wenzelm@42830
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(* Title: HOL/Ln.thy
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avigad@16959
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Author: Jeremy Avigad
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avigad@16959
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*)
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avigad@16959
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avigad@16959
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header {* Properties of ln *}
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avigad@16959
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avigad@16959
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theory Ln
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avigad@16959
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imports Transcendental
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avigad@16959
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begin
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avigad@16959
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avigad@16959
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lemma exp_first_two_terms: "exp x = 1 + x + suminf (%n.
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nipkow@41107
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inverse(fact (n+2)) * (x ^ (n+2)))"
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avigad@16959
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proof -
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nipkow@41107
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have "exp x = suminf (%n. inverse(fact n) * (x ^ n))"
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wenzelm@19765
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by (simp add: exp_def)
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nipkow@41107
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also from summable_exp have "... = (SUM n::nat : {0..<2}.
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nipkow@41107
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inverse(fact n) * (x ^ n)) + suminf (%n.
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nipkow@41107
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inverse(fact(n+2)) * (x ^ (n+2)))" (is "_ = ?a + _")
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avigad@16959
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by (rule suminf_split_initial_segment)
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avigad@16959
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also have "?a = 1 + x"
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avigad@16959
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by (simp add: numerals)
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avigad@16959
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finally show ?thesis .
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avigad@16959
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qed
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avigad@16959
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avigad@16959
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lemma exp_tail_after_first_two_terms_summable:
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nipkow@41107
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"summable (%n. inverse(fact (n+2)) * (x ^ (n+2)))"
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avigad@16959
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proof -
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avigad@16959
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note summable_exp
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avigad@16959
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thus ?thesis
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avigad@16959
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by (frule summable_ignore_initial_segment)
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avigad@16959
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qed
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avigad@16959
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avigad@16959
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lemma aux1: assumes a: "0 <= x" and b: "x <= 1"
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nipkow@41107
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shows "inverse (fact ((n::nat) + 2)) * x ^ (n + 2) <= (x^2/2) * ((1/2)^n)"
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avigad@16959
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proof (induct n)
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nipkow@41107
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show "inverse (fact ((0::nat) + 2)) * x ^ (0 + 2) <=
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avigad@16959
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x ^ 2 / 2 * (1 / 2) ^ 0"
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nipkow@23482
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by (simp add: real_of_nat_Suc power2_eq_square)
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avigad@16959
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next
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avigad@32031
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fix n :: nat
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nipkow@41107
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assume c: "inverse (fact (n + 2)) * x ^ (n + 2)
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avigad@16959
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<= x ^ 2 / 2 * (1 / 2) ^ n"
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nipkow@41107
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show "inverse (fact (Suc n + 2)) * x ^ (Suc n + 2)
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avigad@16959
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<= x ^ 2 / 2 * (1 / 2) ^ Suc n"
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avigad@16959
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proof -
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nipkow@41107
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have "inverse(fact (Suc n + 2)) <= (1/2) * inverse (fact (n+2))"
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avigad@16959
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proof -
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avigad@16959
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have "Suc n + 2 = Suc (n + 2)" by simp
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avigad@16959
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then have "fact (Suc n + 2) = Suc (n + 2) * fact (n + 2)"
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avigad@16959
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by simp
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avigad@16959
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then have "real(fact (Suc n + 2)) = real(Suc (n + 2) * fact (n + 2))"
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avigad@16959
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apply (rule subst)
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avigad@16959
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apply (rule refl)
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avigad@16959
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done
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avigad@16959
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also have "... = real(Suc (n + 2)) * real(fact (n + 2))"
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avigad@16959
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by (rule real_of_nat_mult)
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avigad@16959
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finally have "real (fact (Suc n + 2)) =
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avigad@16959
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real (Suc (n + 2)) * real (fact (n + 2))" .
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nipkow@41107
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then have "inverse(fact (Suc n + 2)) =
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nipkow@41107
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inverse(Suc (n + 2)) * inverse(fact (n + 2))"
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avigad@16959
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apply (rule ssubst)
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avigad@16959
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apply (rule inverse_mult_distrib)
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avigad@16959
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done
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nipkow@41107
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also have "... <= (1/2) * inverse(fact (n + 2))"
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avigad@16959
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apply (rule mult_right_mono)
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avigad@16959
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apply (subst inverse_eq_divide)
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avigad@16959
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apply simp
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avigad@16959
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apply (rule inv_real_of_nat_fact_ge_zero)
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avigad@16959
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done
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avigad@16959
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finally show ?thesis .
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avigad@16959
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qed
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avigad@16959
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moreover have "x ^ (Suc n + 2) <= x ^ (n + 2)"
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avigad@16959
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apply (simp add: mult_compare_simps)
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wenzelm@41798
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apply (simp add: assms)
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avigad@16959
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apply (subgoal_tac "0 <= x * (x * x^n)")
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avigad@16959
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apply force
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avigad@16959
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apply (rule mult_nonneg_nonneg, rule a)+
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avigad@16959
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apply (rule zero_le_power, rule a)
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avigad@16959
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done
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nipkow@41107
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ultimately have "inverse (fact (Suc n + 2)) * x ^ (Suc n + 2) <=
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nipkow@41107
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(1 / 2 * inverse (fact (n + 2))) * x ^ (n + 2)"
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avigad@16959
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apply (rule mult_mono)
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avigad@16959
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apply (rule mult_nonneg_nonneg)
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avigad@16959
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apply simp
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avigad@16959
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apply (subst inverse_nonnegative_iff_nonnegative)
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huffman@27483
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apply (rule real_of_nat_ge_zero)
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avigad@16959
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apply (rule zero_le_power)
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huffman@23441
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apply (rule a)
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avigad@16959
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done
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nipkow@41107
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also have "... = 1 / 2 * (inverse (fact (n + 2)) * x ^ (n + 2))"
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avigad@16959
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by simp
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avigad@16959
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also have "... <= 1 / 2 * (x ^ 2 / 2 * (1 / 2) ^ n)"
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avigad@16959
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apply (rule mult_left_mono)
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wenzelm@41798
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apply (rule c)
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avigad@16959
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apply simp
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avigad@16959
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done
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avigad@16959
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also have "... = x ^ 2 / 2 * (1 / 2 * (1 / 2) ^ n)"
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avigad@16959
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by auto
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avigad@16959
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also have "(1::real) / 2 * (1 / 2) ^ n = (1 / 2) ^ (Suc n)"
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huffman@30269
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by (rule power_Suc [THEN sym])
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avigad@16959
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finally show ?thesis .
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avigad@16959
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qed
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avigad@16959
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qed
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avigad@16959
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huffman@20692
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lemma aux2: "(%n. (x::real) ^ 2 / 2 * (1 / 2) ^ n) sums x^2"
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avigad@16959
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proof -
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huffman@20692
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have "(%n. (1 / 2::real)^n) sums (1 / (1 - (1/2)))"
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avigad@16959
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apply (rule geometric_sums)
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huffman@22998
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by (simp add: abs_less_iff)
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avigad@16959
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also have "(1::real) / (1 - 1/2) = 2"
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avigad@16959
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by simp
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huffman@20692
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finally have "(%n. (1 / 2::real)^n) sums 2" .
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avigad@16959
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then have "(%n. x ^ 2 / 2 * (1 / 2) ^ n) sums (x^2 / 2 * 2)"
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avigad@16959
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by (rule sums_mult)
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avigad@16959
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also have "x^2 / 2 * 2 = x^2"
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avigad@16959
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by simp
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avigad@16959
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finally show ?thesis .
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avigad@16959
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qed
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avigad@16959
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huffman@23114
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lemma exp_bound: "0 <= (x::real) ==> x <= 1 ==> exp x <= 1 + x + x^2"
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avigad@16959
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proof -
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avigad@16959
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assume a: "0 <= x"
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avigad@16959
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assume b: "x <= 1"
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nipkow@41107
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have c: "exp x = 1 + x + suminf (%n. inverse(fact (n+2)) *
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avigad@16959
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(x ^ (n+2)))"
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avigad@16959
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by (rule exp_first_two_terms)
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nipkow@41107
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moreover have "suminf (%n. inverse(fact (n+2)) * (x ^ (n+2))) <= x^2"
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avigad@16959
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proof -
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nipkow@41107
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have "suminf (%n. inverse(fact (n+2)) * (x ^ (n+2))) <=
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avigad@16959
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suminf (%n. (x^2/2) * ((1/2)^n))"
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avigad@16959
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apply (rule summable_le)
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wenzelm@41798
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apply (auto simp only: aux1 a b)
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avigad@16959
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apply (rule exp_tail_after_first_two_terms_summable)
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avigad@16959
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by (rule sums_summable, rule aux2)
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avigad@16959
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also have "... = x^2"
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avigad@16959
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by (rule sums_unique [THEN sym], rule aux2)
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avigad@16959
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finally show ?thesis .
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avigad@16959
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qed
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avigad@16959
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ultimately show ?thesis
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avigad@16959
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by auto
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avigad@16959
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qed
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avigad@16959
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huffman@23114
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lemma aux4: "0 <= (x::real) ==> x <= 1 ==> exp (x - x^2) <= 1 + x"
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avigad@16959
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proof -
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avigad@16959
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assume a: "0 <= x" and b: "x <= 1"
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avigad@16959
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have "exp (x - x^2) = exp x / exp (x^2)"
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avigad@16959
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by (rule exp_diff)
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avigad@16959
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also have "... <= (1 + x + x^2) / exp (x ^2)"
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avigad@16959
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apply (rule divide_right_mono)
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avigad@16959
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apply (rule exp_bound)
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avigad@16959
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apply (rule a, rule b)
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avigad@16959
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apply simp
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avigad@16959
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done
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avigad@16959
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also have "... <= (1 + x + x^2) / (1 + x^2)"
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avigad@16959
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apply (rule divide_left_mono)
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avigad@17013
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apply (auto simp add: exp_ge_add_one_self_aux)
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avigad@16959
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apply (rule add_nonneg_nonneg)
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wenzelm@41798
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using a apply auto
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avigad@16959
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apply (rule mult_pos_pos)
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avigad@16959
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apply auto
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avigad@16959
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apply (rule add_pos_nonneg)
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avigad@16959
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apply auto
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avigad@16959
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done
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avigad@16959
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also from a have "... <= 1 + x"
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wenzelm@41798
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by (simp add: field_simps zero_compare_simps)
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avigad@16959
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finally show ?thesis .
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avigad@16959
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qed
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avigad@16959
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avigad@16959
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lemma ln_one_plus_pos_lower_bound: "0 <= x ==> x <= 1 ==>
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avigad@16959
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x - x^2 <= ln (1 + x)"
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avigad@16959
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proof -
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avigad@16959
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assume a: "0 <= x" and b: "x <= 1"
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avigad@16959
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then have "exp (x - x^2) <= 1 + x"
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avigad@16959
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by (rule aux4)
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avigad@16959
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also have "... = exp (ln (1 + x))"
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avigad@16959
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proof -
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avigad@16959
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from a have "0 < 1 + x" by auto
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avigad@16959
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thus ?thesis
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avigad@16959
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by (auto simp only: exp_ln_iff [THEN sym])
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avigad@16959
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qed
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avigad@16959
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finally have "exp (x - x ^ 2) <= exp (ln (1 + x))" .
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avigad@16959
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thus ?thesis by (auto simp only: exp_le_cancel_iff)
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avigad@16959
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qed
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avigad@16959
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avigad@16959
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lemma ln_one_minus_pos_upper_bound: "0 <= x ==> x < 1 ==> ln (1 - x) <= - x"
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avigad@16959
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proof -
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avigad@16959
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assume a: "0 <= (x::real)" and b: "x < 1"
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avigad@16959
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have "(1 - x) * (1 + x + x^2) = (1 - x^3)"
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nipkow@29667
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by (simp add: algebra_simps power2_eq_square power3_eq_cube)
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avigad@16959
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also have "... <= 1"
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nipkow@25875
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by (auto simp add: a)
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avigad@16959
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finally have "(1 - x) * (1 + x + x ^ 2) <= 1" .
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avigad@16959
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moreover have "0 < 1 + x + x^2"
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avigad@16959
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apply (rule add_pos_nonneg)
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wenzelm@41798
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using a apply auto
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avigad@16959
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done
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avigad@16959
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ultimately have "1 - x <= 1 / (1 + x + x^2)"
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avigad@16959
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by (elim mult_imp_le_div_pos)
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avigad@16959
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also have "... <= 1 / exp x"
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avigad@16959
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apply (rule divide_left_mono)
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avigad@16959
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apply (rule exp_bound, rule a)
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wenzelm@41798
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using a b apply auto
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avigad@16959
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apply (rule mult_pos_pos)
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avigad@16959
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apply (rule add_pos_nonneg)
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avigad@16959
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apply auto
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avigad@16959
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done
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avigad@16959
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also have "... = exp (-x)"
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huffman@36769
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by (auto simp add: exp_minus divide_inverse)
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avigad@16959
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finally have "1 - x <= exp (- x)" .
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avigad@16959
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also have "1 - x = exp (ln (1 - x))"
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avigad@16959
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proof -
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avigad@16959
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have "0 < 1 - x"
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avigad@16959
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by (insert b, auto)
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avigad@16959
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thus ?thesis
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avigad@16959
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by (auto simp only: exp_ln_iff [THEN sym])
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avigad@16959
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qed
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avigad@16959
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finally have "exp (ln (1 - x)) <= exp (- x)" .
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avigad@16959
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thus ?thesis by (auto simp only: exp_le_cancel_iff)
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avigad@16959
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219 |
qed
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avigad@16959
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220 |
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avigad@16959
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lemma aux5: "x < 1 ==> ln(1 - x) = - ln(1 + x / (1 - x))"
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avigad@16959
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proof -
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avigad@16959
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223 |
assume a: "x < 1"
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avigad@16959
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224 |
have "ln(1 - x) = - ln(1 / (1 - x))"
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avigad@16959
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proof -
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avigad@16959
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have "ln(1 - x) = - (- ln (1 - x))"
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avigad@16959
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227 |
by auto
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avigad@16959
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228 |
also have "- ln(1 - x) = ln 1 - ln(1 - x)"
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avigad@16959
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by simp
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avigad@16959
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also have "... = ln(1 / (1 - x))"
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avigad@16959
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apply (rule ln_div [THEN sym])
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avigad@16959
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232 |
by (insert a, auto)
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avigad@16959
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233 |
finally show ?thesis .
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avigad@16959
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234 |
qed
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nipkow@23482
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235 |
also have " 1 / (1 - x) = 1 + x / (1 - x)" using a by(simp add:field_simps)
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avigad@16959
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236 |
finally show ?thesis .
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avigad@16959
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237 |
qed
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avigad@16959
|
238 |
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avigad@16959
|
239 |
lemma ln_one_minus_pos_lower_bound: "0 <= x ==> x <= (1 / 2) ==>
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avigad@16959
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240 |
- x - 2 * x^2 <= ln (1 - x)"
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avigad@16959
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241 |
proof -
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avigad@16959
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242 |
assume a: "0 <= x" and b: "x <= (1 / 2)"
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avigad@16959
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243 |
from b have c: "x < 1"
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avigad@16959
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244 |
by auto
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avigad@16959
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245 |
then have "ln (1 - x) = - ln (1 + x / (1 - x))"
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avigad@16959
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246 |
by (rule aux5)
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avigad@16959
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247 |
also have "- (x / (1 - x)) <= ..."
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avigad@16959
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proof -
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avigad@16959
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249 |
have "ln (1 + x / (1 - x)) <= x / (1 - x)"
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avigad@16959
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250 |
apply (rule ln_add_one_self_le_self)
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avigad@16959
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251 |
apply (rule divide_nonneg_pos)
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avigad@16959
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252 |
by (insert a c, auto)
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avigad@16959
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253 |
thus ?thesis
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avigad@16959
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254 |
by auto
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avigad@16959
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255 |
qed
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avigad@16959
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256 |
also have "- (x / (1 - x)) = -x / (1 - x)"
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avigad@16959
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257 |
by auto
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avigad@16959
|
258 |
finally have d: "- x / (1 - x) <= ln (1 - x)" .
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wenzelm@41798
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259 |
have "0 < 1 - x" using a b by simp
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nipkow@23482
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260 |
hence e: "-x - 2 * x^2 <= - x / (1 - x)"
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wenzelm@41798
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261 |
using mult_right_le_one_le[of "x*x" "2*x"] a b
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wenzelm@41798
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262 |
by (simp add:field_simps power2_eq_square)
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avigad@16959
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263 |
from e d show "- x - 2 * x^2 <= ln (1 - x)"
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avigad@16959
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264 |
by (rule order_trans)
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avigad@16959
|
265 |
qed
|
avigad@16959
|
266 |
|
huffman@23114
|
267 |
lemma exp_ge_add_one_self [simp]: "1 + (x::real) <= exp x"
|
avigad@16959
|
268 |
apply (case_tac "0 <= x")
|
avigad@17013
|
269 |
apply (erule exp_ge_add_one_self_aux)
|
avigad@16959
|
270 |
apply (case_tac "x <= -1")
|
avigad@16959
|
271 |
apply (subgoal_tac "1 + x <= 0")
|
avigad@16959
|
272 |
apply (erule order_trans)
|
avigad@16959
|
273 |
apply simp
|
avigad@16959
|
274 |
apply simp
|
avigad@16959
|
275 |
apply (subgoal_tac "1 + x = exp(ln (1 + x))")
|
avigad@16959
|
276 |
apply (erule ssubst)
|
avigad@16959
|
277 |
apply (subst exp_le_cancel_iff)
|
avigad@16959
|
278 |
apply (subgoal_tac "ln (1 - (- x)) <= - (- x)")
|
avigad@16959
|
279 |
apply simp
|
avigad@16959
|
280 |
apply (rule ln_one_minus_pos_upper_bound)
|
avigad@16959
|
281 |
apply auto
|
avigad@16959
|
282 |
done
|
avigad@16959
|
283 |
|
avigad@16959
|
284 |
lemma ln_add_one_self_le_self2: "-1 < x ==> ln(1 + x) <= x"
|
avigad@16959
|
285 |
apply (subgoal_tac "x = ln (exp x)")
|
avigad@16959
|
286 |
apply (erule ssubst)back
|
avigad@16959
|
287 |
apply (subst ln_le_cancel_iff)
|
avigad@16959
|
288 |
apply auto
|
avigad@16959
|
289 |
done
|
avigad@16959
|
290 |
|
avigad@16959
|
291 |
lemma abs_ln_one_plus_x_minus_x_bound_nonneg:
|
avigad@16959
|
292 |
"0 <= x ==> x <= 1 ==> abs(ln (1 + x) - x) <= x^2"
|
avigad@16959
|
293 |
proof -
|
huffman@23441
|
294 |
assume x: "0 <= x"
|
wenzelm@41798
|
295 |
assume x1: "x <= 1"
|
huffman@23441
|
296 |
from x have "ln (1 + x) <= x"
|
avigad@16959
|
297 |
by (rule ln_add_one_self_le_self)
|
avigad@16959
|
298 |
then have "ln (1 + x) - x <= 0"
|
avigad@16959
|
299 |
by simp
|
avigad@16959
|
300 |
then have "abs(ln(1 + x) - x) = - (ln(1 + x) - x)"
|
avigad@16959
|
301 |
by (rule abs_of_nonpos)
|
avigad@16959
|
302 |
also have "... = x - ln (1 + x)"
|
avigad@16959
|
303 |
by simp
|
avigad@16959
|
304 |
also have "... <= x^2"
|
avigad@16959
|
305 |
proof -
|
wenzelm@41798
|
306 |
from x x1 have "x - x^2 <= ln (1 + x)"
|
avigad@16959
|
307 |
by (intro ln_one_plus_pos_lower_bound)
|
avigad@16959
|
308 |
thus ?thesis
|
avigad@16959
|
309 |
by simp
|
avigad@16959
|
310 |
qed
|
avigad@16959
|
311 |
finally show ?thesis .
|
avigad@16959
|
312 |
qed
|
avigad@16959
|
313 |
|
avigad@16959
|
314 |
lemma abs_ln_one_plus_x_minus_x_bound_nonpos:
|
avigad@16959
|
315 |
"-(1 / 2) <= x ==> x <= 0 ==> abs(ln (1 + x) - x) <= 2 * x^2"
|
avigad@16959
|
316 |
proof -
|
wenzelm@41798
|
317 |
assume a: "-(1 / 2) <= x"
|
wenzelm@41798
|
318 |
assume b: "x <= 0"
|
avigad@16959
|
319 |
have "abs(ln (1 + x) - x) = x - ln(1 - (-x))"
|
avigad@16959
|
320 |
apply (subst abs_of_nonpos)
|
avigad@16959
|
321 |
apply simp
|
avigad@16959
|
322 |
apply (rule ln_add_one_self_le_self2)
|
wenzelm@41798
|
323 |
using a apply auto
|
avigad@16959
|
324 |
done
|
avigad@16959
|
325 |
also have "... <= 2 * x^2"
|
avigad@16959
|
326 |
apply (subgoal_tac "- (-x) - 2 * (-x)^2 <= ln (1 - (-x))")
|
nipkow@29667
|
327 |
apply (simp add: algebra_simps)
|
avigad@16959
|
328 |
apply (rule ln_one_minus_pos_lower_bound)
|
wenzelm@41798
|
329 |
using a b apply auto
|
nipkow@29667
|
330 |
done
|
avigad@16959
|
331 |
finally show ?thesis .
|
avigad@16959
|
332 |
qed
|
avigad@16959
|
333 |
|
avigad@16959
|
334 |
lemma abs_ln_one_plus_x_minus_x_bound:
|
avigad@16959
|
335 |
"abs x <= 1 / 2 ==> abs(ln (1 + x) - x) <= 2 * x^2"
|
avigad@16959
|
336 |
apply (case_tac "0 <= x")
|
avigad@16959
|
337 |
apply (rule order_trans)
|
avigad@16959
|
338 |
apply (rule abs_ln_one_plus_x_minus_x_bound_nonneg)
|
avigad@16959
|
339 |
apply auto
|
avigad@16959
|
340 |
apply (rule abs_ln_one_plus_x_minus_x_bound_nonpos)
|
avigad@16959
|
341 |
apply auto
|
avigad@16959
|
342 |
done
|
avigad@16959
|
343 |
|
avigad@16959
|
344 |
lemma ln_x_over_x_mono: "exp 1 <= x ==> x <= y ==> (ln y / y) <= (ln x / x)"
|
avigad@16959
|
345 |
proof -
|
wenzelm@41798
|
346 |
assume x: "exp 1 <= x" "x <= y"
|
avigad@16959
|
347 |
have a: "0 < x" and b: "0 < y"
|
wenzelm@41798
|
348 |
apply (insert x)
|
huffman@23114
|
349 |
apply (subgoal_tac "0 < exp (1::real)")
|
avigad@16959
|
350 |
apply arith
|
avigad@16959
|
351 |
apply auto
|
huffman@23114
|
352 |
apply (subgoal_tac "0 < exp (1::real)")
|
avigad@16959
|
353 |
apply arith
|
avigad@16959
|
354 |
apply auto
|
avigad@16959
|
355 |
done
|
avigad@16959
|
356 |
have "x * ln y - x * ln x = x * (ln y - ln x)"
|
nipkow@29667
|
357 |
by (simp add: algebra_simps)
|
avigad@16959
|
358 |
also have "... = x * ln(y / x)"
|
avigad@16959
|
359 |
apply (subst ln_div)
|
avigad@16959
|
360 |
apply (rule b, rule a, rule refl)
|
avigad@16959
|
361 |
done
|
avigad@16959
|
362 |
also have "y / x = (x + (y - x)) / x"
|
avigad@16959
|
363 |
by simp
|
wenzelm@41798
|
364 |
also have "... = 1 + (y - x) / x" using x a by (simp add: field_simps)
|
avigad@16959
|
365 |
also have "x * ln(1 + (y - x) / x) <= x * ((y - x) / x)"
|
avigad@16959
|
366 |
apply (rule mult_left_mono)
|
avigad@16959
|
367 |
apply (rule ln_add_one_self_le_self)
|
avigad@16959
|
368 |
apply (rule divide_nonneg_pos)
|
wenzelm@41798
|
369 |
using x a apply simp_all
|
avigad@16959
|
370 |
done
|
nipkow@23482
|
371 |
also have "... = y - x" using a by simp
|
nipkow@23482
|
372 |
also have "... = (y - x) * ln (exp 1)" by simp
|
avigad@16959
|
373 |
also have "... <= (y - x) * ln x"
|
avigad@16959
|
374 |
apply (rule mult_left_mono)
|
avigad@16959
|
375 |
apply (subst ln_le_cancel_iff)
|
avigad@16959
|
376 |
apply force
|
avigad@16959
|
377 |
apply (rule a)
|
wenzelm@41798
|
378 |
apply (rule x)
|
wenzelm@41798
|
379 |
using x apply simp
|
avigad@16959
|
380 |
done
|
avigad@16959
|
381 |
also have "... = y * ln x - x * ln x"
|
avigad@16959
|
382 |
by (rule left_diff_distrib)
|
avigad@16959
|
383 |
finally have "x * ln y <= y * ln x"
|
avigad@16959
|
384 |
by arith
|
wenzelm@41798
|
385 |
then have "ln y <= (y * ln x) / x" using a by (simp add: field_simps)
|
wenzelm@41798
|
386 |
also have "... = y * (ln x / x)" by simp
|
wenzelm@41798
|
387 |
finally show ?thesis using b by (simp add: field_simps)
|
avigad@16959
|
388 |
qed
|
avigad@16959
|
389 |
|
hoelzl@44193
|
390 |
lemma ln_le_minus_one:
|
hoelzl@44193
|
391 |
"0 < x \<Longrightarrow> ln x \<le> x - 1"
|
hoelzl@44193
|
392 |
using exp_ge_add_one_self[of "ln x"] by simp
|
hoelzl@44193
|
393 |
|
hoelzl@44193
|
394 |
lemma ln_eq_minus_one:
|
hoelzl@44193
|
395 |
assumes "0 < x" "ln x = x - 1" shows "x = 1"
|
hoelzl@44193
|
396 |
proof -
|
hoelzl@44193
|
397 |
let "?l y" = "ln y - y + 1"
|
hoelzl@44193
|
398 |
have D: "\<And>x. 0 < x \<Longrightarrow> DERIV ?l x :> (1 / x - 1)"
|
hoelzl@44193
|
399 |
by (auto intro!: DERIV_intros)
|
hoelzl@44193
|
400 |
|
hoelzl@44193
|
401 |
show ?thesis
|
hoelzl@44193
|
402 |
proof (cases rule: linorder_cases)
|
hoelzl@44193
|
403 |
assume "x < 1"
|
hoelzl@44193
|
404 |
from dense[OF `x < 1`] obtain a where "x < a" "a < 1" by blast
|
hoelzl@44193
|
405 |
from `x < a` have "?l x < ?l a"
|
hoelzl@44193
|
406 |
proof (rule DERIV_pos_imp_increasing, safe)
|
hoelzl@44193
|
407 |
fix y assume "x \<le> y" "y \<le> a"
|
hoelzl@44193
|
408 |
with `0 < x` `a < 1` have "0 < 1 / y - 1" "0 < y"
|
hoelzl@44193
|
409 |
by (auto simp: field_simps)
|
hoelzl@44193
|
410 |
with D show "\<exists>z. DERIV ?l y :> z \<and> 0 < z"
|
hoelzl@44193
|
411 |
by auto
|
hoelzl@44193
|
412 |
qed
|
hoelzl@44193
|
413 |
also have "\<dots> \<le> 0"
|
hoelzl@44193
|
414 |
using ln_le_minus_one `0 < x` `x < a` by (auto simp: field_simps)
|
hoelzl@44193
|
415 |
finally show "x = 1" using assms by auto
|
hoelzl@44193
|
416 |
next
|
hoelzl@44193
|
417 |
assume "1 < x"
|
hoelzl@44193
|
418 |
from dense[OF `1 < x`] obtain a where "1 < a" "a < x" by blast
|
hoelzl@44193
|
419 |
from `a < x` have "?l x < ?l a"
|
hoelzl@44193
|
420 |
proof (rule DERIV_neg_imp_decreasing, safe)
|
hoelzl@44193
|
421 |
fix y assume "a \<le> y" "y \<le> x"
|
hoelzl@44193
|
422 |
with `1 < a` have "1 / y - 1 < 0" "0 < y"
|
hoelzl@44193
|
423 |
by (auto simp: field_simps)
|
hoelzl@44193
|
424 |
with D show "\<exists>z. DERIV ?l y :> z \<and> z < 0"
|
hoelzl@44193
|
425 |
by blast
|
hoelzl@44193
|
426 |
qed
|
hoelzl@44193
|
427 |
also have "\<dots> \<le> 0"
|
hoelzl@44193
|
428 |
using ln_le_minus_one `1 < a` by (auto simp: field_simps)
|
hoelzl@44193
|
429 |
finally show "x = 1" using assms by auto
|
hoelzl@44193
|
430 |
qed simp
|
hoelzl@44193
|
431 |
qed
|
hoelzl@44193
|
432 |
|
avigad@16959
|
433 |
end
|