author  nipkow 
Fri, 12 Aug 2016 08:20:17 +0200  
changeset 63663  28d1deca302e 
child 63664  9ddc48a8635e 
permissions  rwrr 
63663
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(* Title: HOL/Library/Log_Nat.thy 
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Author: Johannes Hölzl, Fabian Immler 
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Copyright 2012 TU München 
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*) 
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5 

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section \<open>Logarithm of Natural Numbers\<close> 
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theory Log_Nat 
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imports Complex_Main 
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begin 
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11 

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definition floorlog :: "nat \<Rightarrow> nat \<Rightarrow> nat" where 
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"floorlog b a = (if a > 0 \<and> b > 1 then nat \<lfloor>log b a\<rfloor> + 1 else 0)" 
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14 

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lemma floorlog_mono: "x \<le> y \<Longrightarrow> floorlog b x \<le> floorlog b y" 
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by(auto simp: floorlog_def floor_mono nat_mono) 
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17 

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lemma floorlog_bounds: 
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assumes "x > 0" "b > 1" 
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shows "b ^ (floorlog b x  1) \<le> x \<and> x < b ^ (floorlog b x)" 
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proof 
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22 
show "b ^ (floorlog b x  1) \<le> x" 
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23 
proof  
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have "b ^ nat \<lfloor>log b x\<rfloor> = b powr \<lfloor>log b x\<rfloor>" 
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25 
using powr_realpow[symmetric, of b "nat \<lfloor>log b x\<rfloor>"] \<open>x > 0\<close> \<open>b > 1\<close> 
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by simp 
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also have "\<dots> \<le> b powr log b x" using \<open>b > 1\<close> by simp 
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also have "\<dots> = real_of_int x" using \<open>0 < x\<close> \<open>b > 1\<close> by simp 
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finally have "b ^ nat \<lfloor>log b x\<rfloor> \<le> real_of_int x" by simp 
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then show ?thesis 
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using \<open>0 < x\<close> \<open>b > 1\<close> of_nat_le_iff 
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32 
by (fastforce simp add: floorlog_def) 
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33 
qed 
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show "x < b ^ (floorlog b x)" 
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35 
proof  
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have "x \<le> b powr (log b x)" using \<open>x > 0\<close> \<open>b > 1\<close> by simp 
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also have "\<dots> < b powr (\<lfloor>log b x\<rfloor> + 1)" 
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38 
using assms by (intro powr_less_mono) auto 
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also have "\<dots> = b ^ nat (\<lfloor>log b (real_of_int x)\<rfloor> + 1)" 
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using assms by (simp add: powr_realpow[symmetric]) 
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finally 
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have "x < b ^ nat (\<lfloor>log b (int x)\<rfloor> + 1)" 
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43 
by (rule of_nat_less_imp_less) 
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then show ?thesis 
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using \<open>x > 0\<close> \<open>b > 1\<close> by (simp add: floorlog_def nat_add_distrib) 
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qed 
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qed 
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48 

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lemma floorlog_power[simp]: 
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50 
assumes "a > 0" "b > 1" 
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51 
shows "floorlog b (a * b ^ c) = floorlog b a + c" 
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52 
proof  
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53 
have "\<lfloor>log b a + real c\<rfloor> = \<lfloor>log b a\<rfloor> + c" by arith 
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54 
then show ?thesis using assms 
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by (auto simp: floorlog_def log_mult powr_realpow[symmetric] nat_add_distrib) 
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qed 
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57 

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58 
lemma floor_log_add_eqI: 
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59 
fixes a::nat and b::nat and r::real 
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60 
assumes "b > 1" "a \<ge> 1" "0 \<le> r" "r < 1" 
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61 
shows "\<lfloor>log b (a + r)\<rfloor> = \<lfloor>log b a\<rfloor>" 
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62 
proof (rule floor_eq2) 
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63 
have "log b a \<le> log b (a + r)" using assms by force 
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64 
then show "\<lfloor>log b a\<rfloor> \<le> log b (a + r)" by arith 
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65 
next 
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66 
define l::int where "l = int b ^ (nat \<lfloor>log b a\<rfloor> + 1)" 
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67 
have l_def_real: "l = b powr (\<lfloor>log b a\<rfloor> + 1)" 
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68 
using assms by (simp add: l_def powr_add powr_real_of_int) 
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69 
have "a < l" 
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70 
proof  
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71 
have "a = b powr (log b a)" using assms by simp 
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72 
also have "\<dots> < b powr floor ((log b a) + 1)" 
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using assms(1) by auto 
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74 
also have "\<dots> = l" 
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using assms by (simp add: l_def powr_real_of_int powr_add) 
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76 
finally show ?thesis by simp 
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77 
qed 
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78 
then have "a + r < l" using assms by simp 
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79 
then have "log b (a + r) < log b l" using assms by simp 
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also have "\<dots> = real_of_int \<lfloor>log b a\<rfloor> + 1" 
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81 
using assms by (simp add: l_def_real) 
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82 
finally show "log b (a + r) < real_of_int \<lfloor>log b a\<rfloor> + 1" . 
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83 
qed 
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84 

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85 
lemma divide_nat_diff_div_nat_less_one: 
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86 
fixes x b::nat shows "x / b  x div b < 1" 
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87 
proof  
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88 
have "int 0 \<noteq> \<lfloor>1::real\<rfloor>" by simp 
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89 
thus ?thesis 
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90 
by (metis add_diff_cancel_left' floor_divide_of_nat_eq less_eq_real_def 
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91 
mod_div_trivial real_of_nat_div3 real_of_nat_div_aux) 
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92 
qed 
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93 

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94 
lemma floor_log_div: 
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95 
fixes b x :: nat assumes "b > 1" "x > 0" "x div b > 0" 
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96 
shows "\<lfloor>log b x\<rfloor> = \<lfloor>log b (x div b)\<rfloor> + 1" 
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97 
proof 
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98 
have "\<lfloor>log b x\<rfloor> = \<lfloor>log b (x / b * b)\<rfloor>" using assms by simp 
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99 
also have "\<dots> = \<lfloor>log b (x / b) + log b b\<rfloor>" 
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100 
using assms by (subst log_mult) auto 
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101 
also have "\<dots> = \<lfloor>log b (x / b)\<rfloor> + 1" using assms by simp 
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102 
also have "\<lfloor>log b (x / b)\<rfloor> = \<lfloor>log b (x div b + (x / b  x div b))\<rfloor>" by simp 
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103 
also have "\<dots> = \<lfloor>log b (x div b)\<rfloor>" 
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104 
using assms real_of_nat_div4 divide_nat_diff_div_nat_less_one 
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105 
by (intro floor_log_add_eqI) auto 
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106 
finally show ?thesis . 
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107 
qed 
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108 

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109 
lemma compute_floorlog[code]: 
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"floorlog b x = (if x > 0 \<and> b > 1 then floorlog b (x div b) + 1 else 0)" 
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111 
by (auto simp: floorlog_def floor_log_div[of b x] div_eq_0_iff nat_add_distrib 
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112 
intro!: floor_eq2) 
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113 

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114 
lemma floor_log_eq_if: 
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115 
fixes b x y :: nat 
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116 
assumes "x div b = y div b" "b > 1" "x > 0" "x div b \<ge> 1" 
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117 
shows "floor(log b x) = floor(log b y)" 
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118 
proof  
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119 
have "y > 0" using assms by(auto intro: ccontr) 
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120 
thus ?thesis using assms by (simp add: floor_log_div) 
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121 
qed 
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122 

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123 
lemma floorlog_eq_if: 
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124 
fixes b x y :: nat 
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125 
assumes "x div b = y div b" "b > 1" "x > 0" "x div b \<ge> 1" 
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126 
shows "floorlog b x = floorlog b y" 
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127 
proof  
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128 
have "y > 0" using assms by(auto intro: ccontr) 
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129 
thus ?thesis using assms 
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130 
by(auto simp add: floorlog_def eq_nat_nat_iff intro: floor_log_eq_if) 
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131 
qed 
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132 

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133 

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134 
definition bitlen :: "int \<Rightarrow> int" where "bitlen a = floorlog 2 (nat a)" 
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135 

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136 
lemma bitlen_alt_def: "bitlen a = (if a > 0 then \<lfloor>log 2 a\<rfloor> + 1 else 0)" 
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137 
by (simp add: bitlen_def floorlog_def) 
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138 

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139 
lemma bitlen_nonneg: "0 \<le> bitlen x" 
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140 
by (simp add: bitlen_def) 
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141 

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lemma bitlen_bounds: 
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assumes "x > 0" 
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shows "2 ^ nat (bitlen x  1) \<le> x \<and> x < 2 ^ nat (bitlen x)" 
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145 
proof  
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146 
from assms have "bitlen x \<ge> 1" by (auto simp: bitlen_alt_def) 
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147 
with assms floorlog_bounds[of "nat x" 2] show ?thesis 
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by (auto simp add: bitlen_def le_nat_iff nat_less_iff nat_diff_distrib) 
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149 
qed 
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150 

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151 
lemma bitlen_pow2[simp]: 
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assumes "b > 0" 
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153 
shows "bitlen (b * 2 ^ c) = bitlen b + c" 
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154 
using assms 
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by (simp add: bitlen_def nat_mult_distrib nat_power_eq) 
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156 

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157 
lemma compute_bitlen[code]: 
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"bitlen x = (if x > 0 then bitlen (x div 2) + 1 else 0)" 
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by (simp add: bitlen_def nat_div_distrib compute_floorlog) 
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160 

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161 
end 