author | huffman |
Tue, 04 Mar 2014 14:00:59 -0800 | |
changeset 55909 | df6133adb63f |
parent 54230 | b1d955791529 |
child 56409 | 36489d77c484 |
permissions | -rw-r--r-- |
36648 | 1 |
(* Title: HOL/Library/Convex.thy |
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Author: Armin Heller, TU Muenchen |
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Author: Johannes Hoelzl, TU Muenchen |
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*) |
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header {* Convexity in real vector spaces *} |
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36623 | 8 |
theory Convex |
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imports Product_Vector |
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begin |
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subsection {* Convexity. *} |
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49609 | 14 |
definition convex :: "'a::real_vector set \<Rightarrow> bool" |
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where "convex s \<longleftrightarrow> (\<forall>x\<in>s. \<forall>y\<in>s. \<forall>u\<ge>0. \<forall>v\<ge>0. u + v = 1 \<longrightarrow> u *\<^sub>R x + v *\<^sub>R y \<in> s)" |
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36623 | 16 |
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53676 | 17 |
lemma convexI: |
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assumes "\<And>x y u v. x \<in> s \<Longrightarrow> y \<in> s \<Longrightarrow> 0 \<le> u \<Longrightarrow> 0 \<le> v \<Longrightarrow> u + v = 1 \<Longrightarrow> u *\<^sub>R x + v *\<^sub>R y \<in> s" |
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shows "convex s" |
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using assms unfolding convex_def by fast |
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lemma convexD: |
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assumes "convex s" and "x \<in> s" and "y \<in> s" and "0 \<le> u" and "0 \<le> v" and "u + v = 1" |
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shows "u *\<^sub>R x + v *\<^sub>R y \<in> s" |
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using assms unfolding convex_def by fast |
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36623 | 27 |
lemma convex_alt: |
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"convex s \<longleftrightarrow> (\<forall>x\<in>s. \<forall>y\<in>s. \<forall>u. 0 \<le> u \<and> u \<le> 1 \<longrightarrow> ((1 - u) *\<^sub>R x + u *\<^sub>R y) \<in> s)" |
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(is "_ \<longleftrightarrow> ?alt") |
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proof |
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assume alt[rule_format]: ?alt |
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{ fix x y and u v :: real assume mem: "x \<in> s" "y \<in> s" |
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49609 | 33 |
assume "0 \<le> u" "0 \<le> v" |
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moreover assume "u + v = 1" then have "u = 1 - v" by auto |
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36623 | 35 |
ultimately have "u *\<^sub>R x + v *\<^sub>R y \<in> s" using alt[OF mem] by auto } |
49609 | 36 |
then show "convex s" unfolding convex_def by auto |
36623 | 37 |
qed (auto simp: convex_def) |
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lemma mem_convex: |
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assumes "convex s" "a \<in> s" "b \<in> s" "0 \<le> u" "u \<le> 1" |
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shows "((1 - u) *\<^sub>R a + u *\<^sub>R b) \<in> s" |
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using assms unfolding convex_alt by auto |
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lemma convex_empty[intro]: "convex {}" |
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unfolding convex_def by simp |
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lemma convex_singleton[intro]: "convex {a}" |
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unfolding convex_def by (auto simp: scaleR_left_distrib[symmetric]) |
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lemma convex_UNIV[intro]: "convex UNIV" |
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unfolding convex_def by auto |
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lemma convex_Inter: "(\<forall>s\<in>f. convex s) ==> convex(\<Inter> f)" |
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unfolding convex_def by auto |
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lemma convex_Int: "convex s \<Longrightarrow> convex t \<Longrightarrow> convex (s \<inter> t)" |
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unfolding convex_def by auto |
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lemma convex_INT: "\<forall>i\<in>A. convex (B i) \<Longrightarrow> convex (\<Inter>i\<in>A. B i)" |
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unfolding convex_def by auto |
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lemma convex_Times: "convex s \<Longrightarrow> convex t \<Longrightarrow> convex (s \<times> t)" |
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unfolding convex_def by auto |
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36623 | 65 |
lemma convex_halfspace_le: "convex {x. inner a x \<le> b}" |
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unfolding convex_def |
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44142 | 67 |
by (auto simp: inner_add intro!: convex_bound_le) |
36623 | 68 |
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lemma convex_halfspace_ge: "convex {x. inner a x \<ge> b}" |
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proof - |
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49609 | 71 |
have *: "{x. inner a x \<ge> b} = {x. inner (-a) x \<le> -b}" by auto |
36623 | 72 |
show ?thesis unfolding * using convex_halfspace_le[of "-a" "-b"] by auto |
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qed |
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lemma convex_hyperplane: "convex {x. inner a x = b}" |
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49609 | 76 |
proof - |
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have *: "{x. inner a x = b} = {x. inner a x \<le> b} \<inter> {x. inner a x \<ge> b}" by auto |
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36623 | 78 |
show ?thesis using convex_halfspace_le convex_halfspace_ge |
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by (auto intro!: convex_Int simp: *) |
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qed |
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lemma convex_halfspace_lt: "convex {x. inner a x < b}" |
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unfolding convex_def |
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by (auto simp: convex_bound_lt inner_add) |
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lemma convex_halfspace_gt: "convex {x. inner a x > b}" |
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using convex_halfspace_lt[of "-a" "-b"] by auto |
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lemma convex_real_interval: |
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fixes a b :: "real" |
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shows "convex {a..}" and "convex {..b}" |
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49609 | 92 |
and "convex {a<..}" and "convex {..<b}" |
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and "convex {a..b}" and "convex {a<..b}" |
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and "convex {a..<b}" and "convex {a<..<b}" |
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proof - |
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have "{a..} = {x. a \<le> inner 1 x}" by auto |
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49609 | 97 |
then show 1: "convex {a..}" by (simp only: convex_halfspace_ge) |
36623 | 98 |
have "{..b} = {x. inner 1 x \<le> b}" by auto |
49609 | 99 |
then show 2: "convex {..b}" by (simp only: convex_halfspace_le) |
36623 | 100 |
have "{a<..} = {x. a < inner 1 x}" by auto |
49609 | 101 |
then show 3: "convex {a<..}" by (simp only: convex_halfspace_gt) |
36623 | 102 |
have "{..<b} = {x. inner 1 x < b}" by auto |
49609 | 103 |
then show 4: "convex {..<b}" by (simp only: convex_halfspace_lt) |
36623 | 104 |
have "{a..b} = {a..} \<inter> {..b}" by auto |
49609 | 105 |
then show "convex {a..b}" by (simp only: convex_Int 1 2) |
36623 | 106 |
have "{a<..b} = {a<..} \<inter> {..b}" by auto |
49609 | 107 |
then show "convex {a<..b}" by (simp only: convex_Int 3 2) |
36623 | 108 |
have "{a..<b} = {a..} \<inter> {..<b}" by auto |
49609 | 109 |
then show "convex {a..<b}" by (simp only: convex_Int 1 4) |
36623 | 110 |
have "{a<..<b} = {a<..} \<inter> {..<b}" by auto |
49609 | 111 |
then show "convex {a<..<b}" by (simp only: convex_Int 3 4) |
36623 | 112 |
qed |
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subsection {* Explicit expressions for convexity in terms of arbitrary sums. *} |
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lemma convex_setsum: |
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fixes C :: "'a::real_vector set" |
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assumes "finite s" and "convex C" and "(\<Sum> i \<in> s. a i) = 1" |
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49609 | 119 |
assumes "\<And>i. i \<in> s \<Longrightarrow> a i \<ge> 0" and "\<And>i. i \<in> s \<Longrightarrow> y i \<in> C" |
36623 | 120 |
shows "(\<Sum> j \<in> s. a j *\<^sub>R y j) \<in> C" |
55909 | 121 |
using assms(1,3,4,5) |
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proof (induct arbitrary: a set: finite) |
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49609 | 123 |
case empty |
55909 | 124 |
then show ?case by simp |
36623 | 125 |
next |
55909 | 126 |
case (insert i s) note IH = this(3) |
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have "a i + setsum a s = 1" and "0 \<le> a i" and "\<forall>j\<in>s. 0 \<le> a j" and "y i \<in> C" and "\<forall>j\<in>s. y j \<in> C" |
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using insert.hyps(1,2) insert.prems by simp_all |
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then have "0 \<le> setsum a s" by (simp add: setsum_nonneg) |
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have "a i *\<^sub>R y i + (\<Sum>j\<in>s. a j *\<^sub>R y j) \<in> C" |
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proof (cases) |
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assume z: "setsum a s = 0" |
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with `a i + setsum a s = 1` have "a i = 1" by simp |
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from setsum_nonneg_0 [OF `finite s` _ z] `\<forall>j\<in>s. 0 \<le> a j` have "\<forall>j\<in>s. a j = 0" by simp |
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show ?thesis using `a i = 1` and `\<forall>j\<in>s. a j = 0` and `y i \<in> C` by simp |
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next |
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assume nz: "setsum a s \<noteq> 0" |
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with `0 \<le> setsum a s` have "0 < setsum a s" by simp |
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then have "(\<Sum>j\<in>s. (a j / setsum a s) *\<^sub>R y j) \<in> C" |
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using `\<forall>j\<in>s. 0 \<le> a j` and `\<forall>j\<in>s. y j \<in> C` |
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by (simp add: IH divide_nonneg_pos setsum_divide_distrib [symmetric]) |
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from `convex C` and `y i \<in> C` and this and `0 \<le> a i` |
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and `0 \<le> setsum a s` and `a i + setsum a s = 1` |
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have "a i *\<^sub>R y i + setsum a s *\<^sub>R (\<Sum>j\<in>s. (a j / setsum a s) *\<^sub>R y j) \<in> C" |
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by (rule convexD) |
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then show ?thesis by (simp add: scaleR_setsum_right nz) |
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qed |
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then show ?case using `finite s` and `i \<notin> s` by simp |
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36623 | 149 |
qed |
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lemma convex: |
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49609 | 152 |
"convex s \<longleftrightarrow> (\<forall>(k::nat) u x. (\<forall>i. 1\<le>i \<and> i\<le>k \<longrightarrow> 0 \<le> u i \<and> x i \<in>s) \<and> (setsum u {1..k} = 1) |
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\<longrightarrow> setsum (\<lambda>i. u i *\<^sub>R x i) {1..k} \<in> s)" |
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36623 | 154 |
proof safe |
49609 | 155 |
fix k :: nat |
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fix u :: "nat \<Rightarrow> real" |
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fix x |
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36623 | 158 |
assume "convex s" |
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"\<forall>i. 1 \<le> i \<and> i \<le> k \<longrightarrow> 0 \<le> u i \<and> x i \<in> s" |
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"setsum u {1..k} = 1" |
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from this convex_setsum[of "{1 .. k}" s] |
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show "(\<Sum>j\<in>{1 .. k}. u j *\<^sub>R x j) \<in> s" by auto |
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next |
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assume asm: "\<forall>k u x. (\<forall> i :: nat. 1 \<le> i \<and> i \<le> k \<longrightarrow> 0 \<le> u i \<and> x i \<in> s) \<and> setsum u {1..k} = 1 |
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\<longrightarrow> (\<Sum>i = 1..k. u i *\<^sub>R (x i :: 'a)) \<in> s" |
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49609 | 166 |
{ fix \<mu> :: real |
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fix x y :: 'a |
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assume xy: "x \<in> s" "y \<in> s" |
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assume mu: "\<mu> \<ge> 0" "\<mu> \<le> 1" |
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let ?u = "\<lambda>i. if (i :: nat) = 1 then \<mu> else 1 - \<mu>" |
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let ?x = "\<lambda>i. if (i :: nat) = 1 then x else y" |
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36623 | 172 |
have "{1 :: nat .. 2} \<inter> - {x. x = 1} = {2}" by auto |
49609 | 173 |
then have card: "card ({1 :: nat .. 2} \<inter> - {x. x = 1}) = 1" by simp |
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then have "setsum ?u {1 .. 2} = 1" |
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36623 | 175 |
using setsum_cases[of "{(1 :: nat) .. 2}" "\<lambda> x. x = 1" "\<lambda> x. \<mu>" "\<lambda> x. 1 - \<mu>"] |
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by auto |
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49609 | 177 |
with asm[rule_format, of "2" ?u ?x] have s: "(\<Sum>j \<in> {1..2}. ?u j *\<^sub>R ?x j) \<in> s" |
36623 | 178 |
using mu xy by auto |
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have grarr: "(\<Sum>j \<in> {Suc (Suc 0)..2}. ?u j *\<^sub>R ?x j) = (1 - \<mu>) *\<^sub>R y" |
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using setsum_head_Suc[of "Suc (Suc 0)" 2 "\<lambda> j. (1 - \<mu>) *\<^sub>R y"] by auto |
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from setsum_head_Suc[of "Suc 0" 2 "\<lambda> j. ?u j *\<^sub>R ?x j", simplified this] |
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have "(\<Sum>j \<in> {1..2}. ?u j *\<^sub>R ?x j) = \<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y" by auto |
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49609 | 183 |
then have "(1 - \<mu>) *\<^sub>R y + \<mu> *\<^sub>R x \<in> s" using s by (auto simp:add_commute) |
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} |
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then show "convex s" unfolding convex_alt by auto |
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36623 | 186 |
qed |
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lemma convex_explicit: |
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fixes s :: "'a::real_vector set" |
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shows "convex s \<longleftrightarrow> |
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49609 | 192 |
(\<forall>t u. finite t \<and> t \<subseteq> s \<and> (\<forall>x\<in>t. 0 \<le> u x) \<and> setsum u t = 1 \<longrightarrow> setsum (\<lambda>x. u x *\<^sub>R x) t \<in> s)" |
36623 | 193 |
proof safe |
49609 | 194 |
fix t |
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fix u :: "'a \<Rightarrow> real" |
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36623 | 196 |
assume "convex s" "finite t" |
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"t \<subseteq> s" "\<forall>x\<in>t. 0 \<le> u x" "setsum u t = 1" |
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49609 | 198 |
then show "(\<Sum>x\<in>t. u x *\<^sub>R x) \<in> s" |
36623 | 199 |
using convex_setsum[of t s u "\<lambda> x. x"] by auto |
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next |
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assume asm0: "\<forall>t. \<forall> u. finite t \<and> t \<subseteq> s \<and> (\<forall>x\<in>t. 0 \<le> u x) |
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\<and> setsum u t = 1 \<longrightarrow> (\<Sum>x\<in>t. u x *\<^sub>R x) \<in> s" |
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show "convex s" |
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unfolding convex_alt |
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proof safe |
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49609 | 206 |
fix x y |
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fix \<mu> :: real |
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36623 | 208 |
assume asm: "x \<in> s" "y \<in> s" "0 \<le> \<mu>" "\<mu> \<le> 1" |
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{ assume "x \<noteq> y" |
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49609 | 210 |
then have "(1 - \<mu>) *\<^sub>R x + \<mu> *\<^sub>R y \<in> s" |
36623 | 211 |
using asm0[rule_format, of "{x, y}" "\<lambda> z. if z = x then 1 - \<mu> else \<mu>"] |
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asm by auto } |
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moreover |
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{ assume "x = y" |
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49609 | 215 |
then have "(1 - \<mu>) *\<^sub>R x + \<mu> *\<^sub>R y \<in> s" |
36623 | 216 |
using asm0[rule_format, of "{x, y}" "\<lambda> z. 1"] |
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asm by (auto simp:field_simps real_vector.scale_left_diff_distrib) } |
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ultimately show "(1 - \<mu>) *\<^sub>R x + \<mu> *\<^sub>R y \<in> s" by blast |
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qed |
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qed |
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||
49609 | 222 |
lemma convex_finite: |
223 |
assumes "finite s" |
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36623 | 224 |
shows "convex s \<longleftrightarrow> (\<forall>u. (\<forall>x\<in>s. 0 \<le> u x) \<and> setsum u s = 1 |
225 |
\<longrightarrow> setsum (\<lambda>x. u x *\<^sub>R x) s \<in> s)" |
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unfolding convex_explicit |
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49609 | 227 |
proof safe |
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fix t u |
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assume sum: "\<forall>u. (\<forall>x\<in>s. 0 \<le> u x) \<and> setsum u s = 1 \<longrightarrow> (\<Sum>x\<in>s. u x *\<^sub>R x) \<in> s" |
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36623 | 230 |
and as: "finite t" "t \<subseteq> s" "\<forall>x\<in>t. 0 \<le> u x" "setsum u t = (1::real)" |
49609 | 231 |
have *: "s \<inter> t = t" using as(2) by auto |
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have if_distrib_arg: "\<And>P f g x. (if P then f else g) x = (if P then f x else g x)" |
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by simp |
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36623 | 234 |
show "(\<Sum>x\<in>t. u x *\<^sub>R x) \<in> s" |
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using sum[THEN spec[where x="\<lambda>x. if x\<in>t then u x else 0"]] as * |
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by (auto simp: assms setsum_cases if_distrib if_distrib_arg) |
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qed (erule_tac x=s in allE, erule_tac x=u in allE, auto) |
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238 |
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55909 | 239 |
subsection {* Functions that are convex on a set *} |
240 |
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49609 | 241 |
definition convex_on :: "'a::real_vector set \<Rightarrow> ('a \<Rightarrow> real) \<Rightarrow> bool" |
242 |
where "convex_on s f \<longleftrightarrow> |
|
243 |
(\<forall>x\<in>s. \<forall>y\<in>s. \<forall>u\<ge>0. \<forall>v\<ge>0. u + v = 1 \<longrightarrow> f (u *\<^sub>R x + v *\<^sub>R y) \<le> u * f x + v * f y)" |
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36623 | 244 |
|
245 |
lemma convex_on_subset: "convex_on t f \<Longrightarrow> s \<subseteq> t \<Longrightarrow> convex_on s f" |
|
246 |
unfolding convex_on_def by auto |
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247 |
||
53620
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
248 |
lemma convex_on_add [intro]: |
36623 | 249 |
assumes "convex_on s f" "convex_on s g" |
250 |
shows "convex_on s (\<lambda>x. f x + g x)" |
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49609 | 251 |
proof - |
252 |
{ fix x y |
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253 |
assume "x\<in>s" "y\<in>s" |
|
254 |
moreover |
|
255 |
fix u v :: real |
|
256 |
assume "0 \<le> u" "0 \<le> v" "u + v = 1" |
|
257 |
ultimately |
|
258 |
have "f (u *\<^sub>R x + v *\<^sub>R y) + g (u *\<^sub>R x + v *\<^sub>R y) \<le> (u * f x + v * f y) + (u * g x + v * g y)" |
|
259 |
using assms unfolding convex_on_def by (auto simp add: add_mono) |
|
260 |
then have "f (u *\<^sub>R x + v *\<^sub>R y) + g (u *\<^sub>R x + v *\<^sub>R y) \<le> u * (f x + g x) + v * (f y + g y)" |
|
261 |
by (simp add: field_simps) |
|
262 |
} |
|
263 |
then show ?thesis unfolding convex_on_def by auto |
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36623 | 264 |
qed |
265 |
||
53620
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
266 |
lemma convex_on_cmul [intro]: |
36623 | 267 |
assumes "0 \<le> (c::real)" "convex_on s f" |
268 |
shows "convex_on s (\<lambda>x. c * f x)" |
|
269 |
proof- |
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49609 | 270 |
have *: "\<And>u c fx v fy ::real. u * (c * fx) + v * (c * fy) = c * (u * fx + v * fy)" |
271 |
by (simp add: field_simps) |
|
272 |
show ?thesis using assms(2) and mult_left_mono [OF _ assms(1)] |
|
273 |
unfolding convex_on_def and * by auto |
|
36623 | 274 |
qed |
275 |
||
276 |
lemma convex_lower: |
|
277 |
assumes "convex_on s f" "x\<in>s" "y \<in> s" "0 \<le> u" "0 \<le> v" "u + v = 1" |
|
278 |
shows "f (u *\<^sub>R x + v *\<^sub>R y) \<le> max (f x) (f y)" |
|
279 |
proof- |
|
280 |
let ?m = "max (f x) (f y)" |
|
281 |
have "u * f x + v * f y \<le> u * max (f x) (f y) + v * max (f x) (f y)" |
|
38642
8fa437809c67
dropped type classes mult_mono and mult_mono1; tuned names of technical rule duplicates
haftmann
parents:
36778
diff
changeset
|
282 |
using assms(4,5) by (auto simp add: mult_left_mono add_mono) |
49609 | 283 |
also have "\<dots> = max (f x) (f y)" using assms(6) unfolding distrib[symmetric] by auto |
36623 | 284 |
finally show ?thesis |
44890
22f665a2e91c
new fastforce replacing fastsimp - less confusing name
nipkow
parents:
44282
diff
changeset
|
285 |
using assms unfolding convex_on_def by fastforce |
36623 | 286 |
qed |
287 |
||
53620
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
288 |
lemma convex_on_dist [intro]: |
36623 | 289 |
fixes s :: "'a::real_normed_vector set" |
290 |
shows "convex_on s (\<lambda>x. dist a x)" |
|
49609 | 291 |
proof (auto simp add: convex_on_def dist_norm) |
292 |
fix x y |
|
293 |
assume "x\<in>s" "y\<in>s" |
|
294 |
fix u v :: real |
|
295 |
assume "0 \<le> u" "0 \<le> v" "u + v = 1" |
|
296 |
have "a = u *\<^sub>R a + v *\<^sub>R a" |
|
297 |
unfolding scaleR_left_distrib[symmetric] and `u+v=1` by simp |
|
298 |
then have *: "a - (u *\<^sub>R x + v *\<^sub>R y) = (u *\<^sub>R (a - x)) + (v *\<^sub>R (a - y))" |
|
36623 | 299 |
by (auto simp add: algebra_simps) |
300 |
show "norm (a - (u *\<^sub>R x + v *\<^sub>R y)) \<le> u * norm (a - x) + v * norm (a - y)" |
|
301 |
unfolding * using norm_triangle_ineq[of "u *\<^sub>R (a - x)" "v *\<^sub>R (a - y)"] |
|
302 |
using `0 \<le> u` `0 \<le> v` by auto |
|
303 |
qed |
|
304 |
||
49609 | 305 |
|
36623 | 306 |
subsection {* Arithmetic operations on sets preserve convexity. *} |
49609 | 307 |
|
53620
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
308 |
lemma convex_linear_image: |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
309 |
assumes "linear f" and "convex s" shows "convex (f ` s)" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
310 |
proof - |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
311 |
interpret f: linear f by fact |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
312 |
from `convex s` show "convex (f ` s)" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
313 |
by (simp add: convex_def f.scaleR [symmetric] f.add [symmetric]) |
36623 | 314 |
qed |
315 |
||
53620
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
316 |
lemma convex_linear_vimage: |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
317 |
assumes "linear f" and "convex s" shows "convex (f -` s)" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
318 |
proof - |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
319 |
interpret f: linear f by fact |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
320 |
from `convex s` show "convex (f -` s)" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
321 |
by (simp add: convex_def f.add f.scaleR) |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
322 |
qed |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
323 |
|
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
324 |
lemma convex_scaling: |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
325 |
assumes "convex s" shows "convex ((\<lambda>x. c *\<^sub>R x) ` s)" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
326 |
proof - |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
327 |
have "linear (\<lambda>x. c *\<^sub>R x)" by (simp add: linearI scaleR_add_right) |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
328 |
then show ?thesis using `convex s` by (rule convex_linear_image) |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
329 |
qed |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
330 |
|
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
331 |
lemma convex_negations: |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
332 |
assumes "convex s" shows "convex ((\<lambda>x. - x) ` s)" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
333 |
proof - |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
334 |
have "linear (\<lambda>x. - x)" by (simp add: linearI) |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
335 |
then show ?thesis using `convex s` by (rule convex_linear_image) |
36623 | 336 |
qed |
337 |
||
338 |
lemma convex_sums: |
|
53620
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
339 |
assumes "convex s" and "convex t" |
36623 | 340 |
shows "convex {x + y| x y. x \<in> s \<and> y \<in> t}" |
53620
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
341 |
proof - |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
342 |
have "linear (\<lambda>(x, y). x + y)" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
343 |
by (auto intro: linearI simp add: scaleR_add_right) |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
344 |
with assms have "convex ((\<lambda>(x, y). x + y) ` (s \<times> t))" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
345 |
by (intro convex_linear_image convex_Times) |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
346 |
also have "((\<lambda>(x, y). x + y) ` (s \<times> t)) = {x + y| x y. x \<in> s \<and> y \<in> t}" |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
347 |
by auto |
3c7f5e7926dc
generalized and simplified proofs of several theorems about convex sets
huffman
parents:
53596
diff
changeset
|
348 |
finally show ?thesis . |
36623 | 349 |
qed |
350 |
||
351 |
lemma convex_differences: |
|
352 |
assumes "convex s" "convex t" |
|
353 |
shows "convex {x - y| x y. x \<in> s \<and> y \<in> t}" |
|
354 |
proof - |
|
355 |
have "{x - y| x y. x \<in> s \<and> y \<in> t} = {x + y |x y. x \<in> s \<and> y \<in> uminus ` t}" |
|
54230
b1d955791529
more simplification rules on unary and binary minus
haftmann
parents:
53676
diff
changeset
|
356 |
by (auto simp add: diff_conv_add_uminus simp del: add_uminus_conv_diff) |
49609 | 357 |
then show ?thesis |
358 |
using convex_sums[OF assms(1) convex_negations[OF assms(2)]] by auto |
|
36623 | 359 |
qed |
360 |
||
49609 | 361 |
lemma convex_translation: |
362 |
assumes "convex s" |
|
363 |
shows "convex ((\<lambda>x. a + x) ` s)" |
|
364 |
proof - |
|
365 |
have "{a + y |y. y \<in> s} = (\<lambda>x. a + x) ` s" by auto |
|
366 |
then show ?thesis |
|
367 |
using convex_sums[OF convex_singleton[of a] assms] by auto |
|
368 |
qed |
|
36623 | 369 |
|
49609 | 370 |
lemma convex_affinity: |
371 |
assumes "convex s" |
|
372 |
shows "convex ((\<lambda>x. a + c *\<^sub>R x) ` s)" |
|
373 |
proof - |
|
374 |
have "(\<lambda>x. a + c *\<^sub>R x) ` s = op + a ` op *\<^sub>R c ` s" by auto |
|
375 |
then show ?thesis |
|
376 |
using convex_translation[OF convex_scaling[OF assms], of a c] by auto |
|
377 |
qed |
|
36623 | 378 |
|
49609 | 379 |
lemma pos_is_convex: "convex {0 :: real <..}" |
380 |
unfolding convex_alt |
|
36623 | 381 |
proof safe |
382 |
fix y x \<mu> :: real |
|
383 |
assume asms: "y > 0" "x > 0" "\<mu> \<ge> 0" "\<mu> \<le> 1" |
|
384 |
{ assume "\<mu> = 0" |
|
49609 | 385 |
then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y = y" by simp |
386 |
then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y > 0" using asms by simp } |
|
36623 | 387 |
moreover |
388 |
{ assume "\<mu> = 1" |
|
49609 | 389 |
then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y > 0" using asms by simp } |
36623 | 390 |
moreover |
391 |
{ assume "\<mu> \<noteq> 1" "\<mu> \<noteq> 0" |
|
49609 | 392 |
then have "\<mu> > 0" "(1 - \<mu>) > 0" using asms by auto |
393 |
then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y > 0" using asms |
|
36778
739a9379e29b
avoid using real-specific versions of generic lemmas
huffman
parents:
36648
diff
changeset
|
394 |
by (auto simp add: add_pos_pos mult_pos_pos) } |
44890
22f665a2e91c
new fastforce replacing fastsimp - less confusing name
nipkow
parents:
44282
diff
changeset
|
395 |
ultimately show "(1 - \<mu>) *\<^sub>R y + \<mu> *\<^sub>R x > 0" using assms by fastforce |
36623 | 396 |
qed |
397 |
||
398 |
lemma convex_on_setsum: |
|
399 |
fixes a :: "'a \<Rightarrow> real" |
|
49609 | 400 |
and y :: "'a \<Rightarrow> 'b::real_vector" |
401 |
and f :: "'b \<Rightarrow> real" |
|
36623 | 402 |
assumes "finite s" "s \<noteq> {}" |
49609 | 403 |
and "convex_on C f" |
404 |
and "convex C" |
|
405 |
and "(\<Sum> i \<in> s. a i) = 1" |
|
406 |
and "\<And>i. i \<in> s \<Longrightarrow> a i \<ge> 0" |
|
407 |
and "\<And>i. i \<in> s \<Longrightarrow> y i \<in> C" |
|
36623 | 408 |
shows "f (\<Sum> i \<in> s. a i *\<^sub>R y i) \<le> (\<Sum> i \<in> s. a i * f (y i))" |
49609 | 409 |
using assms |
410 |
proof (induct s arbitrary: a rule: finite_ne_induct) |
|
36623 | 411 |
case (singleton i) |
49609 | 412 |
then have ai: "a i = 1" by auto |
413 |
then show ?case by auto |
|
36623 | 414 |
next |
415 |
case (insert i s) note asms = this |
|
49609 | 416 |
then have "convex_on C f" by simp |
36623 | 417 |
from this[unfolded convex_on_def, rule_format] |
49609 | 418 |
have conv: "\<And>x y \<mu>. x \<in> C \<Longrightarrow> y \<in> C \<Longrightarrow> 0 \<le> \<mu> \<Longrightarrow> \<mu> \<le> 1 |
419 |
\<Longrightarrow> f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y" |
|
36623 | 420 |
by simp |
421 |
{ assume "a i = 1" |
|
49609 | 422 |
then have "(\<Sum> j \<in> s. a j) = 0" |
36623 | 423 |
using asms by auto |
49609 | 424 |
then have "\<And>j. j \<in> s \<Longrightarrow> a j = 0" |
44890
22f665a2e91c
new fastforce replacing fastsimp - less confusing name
nipkow
parents:
44282
diff
changeset
|
425 |
using setsum_nonneg_0[where 'b=real] asms by fastforce |
49609 | 426 |
then have ?case using asms by auto } |
36623 | 427 |
moreover |
428 |
{ assume asm: "a i \<noteq> 1" |
|
429 |
from asms have yai: "y i \<in> C" "a i \<ge> 0" by auto |
|
430 |
have fis: "finite (insert i s)" using asms by auto |
|
49609 | 431 |
then have ai1: "a i \<le> 1" using setsum_nonneg_leq_bound[of "insert i s" a] asms by simp |
432 |
then have "a i < 1" using asm by auto |
|
433 |
then have i0: "1 - a i > 0" by auto |
|
434 |
let ?a = "\<lambda>j. a j / (1 - a i)" |
|
36623 | 435 |
{ fix j assume "j \<in> s" |
49609 | 436 |
then have "?a j \<ge> 0" |
36623 | 437 |
using i0 asms divide_nonneg_pos |
49609 | 438 |
by fastforce } |
439 |
note a_nonneg = this |
|
36623 | 440 |
have "(\<Sum> j \<in> insert i s. a j) = 1" using asms by auto |
49609 | 441 |
then have "(\<Sum> j \<in> s. a j) = 1 - a i" using setsum.insert asms by fastforce |
442 |
then have "(\<Sum> j \<in> s. a j) / (1 - a i) = 1" using i0 by auto |
|
443 |
then have a1: "(\<Sum> j \<in> s. ?a j) = 1" unfolding setsum_divide_distrib by simp |
|
36623 | 444 |
have "convex C" using asms by auto |
49609 | 445 |
then have asum: "(\<Sum> j \<in> s. ?a j *\<^sub>R y j) \<in> C" |
36623 | 446 |
using asms convex_setsum[OF `finite s` |
447 |
`convex C` a1 a_nonneg] by auto |
|
448 |
have asum_le: "f (\<Sum> j \<in> s. ?a j *\<^sub>R y j) \<le> (\<Sum> j \<in> s. ?a j * f (y j))" |
|
449 |
using a_nonneg a1 asms by blast |
|
450 |
have "f (\<Sum> j \<in> insert i s. a j *\<^sub>R y j) = f ((\<Sum> j \<in> s. a j *\<^sub>R y j) + a i *\<^sub>R y i)" |
|
451 |
using setsum.insert[of s i "\<lambda> j. a j *\<^sub>R y j", OF `finite s` `i \<notin> s`] asms |
|
452 |
by (auto simp only:add_commute) |
|
453 |
also have "\<dots> = f (((1 - a i) * inverse (1 - a i)) *\<^sub>R (\<Sum> j \<in> s. a j *\<^sub>R y j) + a i *\<^sub>R y i)" |
|
454 |
using i0 by auto |
|
455 |
also have "\<dots> = f ((1 - a i) *\<^sub>R (\<Sum> j \<in> s. (a j * inverse (1 - a i)) *\<^sub>R y j) + a i *\<^sub>R y i)" |
|
49609 | 456 |
using scaleR_right.setsum[of "inverse (1 - a i)" "\<lambda> j. a j *\<^sub>R y j" s, symmetric] |
457 |
by (auto simp:algebra_simps) |
|
36623 | 458 |
also have "\<dots> = f ((1 - a i) *\<^sub>R (\<Sum> j \<in> s. ?a j *\<^sub>R y j) + a i *\<^sub>R y i)" |
36778
739a9379e29b
avoid using real-specific versions of generic lemmas
huffman
parents:
36648
diff
changeset
|
459 |
by (auto simp: divide_inverse) |
36623 | 460 |
also have "\<dots> \<le> (1 - a i) *\<^sub>R f ((\<Sum> j \<in> s. ?a j *\<^sub>R y j)) + a i * f (y i)" |
461 |
using conv[of "y i" "(\<Sum> j \<in> s. ?a j *\<^sub>R y j)" "a i", OF yai(1) asum yai(2) ai1] |
|
462 |
by (auto simp add:add_commute) |
|
463 |
also have "\<dots> \<le> (1 - a i) * (\<Sum> j \<in> s. ?a j * f (y j)) + a i * f (y i)" |
|
464 |
using add_right_mono[OF mult_left_mono[of _ _ "1 - a i", |
|
465 |
OF asum_le less_imp_le[OF i0]], of "a i * f (y i)"] by simp |
|
466 |
also have "\<dots> = (\<Sum> j \<in> s. (1 - a i) * ?a j * f (y j)) + a i * f (y i)" |
|
44282
f0de18b62d63
remove bounded_(bi)linear locale interpretations, to avoid duplicating so many lemmas
huffman
parents:
44142
diff
changeset
|
467 |
unfolding setsum_right_distrib[of "1 - a i" "\<lambda> j. ?a j * f (y j)"] using i0 by auto |
36623 | 468 |
also have "\<dots> = (\<Sum> j \<in> s. a j * f (y j)) + a i * f (y i)" using i0 by auto |
469 |
also have "\<dots> = (\<Sum> j \<in> insert i s. a j * f (y j))" using asms by auto |
|
470 |
finally have "f (\<Sum> j \<in> insert i s. a j *\<^sub>R y j) \<le> (\<Sum> j \<in> insert i s. a j * f (y j))" |
|
471 |
by simp } |
|
472 |
ultimately show ?case by auto |
|
473 |
qed |
|
474 |
||
475 |
lemma convex_on_alt: |
|
476 |
fixes C :: "'a::real_vector set" |
|
477 |
assumes "convex C" |
|
478 |
shows "convex_on C f = |
|
479 |
(\<forall> x \<in> C. \<forall> y \<in> C. \<forall> \<mu> :: real. \<mu> \<ge> 0 \<and> \<mu> \<le> 1 |
|
480 |
\<longrightarrow> f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y)" |
|
481 |
proof safe |
|
49609 | 482 |
fix x y |
483 |
fix \<mu> :: real |
|
36623 | 484 |
assume asms: "convex_on C f" "x \<in> C" "y \<in> C" "0 \<le> \<mu>" "\<mu> \<le> 1" |
485 |
from this[unfolded convex_on_def, rule_format] |
|
49609 | 486 |
have "\<And>u v. \<lbrakk>0 \<le> u; 0 \<le> v; u + v = 1\<rbrakk> \<Longrightarrow> f (u *\<^sub>R x + v *\<^sub>R y) \<le> u * f x + v * f y" by auto |
36623 | 487 |
from this[of "\<mu>" "1 - \<mu>", simplified] asms |
49609 | 488 |
show "f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y" by auto |
36623 | 489 |
next |
490 |
assume asm: "\<forall>x\<in>C. \<forall>y\<in>C. \<forall>\<mu>. 0 \<le> \<mu> \<and> \<mu> \<le> 1 \<longrightarrow> f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y" |
|
49609 | 491 |
{ fix x y |
492 |
fix u v :: real |
|
36623 | 493 |
assume lasm: "x \<in> C" "y \<in> C" "u \<ge> 0" "v \<ge> 0" "u + v = 1" |
49609 | 494 |
then have[simp]: "1 - u = v" by auto |
36623 | 495 |
from asm[rule_format, of x y u] |
49609 | 496 |
have "f (u *\<^sub>R x + v *\<^sub>R y) \<le> u * f x + v * f y" using lasm by auto |
497 |
} |
|
498 |
then show "convex_on C f" unfolding convex_on_def by auto |
|
36623 | 499 |
qed |
500 |
||
43337 | 501 |
lemma convex_on_diff: |
502 |
fixes f :: "real \<Rightarrow> real" |
|
503 |
assumes f: "convex_on I f" and I: "x\<in>I" "y\<in>I" and t: "x < t" "t < y" |
|
49609 | 504 |
shows "(f x - f t) / (x - t) \<le> (f x - f y) / (x - y)" |
505 |
"(f x - f y) / (x - y) \<le> (f t - f y) / (t - y)" |
|
43337 | 506 |
proof - |
507 |
def a \<equiv> "(t - y) / (x - y)" |
|
508 |
with t have "0 \<le> a" "0 \<le> 1 - a" by (auto simp: field_simps) |
|
509 |
with f `x \<in> I` `y \<in> I` have cvx: "f (a * x + (1 - a) * y) \<le> a * f x + (1 - a) * f y" |
|
510 |
by (auto simp: convex_on_def) |
|
511 |
have "a * x + (1 - a) * y = a * (x - y) + y" by (simp add: field_simps) |
|
512 |
also have "\<dots> = t" unfolding a_def using `x < t` `t < y` by simp |
|
513 |
finally have "f t \<le> a * f x + (1 - a) * f y" using cvx by simp |
|
514 |
also have "\<dots> = a * (f x - f y) + f y" by (simp add: field_simps) |
|
515 |
finally have "f t - f y \<le> a * (f x - f y)" by simp |
|
516 |
with t show "(f x - f t) / (x - t) \<le> (f x - f y) / (x - y)" |
|
44142 | 517 |
by (simp add: le_divide_eq divide_le_eq field_simps a_def) |
43337 | 518 |
with t show "(f x - f y) / (x - y) \<le> (f t - f y) / (t - y)" |
44142 | 519 |
by (simp add: le_divide_eq divide_le_eq field_simps) |
43337 | 520 |
qed |
36623 | 521 |
|
522 |
lemma pos_convex_function: |
|
523 |
fixes f :: "real \<Rightarrow> real" |
|
524 |
assumes "convex C" |
|
49609 | 525 |
and leq: "\<And>x y. \<lbrakk>x \<in> C ; y \<in> C\<rbrakk> \<Longrightarrow> f' x * (y - x) \<le> f y - f x" |
36623 | 526 |
shows "convex_on C f" |
49609 | 527 |
unfolding convex_on_alt[OF assms(1)] |
528 |
using assms |
|
36623 | 529 |
proof safe |
530 |
fix x y \<mu> :: real |
|
531 |
let ?x = "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y" |
|
532 |
assume asm: "convex C" "x \<in> C" "y \<in> C" "\<mu> \<ge> 0" "\<mu> \<le> 1" |
|
49609 | 533 |
then have "1 - \<mu> \<ge> 0" by auto |
534 |
then have xpos: "?x \<in> C" using asm unfolding convex_alt by fastforce |
|
36623 | 535 |
have geq: "\<mu> * (f x - f ?x) + (1 - \<mu>) * (f y - f ?x) |
536 |
\<ge> \<mu> * f' ?x * (x - ?x) + (1 - \<mu>) * f' ?x * (y - ?x)" |
|
38642
8fa437809c67
dropped type classes mult_mono and mult_mono1; tuned names of technical rule duplicates
haftmann
parents:
36778
diff
changeset
|
537 |
using add_mono[OF mult_left_mono[OF leq[OF xpos asm(2)] `\<mu> \<ge> 0`] |
8fa437809c67
dropped type classes mult_mono and mult_mono1; tuned names of technical rule duplicates
haftmann
parents:
36778
diff
changeset
|
538 |
mult_left_mono[OF leq[OF xpos asm(3)] `1 - \<mu> \<ge> 0`]] by auto |
49609 | 539 |
then have "\<mu> * f x + (1 - \<mu>) * f y - f ?x \<ge> 0" |
540 |
by (auto simp add: field_simps) |
|
541 |
then show "f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y" |
|
36623 | 542 |
using convex_on_alt by auto |
543 |
qed |
|
544 |
||
545 |
lemma atMostAtLeast_subset_convex: |
|
546 |
fixes C :: "real set" |
|
547 |
assumes "convex C" |
|
49609 | 548 |
and "x \<in> C" "y \<in> C" "x < y" |
36623 | 549 |
shows "{x .. y} \<subseteq> C" |
550 |
proof safe |
|
551 |
fix z assume zasm: "z \<in> {x .. y}" |
|
552 |
{ assume asm: "x < z" "z < y" |
|
49609 | 553 |
let ?\<mu> = "(y - z) / (y - x)" |
554 |
have "0 \<le> ?\<mu>" "?\<mu> \<le> 1" using assms asm by (auto simp add: field_simps) |
|
555 |
then have comb: "?\<mu> * x + (1 - ?\<mu>) * y \<in> C" |
|
556 |
using assms iffD1[OF convex_alt, rule_format, of C y x ?\<mu>] |
|
557 |
by (simp add: algebra_simps) |
|
36623 | 558 |
have "?\<mu> * x + (1 - ?\<mu>) * y = (y - z) * x / (y - x) + (1 - (y - z) / (y - x)) * y" |
49609 | 559 |
by (auto simp add: field_simps) |
36623 | 560 |
also have "\<dots> = ((y - z) * x + (y - x - (y - z)) * y) / (y - x)" |
49609 | 561 |
using assms unfolding add_divide_distrib by (auto simp: field_simps) |
36623 | 562 |
also have "\<dots> = z" |
49609 | 563 |
using assms by (auto simp: field_simps) |
36623 | 564 |
finally have "z \<in> C" |
49609 | 565 |
using comb by auto } |
566 |
note less = this |
|
36623 | 567 |
show "z \<in> C" using zasm less assms |
568 |
unfolding atLeastAtMost_iff le_less by auto |
|
569 |
qed |
|
570 |
||
571 |
lemma f''_imp_f': |
|
572 |
fixes f :: "real \<Rightarrow> real" |
|
573 |
assumes "convex C" |
|
49609 | 574 |
and f': "\<And>x. x \<in> C \<Longrightarrow> DERIV f x :> (f' x)" |
575 |
and f'': "\<And>x. x \<in> C \<Longrightarrow> DERIV f' x :> (f'' x)" |
|
576 |
and pos: "\<And>x. x \<in> C \<Longrightarrow> f'' x \<ge> 0" |
|
577 |
and "x \<in> C" "y \<in> C" |
|
36623 | 578 |
shows "f' x * (y - x) \<le> f y - f x" |
49609 | 579 |
using assms |
36623 | 580 |
proof - |
49609 | 581 |
{ fix x y :: real |
582 |
assume asm: "x \<in> C" "y \<in> C" "y > x" |
|
583 |
then have ge: "y - x > 0" "y - x \<ge> 0" by auto |
|
36623 | 584 |
from asm have le: "x - y < 0" "x - y \<le> 0" by auto |
585 |
then obtain z1 where z1: "z1 > x" "z1 < y" "f y - f x = (y - x) * f' z1" |
|
586 |
using subsetD[OF atMostAtLeast_subset_convex[OF `convex C` `x \<in> C` `y \<in> C` `x < y`], |
|
587 |
THEN f', THEN MVT2[OF `x < y`, rule_format, unfolded atLeastAtMost_iff[symmetric]]] |
|
588 |
by auto |
|
49609 | 589 |
then have "z1 \<in> C" using atMostAtLeast_subset_convex |
44890
22f665a2e91c
new fastforce replacing fastsimp - less confusing name
nipkow
parents:
44282
diff
changeset
|
590 |
`convex C` `x \<in> C` `y \<in> C` `x < y` by fastforce |
36623 | 591 |
from z1 have z1': "f x - f y = (x - y) * f' z1" |
592 |
by (simp add:field_simps) |
|
593 |
obtain z2 where z2: "z2 > x" "z2 < z1" "f' z1 - f' x = (z1 - x) * f'' z2" |
|
594 |
using subsetD[OF atMostAtLeast_subset_convex[OF `convex C` `x \<in> C` `z1 \<in> C` `x < z1`], |
|
595 |
THEN f'', THEN MVT2[OF `x < z1`, rule_format, unfolded atLeastAtMost_iff[symmetric]]] z1 |
|
596 |
by auto |
|
597 |
obtain z3 where z3: "z3 > z1" "z3 < y" "f' y - f' z1 = (y - z1) * f'' z3" |
|
598 |
using subsetD[OF atMostAtLeast_subset_convex[OF `convex C` `z1 \<in> C` `y \<in> C` `z1 < y`], |
|
599 |
THEN f'', THEN MVT2[OF `z1 < y`, rule_format, unfolded atLeastAtMost_iff[symmetric]]] z1 |
|
600 |
by auto |
|
601 |
have "f' y - (f x - f y) / (x - y) = f' y - f' z1" |
|
602 |
using asm z1' by auto |
|
603 |
also have "\<dots> = (y - z1) * f'' z3" using z3 by auto |
|
604 |
finally have cool': "f' y - (f x - f y) / (x - y) = (y - z1) * f'' z3" by simp |
|
605 |
have A': "y - z1 \<ge> 0" using z1 by auto |
|
606 |
have "z3 \<in> C" using z3 asm atMostAtLeast_subset_convex |
|
44890
22f665a2e91c
new fastforce replacing fastsimp - less confusing name
nipkow
parents:
44282
diff
changeset
|
607 |
`convex C` `x \<in> C` `z1 \<in> C` `x < z1` by fastforce |
49609 | 608 |
then have B': "f'' z3 \<ge> 0" using assms by auto |
36623 | 609 |
from A' B' have "(y - z1) * f'' z3 \<ge> 0" using mult_nonneg_nonneg by auto |
610 |
from cool' this have "f' y - (f x - f y) / (x - y) \<ge> 0" by auto |
|
611 |
from mult_right_mono_neg[OF this le(2)] |
|
612 |
have "f' y * (x - y) - (f x - f y) / (x - y) * (x - y) \<le> 0 * (x - y)" |
|
36778
739a9379e29b
avoid using real-specific versions of generic lemmas
huffman
parents:
36648
diff
changeset
|
613 |
by (simp add: algebra_simps) |
49609 | 614 |
then have "f' y * (x - y) - (f x - f y) \<le> 0" using le by auto |
615 |
then have res: "f' y * (x - y) \<le> f x - f y" by auto |
|
36623 | 616 |
have "(f y - f x) / (y - x) - f' x = f' z1 - f' x" |
617 |
using asm z1 by auto |
|
618 |
also have "\<dots> = (z1 - x) * f'' z2" using z2 by auto |
|
619 |
finally have cool: "(f y - f x) / (y - x) - f' x = (z1 - x) * f'' z2" by simp |
|
620 |
have A: "z1 - x \<ge> 0" using z1 by auto |
|
621 |
have "z2 \<in> C" using z2 z1 asm atMostAtLeast_subset_convex |
|
44890
22f665a2e91c
new fastforce replacing fastsimp - less confusing name
nipkow
parents:
44282
diff
changeset
|
622 |
`convex C` `z1 \<in> C` `y \<in> C` `z1 < y` by fastforce |
49609 | 623 |
then have B: "f'' z2 \<ge> 0" using assms by auto |
36623 | 624 |
from A B have "(z1 - x) * f'' z2 \<ge> 0" using mult_nonneg_nonneg by auto |
625 |
from cool this have "(f y - f x) / (y - x) - f' x \<ge> 0" by auto |
|
626 |
from mult_right_mono[OF this ge(2)] |
|
627 |
have "(f y - f x) / (y - x) * (y - x) - f' x * (y - x) \<ge> 0 * (y - x)" |
|
36778
739a9379e29b
avoid using real-specific versions of generic lemmas
huffman
parents:
36648
diff
changeset
|
628 |
by (simp add: algebra_simps) |
49609 | 629 |
then have "f y - f x - f' x * (y - x) \<ge> 0" using ge by auto |
630 |
then have "f y - f x \<ge> f' x * (y - x)" "f' y * (x - y) \<le> f x - f y" |
|
36623 | 631 |
using res by auto } note less_imp = this |
49609 | 632 |
{ fix x y :: real |
633 |
assume "x \<in> C" "y \<in> C" "x \<noteq> y" |
|
634 |
then have"f y - f x \<ge> f' x * (y - x)" |
|
36623 | 635 |
unfolding neq_iff using less_imp by auto } note neq_imp = this |
636 |
moreover |
|
49609 | 637 |
{ fix x y :: real |
638 |
assume asm: "x \<in> C" "y \<in> C" "x = y" |
|
639 |
then have "f y - f x \<ge> f' x * (y - x)" by auto } |
|
36623 | 640 |
ultimately show ?thesis using assms by blast |
641 |
qed |
|
642 |
||
643 |
lemma f''_ge0_imp_convex: |
|
644 |
fixes f :: "real \<Rightarrow> real" |
|
645 |
assumes conv: "convex C" |
|
49609 | 646 |
and f': "\<And>x. x \<in> C \<Longrightarrow> DERIV f x :> (f' x)" |
647 |
and f'': "\<And>x. x \<in> C \<Longrightarrow> DERIV f' x :> (f'' x)" |
|
648 |
and pos: "\<And>x. x \<in> C \<Longrightarrow> f'' x \<ge> 0" |
|
36623 | 649 |
shows "convex_on C f" |
44890
22f665a2e91c
new fastforce replacing fastsimp - less confusing name
nipkow
parents:
44282
diff
changeset
|
650 |
using f''_imp_f'[OF conv f' f'' pos] assms pos_convex_function by fastforce |
36623 | 651 |
|
652 |
lemma minus_log_convex: |
|
653 |
fixes b :: real |
|
654 |
assumes "b > 1" |
|
655 |
shows "convex_on {0 <..} (\<lambda> x. - log b x)" |
|
656 |
proof - |
|
49609 | 657 |
have "\<And>z. z > 0 \<Longrightarrow> DERIV (log b) z :> 1 / (ln b * z)" using DERIV_log by auto |
658 |
then have f': "\<And>z. z > 0 \<Longrightarrow> DERIV (\<lambda> z. - log b z) z :> - 1 / (ln b * z)" |
|
51642
400ec5ae7f8f
move FrechetDeriv from the Library to HOL/Deriv; base DERIV on FDERIV and both derivatives allow a restricted support set; FDERIV is now an abbreviation of has_derivative
hoelzl
parents:
49609
diff
changeset
|
659 |
by (auto simp: DERIV_minus) |
49609 | 660 |
have "\<And>z :: real. z > 0 \<Longrightarrow> DERIV inverse z :> - (inverse z ^ Suc (Suc 0))" |
36623 | 661 |
using less_imp_neq[THEN not_sym, THEN DERIV_inverse] by auto |
662 |
from this[THEN DERIV_cmult, of _ "- 1 / ln b"] |
|
49609 | 663 |
have "\<And>z :: real. z > 0 \<Longrightarrow> |
664 |
DERIV (\<lambda> z. (- 1 / ln b) * inverse z) z :> (- 1 / ln b) * (- (inverse z ^ Suc (Suc 0)))" |
|
36623 | 665 |
by auto |
49609 | 666 |
then have f''0: "\<And>z :: real. z > 0 \<Longrightarrow> DERIV (\<lambda> z. - 1 / (ln b * z)) z :> 1 / (ln b * z * z)" |
36778
739a9379e29b
avoid using real-specific versions of generic lemmas
huffman
parents:
36648
diff
changeset
|
667 |
unfolding inverse_eq_divide by (auto simp add: mult_assoc) |
49609 | 668 |
have f''_ge0: "\<And>z :: real. z > 0 \<Longrightarrow> 1 / (ln b * z * z) \<ge> 0" |
669 |
using `b > 1` by (auto intro!:less_imp_le simp add: divide_pos_pos[of 1] mult_pos_pos) |
|
36623 | 670 |
from f''_ge0_imp_convex[OF pos_is_convex, |
671 |
unfolded greaterThan_iff, OF f' f''0 f''_ge0] |
|
672 |
show ?thesis by auto |
|
673 |
qed |
|
674 |
||
675 |
end |