author  haftmann 
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parent 30960  fec1a04b7220 
child 30997  081e825c2218 
permissions  rwrr 
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(* Title: HOL/Power.thy 
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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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15131  8 
theory Power 
21413  9 
imports Nat 
15131  10 
begin 
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subsection {* Powers for Arbitrary Monoids *} 
13 

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class power = one + times 
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begin 
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primrec power :: "'a \<Rightarrow> nat \<Rightarrow> 'a" (infixr "^" 80) where 
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power_0: "a ^ 0 = 1" 

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 power_Suc: "a ^ Suc n = a * a ^ n" 

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notation (latex output) 
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power ("(_\<^bsup>_\<^esup>)" [1000] 1000) 

23 

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notation (HTML output) 

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power ("(_\<^bsup>_\<^esup>)" [1000] 1000) 

26 

30960  27 
end 
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context monoid_mult 
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begin 

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subclass power .. 
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lemma power_one [simp]: 
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"1 ^ n = 1" 

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by (induct n) simp_all 
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lemma power_one_right [simp]: 
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"a ^ 1 = a * 1" 

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by simp 

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lemma power_commutes: 
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"a ^ n * a = a * a ^ n" 

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by (induct n) (simp_all add: mult_assoc) 
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lemma power_Suc2: 
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"a ^ Suc n = a ^ n * a" 

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by (simp add: power_commutes) 
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lemma power_add: 
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"a ^ (m + n) = a ^ m * a ^ n" 

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by (induct m) (simp_all add: algebra_simps) 

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lemma power_mult: 
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"a ^ (m * n) = (a ^ m) ^ n" 

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by (induct n) (simp_all add: power_add) 
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end 
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context comm_monoid_mult 

61 
begin 

62 

63 
lemma power_mult_distrib: 

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"(a * b) ^ n = (a ^ n) * (b ^ n)" 

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by (induct n) (simp_all add: mult_ac) 
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end 
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context semiring_1 

70 
begin 

71 

72 
lemma of_nat_power: 

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"of_nat (m ^ n) = of_nat m ^ n" 

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by (induct n) (simp_all add: of_nat_mult) 

75 

76 
end 

77 

78 
context comm_semiring_1 

79 
begin 

80 

81 
text {* The divides relation *} 

82 

83 
lemma le_imp_power_dvd: 

84 
assumes "m \<le> n" shows "a ^ m dvd a ^ n" 

85 
proof 

86 
have "a ^ n = a ^ (m + (n  m))" 

87 
using `m \<le> n` by simp 

88 
also have "\<dots> = a ^ m * a ^ (n  m)" 

89 
by (rule power_add) 

90 
finally show "a ^ n = a ^ m * a ^ (n  m)" . 

91 
qed 

92 

93 
lemma power_le_dvd: 

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"a ^ n dvd b \<Longrightarrow> m \<le> n \<Longrightarrow> a ^ m dvd b" 

95 
by (rule dvd_trans [OF le_imp_power_dvd]) 

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97 
lemma dvd_power_same: 

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"x dvd y \<Longrightarrow> x ^ n dvd y ^ n" 

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by (induct n) (auto simp add: mult_dvd_mono) 

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101 
lemma dvd_power_le: 

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"x dvd y \<Longrightarrow> m \<ge> n \<Longrightarrow> x ^ n dvd y ^ m" 

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by (rule power_le_dvd [OF dvd_power_same]) 

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lemma dvd_power [simp]: 
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assumes "n > (0::nat) \<or> x = 1" 

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shows "x dvd (x ^ n)" 

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using assms proof 

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assume "0 < n" 

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then have "x ^ n = x ^ Suc (n  1)" by simp 

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then show "x dvd (x ^ n)" by simp 

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next 

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assume "x = 1" 

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then show "x dvd (x ^ n)" by simp 

115 
qed 

116 

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end 

118 

119 
context ring_1 

120 
begin 

121 

122 
lemma power_minus: 

123 
"( a) ^ n = ( 1) ^ n * a ^ n" 

124 
proof (induct n) 

125 
case 0 show ?case by simp 

126 
next 

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case (Suc n) then show ?case 

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by (simp del: power_Suc add: power_Suc2 mult_assoc) 

129 
qed 

130 

131 
end 

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133 
context ordered_semidom 

134 
begin 

135 

136 
lemma zero_less_power [simp]: 

137 
"0 < a \<Longrightarrow> 0 < a ^ n" 

138 
by (induct n) (simp_all add: mult_pos_pos) 

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140 
lemma zero_le_power [simp]: 

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"0 \<le> a \<Longrightarrow> 0 \<le> a ^ n" 

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by (induct n) (simp_all add: mult_nonneg_nonneg) 

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lemma one_le_power[simp]: 
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"1 \<le> a \<Longrightarrow> 1 \<le> a ^ n" 
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apply (induct n) 

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apply simp_all 

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apply (rule order_trans [OF _ mult_mono [of 1 _ 1]]) 

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apply (simp_all add: order_trans [OF zero_le_one]) 

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done 

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lemma power_gt1_lemma: 
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assumes gt1: "1 < a" 
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shows "1 < a * a ^ n" 

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proof  
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from gt1 have "0 \<le> a" 
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by (fact order_trans [OF zero_le_one less_imp_le]) 

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have "1 * 1 < a * 1" using gt1 by simp 

159 
also have "\<dots> \<le> a * a ^ n" using gt1 

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by (simp only: mult_mono `0 \<le> a` 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 \<Longrightarrow> 1 < a ^ Suc n" 

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by (simp add: power_gt1_lemma) 

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lemma one_less_power [simp]: 
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"1 < a \<Longrightarrow> 0 < n \<Longrightarrow> 1 < a ^ n" 

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by (cases n) (simp_all add: power_gt1_lemma) 

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lemma power_le_imp_le_exp: 
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assumes gt1: "1 < a" 
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shows "a ^ m \<le> a ^ n \<Longrightarrow> m \<le> n" 

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proof (induct m arbitrary: n) 

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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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with Suc.prems Suc.hyps have "a * a ^ m \<le> 1" by simp 
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with gt1 show ?thesis 
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by (force simp only: power_gt1_lemma 

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not_less [symmetric]) 
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next 
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case (Suc n) 

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with Suc.prems Suc.hyps show ?thesis 
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by (force dest: mult_left_le_imp_le 
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simp add: 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 \<Longrightarrow> a ^ m = a ^ n \<longleftrightarrow> 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 \<Longrightarrow> a ^ m < a ^ n \<Longrightarrow> m < n" 
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by (simp add: order_less_le [of m n] less_le [of "a^m" "a^n"] 

206 
power_le_imp_le_exp) 

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lemma power_mono: 
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"a \<le> b \<Longrightarrow> 0 \<le> a \<Longrightarrow> a ^ n \<le> b ^ n" 
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by (induct n) 

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(auto intro: mult_mono order_trans [of 0 a b]) 

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lemma power_strict_mono [rule_format]: 
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"a < b \<Longrightarrow> 0 \<le> a \<Longrightarrow> 0 < n \<longrightarrow> a ^ n < b ^ n" 
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by (induct n) 

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(auto simp add: mult_strict_mono le_less_trans [of 0 a b]) 

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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 \<Longrightarrow> a < 1 \<Longrightarrow> a * a ^ n < a ^ n" 
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by (induct n) 

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(auto simp add: mult_strict_left_mono) 

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lemma power_strict_decreasing [rule_format]: 
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"n < N \<Longrightarrow> 0 < a \<Longrightarrow> a < 1 \<longrightarrow> a ^ N < a ^ n" 

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proof (induct N) 

227 
case 0 then show ?case by simp 

228 
next 

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case (Suc N) then show ?case 

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apply (auto simp add: power_Suc_less less_Suc_eq) 

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apply (subgoal_tac "a * a^N < 1 * a^n") 

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apply simp 

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apply (rule mult_strict_mono) apply auto 

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done 

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qed 

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text{*Proof resembles that of @{text power_strict_decreasing}*} 
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lemma power_decreasing [rule_format]: 
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"n \<le> N \<Longrightarrow> 0 \<le> a \<Longrightarrow> a \<le> 1 \<longrightarrow> a ^ N \<le> a ^ n" 

240 
proof (induct N) 

241 
case 0 then show ?case by simp 

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next 

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case (Suc N) then show ?case 

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apply (auto simp add: le_Suc_eq) 

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apply (subgoal_tac "a * a^N \<le> 1 * a^n", simp) 

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apply (rule mult_mono) apply auto 

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done 

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qed 

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lemma power_Suc_less_one: 
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"0 < a \<Longrightarrow> a < 1 \<Longrightarrow> a ^ Suc n < 1" 
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using power_strict_decreasing [of 0 "Suc n" a] by simp 

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text{*Proof again resembles that of @{text power_strict_decreasing}*} 
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lemma power_increasing [rule_format]: 
256 
"n \<le> N \<Longrightarrow> 1 \<le> a \<Longrightarrow> a ^ n \<le> a ^ N" 

257 
proof (induct N) 

258 
case 0 then show ?case by simp 

259 
next 

260 
case (Suc N) then show ?case 

261 
apply (auto simp add: le_Suc_eq) 

262 
apply (subgoal_tac "1 * a^n \<le> a * a^N", simp) 

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apply (rule mult_mono) apply (auto simp add: order_trans [OF zero_le_one]) 

264 
done 

265 
qed 

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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 \<Longrightarrow> a ^ n < a * a ^ n" 
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by (induct n) (auto simp add: mult_strict_left_mono less_trans [OF zero_less_one]) 

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lemma power_strict_increasing [rule_format]: 
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"n < N \<Longrightarrow> 1 < a \<longrightarrow> a ^ n < a ^ N" 

274 
proof (induct N) 

275 
case 0 then show ?case by simp 

276 
next 

277 
case (Suc N) then show ?case 

278 
apply (auto simp add: power_less_power_Suc less_Suc_eq) 

279 
apply (subgoal_tac "1 * a^n < a * a^N", simp) 

280 
apply (rule mult_strict_mono) apply (auto simp add: less_trans [OF zero_less_one] less_imp_le) 

281 
done 

282 
qed 

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lemma power_increasing_iff [simp]: 
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"1 < b \<Longrightarrow> b ^ x \<le> b ^ y \<longleftrightarrow> x \<le> y" 
286 
by (blast intro: power_le_imp_le_exp power_increasing less_imp_le) 

15066  287 

288 
lemma power_strict_increasing_iff [simp]: 

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"1 < b \<Longrightarrow> b ^ x < b ^ y \<longleftrightarrow> 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" 
294 
and ynonneg: "0 \<le> b" 

295 
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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22853  305 
lemma power_less_imp_less_base: 
306 
assumes less: "a ^ n < b ^ n" 

307 
assumes nonneg: "0 \<le> b" 

308 
shows "a < b" 

309 
proof (rule contrapos_pp [OF less]) 

310 
assume "~ a < b" 

311 
hence "b \<le> a" by (simp only: linorder_not_less) 

312 
hence "b ^ n \<le> a ^ n" using nonneg by (rule power_mono) 

30996  313 
thus "\<not> a ^ n < b ^ n" by (simp only: linorder_not_less) 
22853  314 
qed 
315 

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lemma power_inject_base: 
30996  317 
"a ^ Suc n = b ^ Suc n \<Longrightarrow> 0 \<le> a \<Longrightarrow> 0 \<le> b \<Longrightarrow> a = b" 
318 
by (blast intro: power_le_imp_le_base antisym eq_refl sym) 

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22955  320 
lemma power_eq_imp_eq_base: 
30996  321 
"a ^ n = b ^ n \<Longrightarrow> 0 \<le> a \<Longrightarrow> 0 \<le> b \<Longrightarrow> 0 < n \<Longrightarrow> a = b" 
322 
by (cases n) (simp_all del: power_Suc, rule power_inject_base) 

22955  323 

30996  324 
end 
325 

326 
context ordered_idom 

327 
begin 

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30996  329 
lemma power_abs: 
330 
"abs (a ^ n) = abs a ^ n" 

331 
by (induct n) (auto simp add: abs_mult) 

332 

333 
lemma abs_power_minus [simp]: 

334 
"abs ((a) ^ n) = abs (a ^ n)" 

335 
by (simp add: abs_minus_cancel power_abs) 

336 

337 
lemma zero_less_power_abs_iff [simp, noatp]: 

338 
"0 < abs a ^ n \<longleftrightarrow> a \<noteq> 0 \<or> n = 0" 

339 
proof (induct n) 

340 
case 0 show ?case by simp 

341 
next 

342 
case (Suc n) show ?case by (auto simp add: Suc zero_less_mult_iff) 

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qed 
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30996  345 
lemma zero_le_power_abs [simp]: 
346 
"0 \<le> abs a ^ n" 

347 
by (rule zero_le_power [OF abs_ge_zero]) 

348 

349 
end 

350 

351 
context ring_1_no_zero_divisors 

352 
begin 

353 

354 
lemma field_power_not_zero: 

355 
"a \<noteq> 0 \<Longrightarrow> a ^ n \<noteq> 0" 

356 
by (induct n) auto 

357 

358 
end 

359 

360 
context division_ring 

361 
begin 

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30996  363 
text {* FIXME reorient or rename to nonzero_inverse_power *} 
364 
lemma nonzero_power_inverse: 

365 
"a \<noteq> 0 \<Longrightarrow> inverse (a ^ n) = (inverse a) ^ n" 

366 
by (induct n) 

367 
(simp_all add: nonzero_inverse_mult_distrib power_commutes field_power_not_zero) 

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30996  369 
end 
370 

371 
context field 

372 
begin 

373 

374 
lemma nonzero_power_divide: 

375 
"b \<noteq> 0 \<Longrightarrow> (a / b) ^ n = a ^ n / b ^ n" 

376 
by (simp add: divide_inverse power_mult_distrib nonzero_power_inverse) 

377 

378 
end 

379 

380 
lemma power_0_Suc [simp]: 

381 
"(0::'a::{power, semiring_0}) ^ Suc n = 0" 

382 
by simp 

30313  383 

30996  384 
text{*It looks plausible as a simprule, but its effect can be strange.*} 
385 
lemma power_0_left: 

386 
"0 ^ n = (if n = 0 then 1 else (0::'a::{power, semiring_0}))" 

387 
by (induct n) simp_all 

388 

389 
lemma power_eq_0_iff [simp]: 

390 
"a ^ n = 0 \<longleftrightarrow> 

391 
a = (0::'a::{mult_zero,zero_neq_one,no_zero_divisors,power}) \<and> n \<noteq> 0" 

392 
by (induct n) 

393 
(auto simp add: no_zero_divisors elim: contrapos_pp) 

394 

395 
lemma power_diff: 

396 
fixes a :: "'a::field" 

397 
assumes nz: "a \<noteq> 0" 

398 
shows "n \<le> m \<Longrightarrow> a ^ (m  n) = a ^ m / a ^ n" 

399 
by (induct m n rule: diff_induct) (simp_all add: nz) 

30313  400 

30996  401 
text{*Perhaps these should be simprules.*} 
402 
lemma power_inverse: 

403 
fixes a :: "'a::{division_ring,division_by_zero,power}" 

404 
shows "inverse (a ^ n) = (inverse a) ^ n" 

405 
apply (cases "a = 0") 

406 
apply (simp add: power_0_left) 

407 
apply (simp add: nonzero_power_inverse) 

408 
done (* TODO: reorient or rename to inverse_power *) 

409 

410 
lemma power_one_over: 

411 
"1 / (a::'a::{field,division_by_zero, power}) ^ n = (1 / a) ^ n" 

412 
by (simp add: divide_inverse) (rule power_inverse) 

413 

414 
lemma power_divide: 

415 
"(a / b) ^ n = (a::'a::{field,division_by_zero}) ^ n / b ^ n" 

416 
apply (cases "b = 0") 

417 
apply (simp add: power_0_left) 

418 
apply (rule nonzero_power_divide) 

419 
apply assumption 

30313  420 
done 
421 

30996  422 
class recpower = monoid_mult 
423 

30313  424 

30960  425 
subsection {* Exponentiation for the Natural Numbers *} 
14577  426 

30960  427 
instance nat :: recpower .. 
25836  428 

30996  429 
lemma nat_one_le_power [simp]: 
430 
"Suc 0 \<le> i \<Longrightarrow> Suc 0 \<le> i ^ n" 

431 
by (rule one_le_power [of i n, unfolded One_nat_def]) 

23305  432 

30996  433 
lemma nat_zero_less_power_iff [simp]: 
434 
"x ^ n > 0 \<longleftrightarrow> x > (0::nat) \<or> n = 0" 

435 
by (induct n) auto 

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30056  437 
lemma nat_power_eq_Suc_0_iff [simp]: 
30996  438 
"x ^ m = Suc 0 \<longleftrightarrow> m = 0 \<or> x = Suc 0" 
439 
by (induct m) auto 

30056  440 

30996  441 
lemma power_Suc_0 [simp]: 
442 
"Suc 0 ^ n = Suc 0" 

443 
by simp 

30056  444 

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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"}.*} 
21413  448 
lemma nat_power_less_imp_less: 
449 
assumes nonneg: "0 < (i\<Colon>nat)" 

30996  450 
assumes less: "i ^ m < i ^ n" 
21413  451 
shows "m < n" 
452 
proof (cases "i = 1") 

453 
case True with less power_one [where 'a = nat] show ?thesis by simp 

454 
next 

455 
case False with nonneg have "1 < i" by auto 

456 
from power_strict_increasing_iff [OF this] less show ?thesis .. 

457 
qed 

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458 

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end 