src/HOL/Log.thy
author wenzelm
Mon Mar 22 20:58:52 2010 +0100 (2010-03-22)
changeset 35898 c890a3835d15
parent 33716 c7b42ad3fadf
child 36622 e393a91f86df
permissions -rw-r--r--
recovered header;
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(*  Title       : Log.thy
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    Author      : Jacques D. Fleuriot
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                  Additional contributions by Jeremy Avigad
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    Copyright   : 2000,2001 University of Edinburgh
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*)
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header{*Logarithms: Standard Version*}
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theory Log
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imports Transcendental
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begin
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definition
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  powr  :: "[real,real] => real"     (infixr "powr" 80) where
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    --{*exponentation with real exponent*}
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  "x powr a = exp(a * ln x)"
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definition
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  log :: "[real,real] => real" where
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    --{*logarithm of @{term x} to base @{term a}*}
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  "log a x = ln x / ln a"
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lemma powr_one_eq_one [simp]: "1 powr a = 1"
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by (simp add: powr_def)
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lemma powr_zero_eq_one [simp]: "x powr 0 = 1"
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by (simp add: powr_def)
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lemma powr_one_gt_zero_iff [simp]: "(x powr 1 = x) = (0 < x)"
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by (simp add: powr_def)
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declare powr_one_gt_zero_iff [THEN iffD2, simp]
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lemma powr_mult: 
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      "[| 0 < x; 0 < y |] ==> (x * y) powr a = (x powr a) * (y powr a)"
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by (simp add: powr_def exp_add [symmetric] ln_mult right_distrib)
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lemma powr_gt_zero [simp]: "0 < x powr a"
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by (simp add: powr_def)
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lemma powr_ge_pzero [simp]: "0 <= x powr y"
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by (rule order_less_imp_le, rule powr_gt_zero)
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lemma powr_not_zero [simp]: "x powr a \<noteq> 0"
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by (simp add: powr_def)
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lemma powr_divide:
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     "[| 0 < x; 0 < y |] ==> (x / y) powr a = (x powr a)/(y powr a)"
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apply (simp add: divide_inverse positive_imp_inverse_positive powr_mult)
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apply (simp add: powr_def exp_minus [symmetric] exp_add [symmetric] ln_inverse)
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done
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lemma powr_divide2: "x powr a / x powr b = x powr (a - b)"
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  apply (simp add: powr_def)
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  apply (subst exp_diff [THEN sym])
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  apply (simp add: left_diff_distrib)
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done
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lemma powr_add: "x powr (a + b) = (x powr a) * (x powr b)"
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by (simp add: powr_def exp_add [symmetric] left_distrib)
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lemma powr_powr: "(x powr a) powr b = x powr (a * b)"
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by (simp add: powr_def)
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lemma powr_powr_swap: "(x powr a) powr b = (x powr b) powr a"
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by (simp add: powr_powr real_mult_commute)
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lemma powr_minus: "x powr (-a) = inverse (x powr a)"
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by (simp add: powr_def exp_minus [symmetric])
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lemma powr_minus_divide: "x powr (-a) = 1/(x powr a)"
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by (simp add: divide_inverse powr_minus)
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lemma powr_less_mono: "[| a < b; 1 < x |] ==> x powr a < x powr b"
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by (simp add: powr_def)
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lemma powr_less_cancel: "[| x powr a < x powr b; 1 < x |] ==> a < b"
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by (simp add: powr_def)
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lemma powr_less_cancel_iff [simp]: "1 < x ==> (x powr a < x powr b) = (a < b)"
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by (blast intro: powr_less_cancel powr_less_mono)
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lemma powr_le_cancel_iff [simp]: "1 < x ==> (x powr a \<le> x powr b) = (a \<le> b)"
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by (simp add: linorder_not_less [symmetric])
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lemma log_ln: "ln x = log (exp(1)) x"
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by (simp add: log_def)
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lemma DERIV_log: "x > 0 ==> DERIV (%y. log b y) x :> 1 / (ln b * x)"
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  apply (subst log_def)
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  apply (subgoal_tac "(%y. ln y / ln b) = (%y. (1 / ln b) * ln y)")
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  apply (erule ssubst)
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  apply (subgoal_tac "1 / (ln b * x) = (1 / ln b) * (1 / x)")
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  apply (erule ssubst)
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  apply (rule DERIV_cmult)
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  apply (erule DERIV_ln_divide)
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  apply auto
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done
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lemma powr_log_cancel [simp]:
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     "[| 0 < a; a \<noteq> 1; 0 < x |] ==> a powr (log a x) = x"
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by (simp add: powr_def log_def)
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lemma log_powr_cancel [simp]: "[| 0 < a; a \<noteq> 1 |] ==> log a (a powr y) = y"
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by (simp add: log_def powr_def)
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lemma log_mult: 
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     "[| 0 < a; a \<noteq> 1; 0 < x; 0 < y |]  
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      ==> log a (x * y) = log a x + log a y"
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by (simp add: log_def ln_mult divide_inverse left_distrib)
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lemma log_eq_div_ln_mult_log: 
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     "[| 0 < a; a \<noteq> 1; 0 < b; b \<noteq> 1; 0 < x |]  
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      ==> log a x = (ln b/ln a) * log b x"
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by (simp add: log_def divide_inverse)
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text{*Base 10 logarithms*}
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lemma log_base_10_eq1: "0 < x ==> log 10 x = (ln (exp 1) / ln 10) * ln x"
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by (simp add: log_def)
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lemma log_base_10_eq2: "0 < x ==> log 10 x = (log 10 (exp 1)) * ln x"
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by (simp add: log_def)
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lemma log_one [simp]: "log a 1 = 0"
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by (simp add: log_def)
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lemma log_eq_one [simp]: "[| 0 < a; a \<noteq> 1 |] ==> log a a = 1"
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by (simp add: log_def)
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lemma log_inverse:
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     "[| 0 < a; a \<noteq> 1; 0 < x |] ==> log a (inverse x) = - log a x"
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apply (rule_tac a1 = "log a x" in add_left_cancel [THEN iffD1])
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apply (simp add: log_mult [symmetric])
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done
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lemma log_divide:
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     "[|0 < a; a \<noteq> 1; 0 < x; 0 < y|] ==> log a (x/y) = log a x - log a y"
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by (simp add: log_mult divide_inverse log_inverse)
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lemma log_less_cancel_iff [simp]:
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     "[| 1 < a; 0 < x; 0 < y |] ==> (log a x < log a y) = (x < y)"
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apply safe
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apply (rule_tac [2] powr_less_cancel)
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apply (drule_tac a = "log a x" in powr_less_mono, auto)
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done
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lemma log_le_cancel_iff [simp]:
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     "[| 1 < a; 0 < x; 0 < y |] ==> (log a x \<le> log a y) = (x \<le> y)"
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by (simp add: linorder_not_less [symmetric])
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lemma powr_realpow: "0 < x ==> x powr (real n) = x^n"
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  apply (induct n, simp)
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  apply (subgoal_tac "real(Suc n) = real n + 1")
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  apply (erule ssubst)
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  apply (subst powr_add, simp, simp)
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done
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lemma powr_realpow2: "0 <= x ==> 0 < n ==> x^n = (if (x = 0) then 0
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  else x powr (real n))"
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  apply (case_tac "x = 0", simp, simp)
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  apply (rule powr_realpow [THEN sym], simp)
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done
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lemma ln_powr: "0 < x ==> 0 < y ==> ln(x powr y) = y * ln x"
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by (unfold powr_def, simp)
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lemma log_powr: "0 < x ==> 0 \<le> y ==> log b (x powr y) = y * log b x"
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  apply (case_tac "y = 0")
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  apply force
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  apply (auto simp add: log_def ln_powr field_simps)
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done
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lemma log_nat_power: "0 < x ==> log b (x^n) = real n * log b x"
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  apply (subst powr_realpow [symmetric])
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  apply (auto simp add: log_powr)
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done
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lemma ln_bound: "1 <= x ==> ln x <= x"
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  apply (subgoal_tac "ln(1 + (x - 1)) <= x - 1")
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  apply simp
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  apply (rule ln_add_one_self_le_self, simp)
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done
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lemma powr_mono: "a <= b ==> 1 <= x ==> x powr a <= x powr b"
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  apply (case_tac "x = 1", simp)
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  apply (case_tac "a = b", simp)
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  apply (rule order_less_imp_le)
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  apply (rule powr_less_mono, auto)
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done
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lemma ge_one_powr_ge_zero: "1 <= x ==> 0 <= a ==> 1 <= x powr a"
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  apply (subst powr_zero_eq_one [THEN sym])
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  apply (rule powr_mono, assumption+)
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done
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lemma powr_less_mono2: "0 < a ==> 0 < x ==> x < y ==> x powr a <
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    y powr a"
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  apply (unfold powr_def)
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  apply (rule exp_less_mono)
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  apply (rule mult_strict_left_mono)
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  apply (subst ln_less_cancel_iff, assumption)
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  apply (rule order_less_trans)
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  prefer 2
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  apply assumption+
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done
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lemma powr_less_mono2_neg: "a < 0 ==> 0 < x ==> x < y ==> y powr a <
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    x powr a"
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  apply (unfold powr_def)
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  apply (rule exp_less_mono)
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  apply (rule mult_strict_left_mono_neg)
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  apply (subst ln_less_cancel_iff)
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  apply assumption
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  apply (rule order_less_trans)
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  prefer 2
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  apply assumption+
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done
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lemma powr_mono2: "0 <= a ==> 0 < x ==> x <= y ==> x powr a <= y powr a"
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  apply (case_tac "a = 0", simp)
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  apply (case_tac "x = y", simp)
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  apply (rule order_less_imp_le)
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  apply (rule powr_less_mono2, auto)
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done
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lemma ln_powr_bound: "1 <= x ==> 0 < a ==> ln x <= (x powr a) / a"
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  apply (rule mult_imp_le_div_pos)
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  apply (assumption)
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  apply (subst mult_commute)
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  apply (subst ln_powr [THEN sym])
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  apply auto
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  apply (rule ln_bound)
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  apply (erule ge_one_powr_ge_zero)
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  apply (erule order_less_imp_le)
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done
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lemma ln_powr_bound2: "1 < x ==> 0 < a ==> (ln x) powr a <= (a powr a) * x"
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proof -
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  assume "1 < x" and "0 < a"
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  then have "ln x <= (x powr (1 / a)) / (1 / a)"
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    apply (intro ln_powr_bound)
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    apply (erule order_less_imp_le)
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    apply (rule divide_pos_pos)
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    apply simp_all
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    done
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  also have "... = a * (x powr (1 / a))"
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    by simp
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  finally have "(ln x) powr a <= (a * (x powr (1 / a))) powr a"
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    apply (intro powr_mono2)
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    apply (rule order_less_imp_le, rule prems)
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    apply (rule ln_gt_zero)
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    apply (rule prems)
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    apply assumption
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    done
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  also have "... = (a powr a) * ((x powr (1 / a)) powr a)"
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    apply (rule powr_mult)
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    apply (rule prems)
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    apply (rule powr_gt_zero)
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    done
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  also have "(x powr (1 / a)) powr a = x powr ((1 / a) * a)"
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    by (rule powr_powr)
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  also have "... = x"
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    apply simp
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    apply (subgoal_tac "a ~= 0")
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    apply (insert prems, auto)
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    done
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  finally show ?thesis .
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qed
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lemma LIMSEQ_neg_powr: "0 < s ==> (%x. (real x) powr - s) ----> 0"
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  apply (unfold LIMSEQ_iff)
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  apply clarsimp
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  apply (rule_tac x = "natfloor(r powr (1 / - s)) + 1" in exI)
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  apply clarify
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  proof -
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    fix r fix n
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    assume "0 < s" and "0 < r" and "natfloor (r powr (1 / - s)) + 1 <= n"
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    have "r powr (1 / - s) < real(natfloor(r powr (1 / - s))) + 1"
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      by (rule real_natfloor_add_one_gt)
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    also have "... = real(natfloor(r powr (1 / -s)) + 1)"
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      by simp
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    also have "... <= real n"
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      apply (subst real_of_nat_le_iff)
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      apply (rule prems)
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      done
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    finally have "r powr (1 / - s) < real n".
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    then have "real n powr (- s) < (r powr (1 / - s)) powr - s" 
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      apply (intro powr_less_mono2_neg)
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      apply (auto simp add: prems)
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      done
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    also have "... = r"
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      by (simp add: powr_powr prems less_imp_neq [THEN not_sym])
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    finally show "real n powr - s < r" .
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  qed
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end