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proof
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  fix z n :: nat
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  show "z^0 = 1" by simp
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  show "z^(Suc n) = z * (z^n)" by simp
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qed
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lemma nat_one_le_power [simp]: "1 \<le> i ==> Suc 0 \<le> i^n"
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by (insert one_le_power [of i n], simp)
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lemma nat_zero_less_power_iff [simp]: "(0 < x^n) = (x \<noteq> (0::nat) | n=0)"
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by (induct "n", auto)
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text{*Valid for the naturals, but what if @{text"0<i<1"}?
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Premises cannot be weakened: consider the case where @{term "i=0"},
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@{term "m=1"} and @{term "n=0"}.*}
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lemma nat_power_less_imp_less:
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  assumes nonneg: "0 < (i\<Colon>nat)"
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  assumes less: "i^m < i^n"
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  shows "m < n"
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proof (cases "i = 1")
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  case True with less power_one [where 'a = nat] show ?thesis by simp
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next
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  case False with nonneg have "1 < i" by auto
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  from power_strict_increasing_iff [OF this] less show ?thesis ..
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qed
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lemma power_diff:
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  assumes nz: "a ~= 0"
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  shows "n <= m ==> (a::'a::{recpower, field}) ^ (m-n) = (a^m) / (a^n)"
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  by (induct m n rule: diff_induct)
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    (simp_all add: power_Suc nonzero_mult_divide_cancel_left nz)
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text{*ML bindings for the general exponentiation theorems*}
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ML
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{*
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val power_0 = thm"power_0";
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val power_Suc = thm"power_Suc";
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val power_0_Suc = thm"power_0_Suc";
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val power_0_left = thm"power_0_left";
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val power_one = thm"power_one";
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val power_one_right = thm"power_one_right";
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val power_add = thm"power_add";
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val power_mult = thm"power_mult";
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val power_mult_distrib = thm"power_mult_distrib";
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val zero_less_power = thm"zero_less_power";
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val zero_le_power = thm"zero_le_power";
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val one_le_power = thm"one_le_power";
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val gt1_imp_ge0 = thm"gt1_imp_ge0";
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val power_gt1_lemma = thm"power_gt1_lemma";
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val power_gt1 = thm"power_gt1";
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val power_le_imp_le_exp = thm"power_le_imp_le_exp";
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val power_inject_exp = thm"power_inject_exp";
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val power_less_imp_less_exp = thm"power_less_imp_less_exp";
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val power_mono = thm"power_mono";
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val power_strict_mono = thm"power_strict_mono";
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val power_eq_0_iff = thm"power_eq_0_iff";
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val field_power_eq_0_iff = thm"field_power_eq_0_iff";
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val field_power_not_zero = thm"field_power_not_zero";
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val power_inverse = thm"power_inverse";
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val nonzero_power_divide = thm"nonzero_power_divide";
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val power_divide = thm"power_divide";
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val power_abs = thm"power_abs";
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val zero_less_power_abs_iff = thm"zero_less_power_abs_iff";
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val zero_le_power_abs = thm "zero_le_power_abs";
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val power_minus = thm"power_minus";
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val power_Suc_less = thm"power_Suc_less";
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val power_strict_decreasing = thm"power_strict_decreasing";
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val power_decreasing = thm"power_decreasing";
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val power_Suc_less_one = thm"power_Suc_less_one";
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val power_increasing = thm"power_increasing";
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val power_strict_increasing = thm"power_strict_increasing";
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val power_le_imp_le_base = thm"power_le_imp_le_base";
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val power_inject_base = thm"power_inject_base";
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*}
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text{*ML bindings for the remaining theorems*}
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ML
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{*
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val nat_one_le_power = thm"nat_one_le_power";
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val nat_power_less_imp_less = thm"nat_power_less_imp_less";
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val nat_zero_less_power_iff = thm"nat_zero_less_power_iff";
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*}
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end
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