(*  Title:      HOL/Library/Convex.thy

    Author:     Armin Heller, TU Muenchen

    Author:     Johannes Hoelzl, TU Muenchen

*)

header {* Convexity in real vector spaces *}

theory Convex

imports Product_Vector

begin

subsection {* Convexity. *}

definition convex :: "'a::real_vector set \<Rightarrow> bool"

  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)"

lemma convexI:

  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"

  shows "convex s"

  using assms unfolding convex_def by fast

lemma convexD:

  assumes "convex s" and "x \<in> s" and "y \<in> s" and "0 \<le> u" and "0 \<le> v" and "u + v = 1"

  shows "u *\<^sub>R x + v *\<^sub>R y \<in> s"

  using assms unfolding convex_def by fast

lemma convex_alt:

  "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)"

  (is "_ \<longleftrightarrow> ?alt")

proof

  assume alt[rule_format]: ?alt

  { fix x y and u v :: real assume mem: "x \<in> s" "y \<in> s"

    assume "0 \<le> u" "0 \<le> v"

    moreover assume "u + v = 1" then have "u = 1 - v" by auto

    ultimately have "u *\<^sub>R x + v *\<^sub>R y \<in> s" using alt[OF mem] by auto }

  then show "convex s" unfolding convex_def by auto

qed (auto simp: convex_def)

lemma mem_convex:

  assumes "convex s" "a \<in> s" "b \<in> s" "0 \<le> u" "u \<le> 1"

  shows "((1 - u) *\<^sub>R a + u *\<^sub>R b) \<in> s"

  using assms unfolding convex_alt by auto

lemma convex_empty[intro]: "convex {}"

  unfolding convex_def by simp

lemma convex_singleton[intro]: "convex {a}"

  unfolding convex_def by (auto simp: scaleR_left_distrib[symmetric])

lemma convex_UNIV[intro]: "convex UNIV"

  unfolding convex_def by auto

lemma convex_Inter: "(\<forall>s\<in>f. convex s) ==> convex(\<Inter> f)"

  unfolding convex_def by auto

lemma convex_Int: "convex s \<Longrightarrow> convex t \<Longrightarrow> convex (s \<inter> t)"

  unfolding convex_def by auto

lemma convex_INT: "\<forall>i\<in>A. convex (B i) \<Longrightarrow> convex (\<Inter>i\<in>A. B i)"

  unfolding convex_def by auto

lemma convex_Times: "convex s \<Longrightarrow> convex t \<Longrightarrow> convex (s \<times> t)"

  unfolding convex_def by auto

lemma convex_halfspace_le: "convex {x. inner a x \<le> b}"

  unfolding convex_def

  by (auto simp: inner_add intro!: convex_bound_le)

lemma convex_halfspace_ge: "convex {x. inner a x \<ge> b}"

proof -

  have *: "{x. inner a x \<ge> b} = {x. inner (-a) x \<le> -b}" by auto

  show ?thesis unfolding * using convex_halfspace_le[of "-a" "-b"] by auto

qed

lemma convex_hyperplane: "convex {x. inner a x = b}"

proof -

  have *: "{x. inner a x = b} = {x. inner a x \<le> b} \<inter> {x. inner a x \<ge> b}" by auto

  show ?thesis using convex_halfspace_le convex_halfspace_ge

    by (auto intro!: convex_Int simp: *)

qed

lemma convex_halfspace_lt: "convex {x. inner a x < b}"

  unfolding convex_def

  by (auto simp: convex_bound_lt inner_add)

lemma convex_halfspace_gt: "convex {x. inner a x > b}"

   using convex_halfspace_lt[of "-a" "-b"] by auto

lemma convex_real_interval:

  fixes a b :: "real"

  shows "convex {a..}" and "convex {..b}"

    and "convex {a<..}" and "convex {..<b}"

    and "convex {a..b}" and "convex {a<..b}"

    and "convex {a..<b}" and "convex {a<..<b}"

proof -

  have "{a..} = {x. a \<le> inner 1 x}" by auto

  then show 1: "convex {a..}" by (simp only: convex_halfspace_ge)

  have "{..b} = {x. inner 1 x \<le> b}" by auto

  then show 2: "convex {..b}" by (simp only: convex_halfspace_le)

  have "{a<..} = {x. a < inner 1 x}" by auto

  then show 3: "convex {a<..}" by (simp only: convex_halfspace_gt)

  have "{..<b} = {x. inner 1 x < b}" by auto

  then show 4: "convex {..<b}" by (simp only: convex_halfspace_lt)

  have "{a..b} = {a..} \<inter> {..b}" by auto

  then show "convex {a..b}" by (simp only: convex_Int 1 2)

  have "{a<..b} = {a<..} \<inter> {..b}" by auto

  then show "convex {a<..b}" by (simp only: convex_Int 3 2)

  have "{a..<b} = {a..} \<inter> {..<b}" by auto

  then show "convex {a..<b}" by (simp only: convex_Int 1 4)

  have "{a<..<b} = {a<..} \<inter> {..<b}" by auto

  then show "convex {a<..<b}" by (simp only: convex_Int 3 4)

qed

subsection {* Explicit expressions for convexity in terms of arbitrary sums. *}

lemma convex_setsum:

  fixes C :: "'a::real_vector set"

  assumes "finite s" and "convex C" and "(\<Sum> i \<in> s. a i) = 1"

  assumes "\<And>i. i \<in> s \<Longrightarrow> a i \<ge> 0" and "\<And>i. i \<in> s \<Longrightarrow> y i \<in> C"

  shows "(\<Sum> j \<in> s. a j *\<^sub>R y j) \<in> C"

  using assms(1,3,4,5)

proof (induct arbitrary: a set: finite)

  case empty

  then show ?case by simp

next

  case (insert i s) note IH = this(3)

  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"

    using insert.hyps(1,2) insert.prems by simp_all

  then have "0 \<le> setsum a s" by (simp add: setsum_nonneg)

  have "a i *\<^sub>R y i + (\<Sum>j\<in>s. a j *\<^sub>R y j) \<in> C"

  proof (cases)

    assume z: "setsum a s = 0"

    with `a i + setsum a s = 1` have "a i = 1" by simp

    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

    show ?thesis using `a i = 1` and `\<forall>j\<in>s. a j = 0` and `y i \<in> C` by simp

  next

    assume nz: "setsum a s \<noteq> 0"

    with `0 \<le> setsum a s` have "0 < setsum a s" by simp

    then have "(\<Sum>j\<in>s. (a j / setsum a s) *\<^sub>R y j) \<in> C"

      using `\<forall>j\<in>s. 0 \<le> a j` and `\<forall>j\<in>s. y j \<in> C`

      by (simp add: IH divide_nonneg_pos setsum_divide_distrib [symmetric])

    from `convex C` and `y i \<in> C` and this and `0 \<le> a i`

      and `0 \<le> setsum a s` and `a i + setsum a s = 1`

    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"

      by (rule convexD)

    then show ?thesis by (simp add: scaleR_setsum_right nz)

  qed

  then show ?case using `finite s` and `i \<notin> s` by simp

qed

lemma convex:

  "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)

      \<longrightarrow> setsum (\<lambda>i. u i *\<^sub>R x i) {1..k} \<in> s)"

proof safe

  fix k :: nat

  fix u :: "nat \<Rightarrow> real"

  fix x

  assume "convex s"

    "\<forall>i. 1 \<le> i \<and> i \<le> k \<longrightarrow> 0 \<le> u i \<and> x i \<in> s"

    "setsum u {1..k} = 1"

  from this convex_setsum[of "{1 .. k}" s]

  show "(\<Sum>j\<in>{1 .. k}. u j *\<^sub>R x j) \<in> s" by auto

next

  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

    \<longrightarrow> (\<Sum>i = 1..k. u i *\<^sub>R (x i :: 'a)) \<in> s"

  { fix \<mu> :: real

    fix x y :: 'a

    assume xy: "x \<in> s" "y \<in> s"

    assume mu: "\<mu> \<ge> 0" "\<mu> \<le> 1"

    let ?u = "\<lambda>i. if (i :: nat) = 1 then \<mu> else 1 - \<mu>"

    let ?x = "\<lambda>i. if (i :: nat) = 1 then x else y"

    have "{1 :: nat .. 2} \<inter> - {x. x = 1} = {2}" by auto

    then have card: "card ({1 :: nat .. 2} \<inter> - {x. x = 1}) = 1" by simp

    then have "setsum ?u {1 .. 2} = 1"

      using setsum_cases[of "{(1 :: nat) .. 2}" "\<lambda> x. x = 1" "\<lambda> x. \<mu>" "\<lambda> x. 1 - \<mu>"]

      by auto

    with asm[rule_format, of "2" ?u ?x] have s: "(\<Sum>j \<in> {1..2}. ?u j *\<^sub>R ?x j) \<in> s"

      using mu xy by auto

    have grarr: "(\<Sum>j \<in> {Suc (Suc 0)..2}. ?u j *\<^sub>R ?x j) = (1 - \<mu>) *\<^sub>R y"

      using setsum_head_Suc[of "Suc (Suc 0)" 2 "\<lambda> j. (1 - \<mu>) *\<^sub>R y"] by auto

    from setsum_head_Suc[of "Suc 0" 2 "\<lambda> j. ?u j *\<^sub>R ?x j", simplified this]

    have "(\<Sum>j \<in> {1..2}. ?u j *\<^sub>R ?x j) = \<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y" by auto

    then have "(1 - \<mu>) *\<^sub>R y + \<mu> *\<^sub>R x \<in> s" using s by (auto simp:add_commute)

  }

  then show "convex s" unfolding convex_alt by auto

qed

lemma convex_explicit:

  fixes s :: "'a::real_vector set"

  shows "convex s \<longleftrightarrow>

    (\<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)"

proof safe

  fix t

  fix u :: "'a \<Rightarrow> real"

  assume "convex s" "finite t"

    "t \<subseteq> s" "\<forall>x\<in>t. 0 \<le> u x" "setsum u t = 1"

  then show "(\<Sum>x\<in>t. u x *\<^sub>R x) \<in> s"

    using convex_setsum[of t s u "\<lambda> x. x"] by auto

next

  assume asm0: "\<forall>t. \<forall> u. finite t \<and> t \<subseteq> s \<and> (\<forall>x\<in>t. 0 \<le> u x)

    \<and> setsum u t = 1 \<longrightarrow> (\<Sum>x\<in>t. u x *\<^sub>R x) \<in> s"

  show "convex s"

    unfolding convex_alt

  proof safe

    fix x y

    fix \<mu> :: real

    assume asm: "x \<in> s" "y \<in> s" "0 \<le> \<mu>" "\<mu> \<le> 1"

    { assume "x \<noteq> y"

      then have "(1 - \<mu>) *\<^sub>R x + \<mu> *\<^sub>R y \<in> s"

        using asm0[rule_format, of "{x, y}" "\<lambda> z. if z = x then 1 - \<mu> else \<mu>"]

          asm by auto }

    moreover

    { assume "x = y"

      then have "(1 - \<mu>) *\<^sub>R x + \<mu> *\<^sub>R y \<in> s"

        using asm0[rule_format, of "{x, y}" "\<lambda> z. 1"]

          asm by (auto simp:field_simps real_vector.scale_left_diff_distrib) }

    ultimately show "(1 - \<mu>) *\<^sub>R x + \<mu> *\<^sub>R y \<in> s" by blast

  qed

qed

lemma convex_finite:

  assumes "finite s"

  shows "convex s \<longleftrightarrow> (\<forall>u. (\<forall>x\<in>s. 0 \<le> u x) \<and> setsum u s = 1

                      \<longrightarrow> setsum (\<lambda>x. u x *\<^sub>R x) s \<in> s)"

  unfolding convex_explicit

proof safe

  fix t u

  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"

    and as: "finite t" "t \<subseteq> s" "\<forall>x\<in>t. 0 \<le> u x" "setsum u t = (1::real)"

  have *: "s \<inter> t = t" using as(2) by auto

  have if_distrib_arg: "\<And>P f g x. (if P then f else g) x = (if P then f x else g x)"

    by simp

  show "(\<Sum>x\<in>t. u x *\<^sub>R x) \<in> s"

   using sum[THEN spec[where x="\<lambda>x. if x\<in>t then u x else 0"]] as *

   by (auto simp: assms setsum_cases if_distrib if_distrib_arg)

qed (erule_tac x=s in allE, erule_tac x=u in allE, auto)

subsection {* Functions that are convex on a set *}

definition convex_on :: "'a::real_vector set \<Rightarrow> ('a \<Rightarrow> real) \<Rightarrow> bool"

  where "convex_on s f \<longleftrightarrow>

    (\<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)"

lemma convex_on_subset: "convex_on t f \<Longrightarrow> s \<subseteq> t \<Longrightarrow> convex_on s f"

  unfolding convex_on_def by auto

lemma convex_on_add [intro]:

  assumes "convex_on s f" "convex_on s g"

  shows "convex_on s (\<lambda>x. f x + g x)"

proof -

  { fix x y

    assume "x\<in>s" "y\<in>s"

    moreover

    fix u v :: real

    assume "0 \<le> u" "0 \<le> v" "u + v = 1"

    ultimately

    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)"

      using assms unfolding convex_on_def by (auto simp add: add_mono)

    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)"

      by (simp add: field_simps)

  }

  then show ?thesis unfolding convex_on_def by auto

qed

lemma convex_on_cmul [intro]:

  assumes "0 \<le> (c::real)" "convex_on s f"

  shows "convex_on s (\<lambda>x. c * f x)"

proof-

  have *: "\<And>u c fx v fy ::real. u * (c * fx) + v * (c * fy) = c * (u * fx + v * fy)"

    by (simp add: field_simps)

  show ?thesis using assms(2) and mult_left_mono [OF _ assms(1)]

    unfolding convex_on_def and * by auto

qed

lemma convex_lower:

  assumes "convex_on s f"  "x\<in>s"  "y \<in> s"  "0 \<le> u"  "0 \<le> v"  "u + v = 1"

  shows "f (u *\<^sub>R x + v *\<^sub>R y) \<le> max (f x) (f y)"

proof-

  let ?m = "max (f x) (f y)"

  have "u * f x + v * f y \<le> u * max (f x) (f y) + v * max (f x) (f y)"

    using assms(4,5) by (auto simp add: mult_left_mono add_mono)

  also have "\<dots> = max (f x) (f y)" using assms(6) unfolding distrib[symmetric] by auto

  finally show ?thesis

    using assms unfolding convex_on_def by fastforce

qed

lemma convex_on_dist [intro]:

  fixes s :: "'a::real_normed_vector set"

  shows "convex_on s (\<lambda>x. dist a x)"

proof (auto simp add: convex_on_def dist_norm)

  fix x y

  assume "x\<in>s" "y\<in>s"

  fix u v :: real

  assume "0 \<le> u" "0 \<le> v" "u + v = 1"

  have "a = u *\<^sub>R a + v *\<^sub>R a"

    unfolding scaleR_left_distrib[symmetric] and `u+v=1` by simp

  then have *: "a - (u *\<^sub>R x + v *\<^sub>R y) = (u *\<^sub>R (a - x)) + (v *\<^sub>R (a - y))"

    by (auto simp add: algebra_simps)

  show "norm (a - (u *\<^sub>R x + v *\<^sub>R y)) \<le> u * norm (a - x) + v * norm (a - y)"

    unfolding * using norm_triangle_ineq[of "u *\<^sub>R (a - x)" "v *\<^sub>R (a - y)"]

    using `0 \<le> u` `0 \<le> v` by auto

qed

subsection {* Arithmetic operations on sets preserve convexity. *}

lemma convex_linear_image:

  assumes "linear f" and "convex s" shows "convex (f ` s)"

proof -

  interpret f: linear f by fact

  from `convex s` show "convex (f ` s)"

    by (simp add: convex_def f.scaleR [symmetric] f.add [symmetric])

qed

lemma convex_linear_vimage:

  assumes "linear f" and "convex s" shows "convex (f -` s)"

proof -

  interpret f: linear f by fact

  from `convex s` show "convex (f -` s)"

    by (simp add: convex_def f.add f.scaleR)

qed

lemma convex_scaling:

  assumes "convex s" shows "convex ((\<lambda>x. c *\<^sub>R x) ` s)"

proof -

  have "linear (\<lambda>x. c *\<^sub>R x)" by (simp add: linearI scaleR_add_right)

  then show ?thesis using `convex s` by (rule convex_linear_image)

qed

lemma convex_negations:

  assumes "convex s" shows "convex ((\<lambda>x. - x) ` s)"

proof -

  have "linear (\<lambda>x. - x)" by (simp add: linearI)

  then show ?thesis using `convex s` by (rule convex_linear_image)

qed

lemma convex_sums:

  assumes "convex s" and "convex t"

  shows "convex {x + y| x y. x \<in> s \<and> y \<in> t}"

proof -

  have "linear (\<lambda>(x, y). x + y)"

    by (auto intro: linearI simp add: scaleR_add_right)

  with assms have "convex ((\<lambda>(x, y). x + y) ` (s \<times> t))"

    by (intro convex_linear_image convex_Times)

  also have "((\<lambda>(x, y). x + y) ` (s \<times> t)) = {x + y| x y. x \<in> s \<and> y \<in> t}"

    by auto

  finally show ?thesis .

qed

lemma convex_differences:

  assumes "convex s" "convex t"

  shows "convex {x - y| x y. x \<in> s \<and> y \<in> t}"

proof -

  have "{x - y| x y. x \<in> s \<and> y \<in> t} = {x + y |x y. x \<in> s \<and> y \<in> uminus ` t}"

    by (auto simp add: diff_conv_add_uminus simp del: add_uminus_conv_diff)

  then show ?thesis

    using convex_sums[OF assms(1) convex_negations[OF assms(2)]] by auto

qed

lemma convex_translation:

  assumes "convex s"

  shows "convex ((\<lambda>x. a + x) ` s)"

proof -

  have "{a + y |y. y \<in> s} = (\<lambda>x. a + x) ` s" by auto

  then show ?thesis

    using convex_sums[OF convex_singleton[of a] assms] by auto

qed

lemma convex_affinity:

  assumes "convex s"

  shows "convex ((\<lambda>x. a + c *\<^sub>R x) ` s)"

proof -

  have "(\<lambda>x. a + c *\<^sub>R x) ` s = op + a ` op *\<^sub>R c ` s" by auto

  then show ?thesis

    using convex_translation[OF convex_scaling[OF assms], of a c] by auto

qed

lemma pos_is_convex: "convex {0 :: real <..}"

  unfolding convex_alt

proof safe

  fix y x \<mu> :: real

  assume asms: "y > 0" "x > 0" "\<mu> \<ge> 0" "\<mu> \<le> 1"

  { assume "\<mu> = 0"

    then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y = y" by simp

    then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y > 0" using asms by simp }

  moreover

  { assume "\<mu> = 1"

    then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y > 0" using asms by simp }

  moreover

  { assume "\<mu> \<noteq> 1" "\<mu> \<noteq> 0"

    then have "\<mu> > 0" "(1 - \<mu>) > 0" using asms by auto

    then have "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y > 0" using asms

      by (auto simp add: add_pos_pos mult_pos_pos) }

  ultimately show "(1 - \<mu>) *\<^sub>R y + \<mu> *\<^sub>R x > 0" using assms by fastforce

qed

lemma convex_on_setsum:

  fixes a :: "'a \<Rightarrow> real"

    and y :: "'a \<Rightarrow> 'b::real_vector"

    and f :: "'b \<Rightarrow> real"

  assumes "finite s" "s \<noteq> {}"

    and "convex_on C f"

    and "convex C"

    and "(\<Sum> i \<in> s. a i) = 1"

    and "\<And>i. i \<in> s \<Longrightarrow> a i \<ge> 0"

    and "\<And>i. i \<in> s \<Longrightarrow> y i \<in> C"

  shows "f (\<Sum> i \<in> s. a i *\<^sub>R y i) \<le> (\<Sum> i \<in> s. a i * f (y i))"

  using assms

proof (induct s arbitrary: a rule: finite_ne_induct)

  case (singleton i)

  then have ai: "a i = 1" by auto

  then show ?case by auto

next

  case (insert i s) note asms = this

  then have "convex_on C f" by simp

  from this[unfolded convex_on_def, rule_format]

  have conv: "\<And>x y \<mu>. x \<in> C \<Longrightarrow> y \<in> C \<Longrightarrow> 0 \<le> \<mu> \<Longrightarrow> \<mu> \<le> 1

      \<Longrightarrow> f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y"

    by simp

  { assume "a i = 1"

    then have "(\<Sum> j \<in> s. a j) = 0"

      using asms by auto

    then have "\<And>j. j \<in> s \<Longrightarrow> a j = 0"

      using setsum_nonneg_0[where 'b=real] asms by fastforce

    then have ?case using asms by auto }

  moreover

  { assume asm: "a i \<noteq> 1"

    from asms have yai: "y i \<in> C" "a i \<ge> 0" by auto

    have fis: "finite (insert i s)" using asms by auto

    then have ai1: "a i \<le> 1" using setsum_nonneg_leq_bound[of "insert i s" a] asms by simp

    then have "a i < 1" using asm by auto

    then have i0: "1 - a i > 0" by auto

    let ?a = "\<lambda>j. a j / (1 - a i)"

    { fix j assume "j \<in> s"

      then have "?a j \<ge> 0"

        using i0 asms divide_nonneg_pos

        by fastforce }

    note a_nonneg = this

    have "(\<Sum> j \<in> insert i s. a j) = 1" using asms by auto

    then have "(\<Sum> j \<in> s. a j) = 1 - a i" using setsum.insert asms by fastforce

    then have "(\<Sum> j \<in> s. a j) / (1 - a i) = 1" using i0 by auto

    then have a1: "(\<Sum> j \<in> s. ?a j) = 1" unfolding setsum_divide_distrib by simp

    have "convex C" using asms by auto

    then have asum: "(\<Sum> j \<in> s. ?a j *\<^sub>R y j) \<in> C"

      using asms convex_setsum[OF `finite s`

        `convex C` a1 a_nonneg] by auto

    have asum_le: "f (\<Sum> j \<in> s. ?a j *\<^sub>R y j) \<le> (\<Sum> j \<in> s. ?a j * f (y j))"

      using a_nonneg a1 asms by blast

    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)"

      using setsum.insert[of s i "\<lambda> j. a j *\<^sub>R y j", OF `finite s` `i \<notin> s`] asms

      by (auto simp only:add_commute)

    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)"

      using i0 by auto

    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)"

      using scaleR_right.setsum[of "inverse (1 - a i)" "\<lambda> j. a j *\<^sub>R y j" s, symmetric]

      by (auto simp:algebra_simps)

    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)"

      by (auto simp: divide_inverse)

    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)"

      using conv[of "y i" "(\<Sum> j \<in> s. ?a j *\<^sub>R y j)" "a i", OF yai(1) asum yai(2) ai1]

      by (auto simp add:add_commute)

    also have "\<dots> \<le> (1 - a i) * (\<Sum> j \<in> s. ?a j * f (y j)) + a i * f (y i)"

      using add_right_mono[OF mult_left_mono[of _ _ "1 - a i",

        OF asum_le less_imp_le[OF i0]], of "a i * f (y i)"] by simp

    also have "\<dots> = (\<Sum> j \<in> s. (1 - a i) * ?a j * f (y j)) + a i * f (y i)"

      unfolding setsum_right_distrib[of "1 - a i" "\<lambda> j. ?a j * f (y j)"] using i0 by auto

    also have "\<dots> = (\<Sum> j \<in> s. a j * f (y j)) + a i * f (y i)" using i0 by auto

    also have "\<dots> = (\<Sum> j \<in> insert i s. a j * f (y j))" using asms by auto

    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))"

      by simp }

  ultimately show ?case by auto

qed

lemma convex_on_alt:

  fixes C :: "'a::real_vector set"

  assumes "convex C"

  shows "convex_on C f =

  (\<forall> x \<in> C. \<forall> y \<in> C. \<forall> \<mu> :: real. \<mu> \<ge> 0 \<and> \<mu> \<le> 1

      \<longrightarrow> f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y)"

proof safe

  fix x y

  fix \<mu> :: real

  assume asms: "convex_on C f" "x \<in> C" "y \<in> C" "0 \<le> \<mu>" "\<mu> \<le> 1"

  from this[unfolded convex_on_def, rule_format]

  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

  from this[of "\<mu>" "1 - \<mu>", simplified] asms

  show "f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y" by auto

next

  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"

  { fix x y

    fix u v :: real

    assume lasm: "x \<in> C" "y \<in> C" "u \<ge> 0" "v \<ge> 0" "u + v = 1"

    then have[simp]: "1 - u = v" by auto

    from asm[rule_format, of x y u]

    have "f (u *\<^sub>R x + v *\<^sub>R y) \<le> u * f x + v * f y" using lasm by auto

  }

  then show "convex_on C f" unfolding convex_on_def by auto

qed

lemma convex_on_diff:

  fixes f :: "real \<Rightarrow> real"

  assumes f: "convex_on I f" and I: "x\<in>I" "y\<in>I" and t: "x < t" "t < y"

  shows "(f x - f t) / (x - t) \<le> (f x - f y) / (x - y)"

    "(f x - f y) / (x - y) \<le> (f t - f y) / (t - y)"

proof -

  def a \<equiv> "(t - y) / (x - y)"

  with t have "0 \<le> a" "0 \<le> 1 - a" by (auto simp: field_simps)

  with f `x \<in> I` `y \<in> I` have cvx: "f (a * x + (1 - a) * y) \<le> a * f x + (1 - a) * f y"

    by (auto simp: convex_on_def)

  have "a * x + (1 - a) * y = a * (x - y) + y" by (simp add: field_simps)

  also have "\<dots> = t" unfolding a_def using `x < t` `t < y` by simp

  finally have "f t \<le> a * f x + (1 - a) * f y" using cvx by simp

  also have "\<dots> = a * (f x - f y) + f y" by (simp add: field_simps)

  finally have "f t - f y \<le> a * (f x - f y)" by simp

  with t show "(f x - f t) / (x - t) \<le> (f x - f y) / (x - y)"

    by (simp add: le_divide_eq divide_le_eq field_simps a_def)

  with t show "(f x - f y) / (x - y) \<le> (f t - f y) / (t - y)"

    by (simp add: le_divide_eq divide_le_eq field_simps)

qed

lemma pos_convex_function:

  fixes f :: "real \<Rightarrow> real"

  assumes "convex C"

    and leq: "\<And>x y. \<lbrakk>x \<in> C ; y \<in> C\<rbrakk> \<Longrightarrow> f' x * (y - x) \<le> f y - f x"

  shows "convex_on C f"

  unfolding convex_on_alt[OF assms(1)]

  using assms

proof safe

  fix x y \<mu> :: real

  let ?x = "\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y"

  assume asm: "convex C" "x \<in> C" "y \<in> C" "\<mu> \<ge> 0" "\<mu> \<le> 1"

  then have "1 - \<mu> \<ge> 0" by auto

  then have xpos: "?x \<in> C" using asm unfolding convex_alt by fastforce

  have geq: "\<mu> * (f x - f ?x) + (1 - \<mu>) * (f y - f ?x)

            \<ge> \<mu> * f' ?x * (x - ?x) + (1 - \<mu>) * f' ?x * (y - ?x)"

    using add_mono[OF mult_left_mono[OF leq[OF xpos asm(2)] `\<mu> \<ge> 0`]

      mult_left_mono[OF leq[OF xpos asm(3)] `1 - \<mu> \<ge> 0`]] by auto

  then have "\<mu> * f x + (1 - \<mu>) * f y - f ?x \<ge> 0"

    by (auto simp add: field_simps)

  then show "f (\<mu> *\<^sub>R x + (1 - \<mu>) *\<^sub>R y) \<le> \<mu> * f x + (1 - \<mu>) * f y"

    using convex_on_alt by auto

qed

lemma atMostAtLeast_subset_convex:

  fixes C :: "real set"

  assumes "convex C"

    and "x \<in> C" "y \<in> C" "x < y"

  shows "{x .. y} \<subseteq> C"

proof safe

  fix z assume zasm: "z \<in> {x .. y}"

  { assume asm: "x < z" "z < y"

    let ?\<mu> = "(y - z) / (y - x)"

    have "0 \<le> ?\<mu>" "?\<mu> \<le> 1" using assms asm by (auto simp add: field_simps)

    then have comb: "?\<mu> * x + (1 - ?\<mu>) * y \<in> C"

      using assms iffD1[OF convex_alt, rule_format, of C y x ?\<mu>]

      by (simp add: algebra_simps)

    have "?\<mu> * x + (1 - ?\<mu>) * y = (y - z) * x / (y - x) + (1 - (y - z) / (y - x)) * y"

      by (auto simp add: field_simps)

    also have "\<dots> = ((y - z) * x + (y - x - (y - z)) * y) / (y - x)"

      using assms unfolding add_divide_distrib by (auto simp: field_simps)

    also have "\<dots> = z"

      using assms by (auto simp: field_simps)

    finally have "z \<in> C"

      using comb by auto }

  note less = this

  show "z \<in> C" using zasm less assms

    unfolding atLeastAtMost_iff le_less by auto

qed

lemma f''_imp_f':

  fixes f :: "real \<Rightarrow> real"

  assumes "convex C"

    and f': "\<And>x. x \<in> C \<Longrightarrow> DERIV f x :> (f' x)"

    and f'': "\<And>x. x \<in> C \<Longrightarrow> DERIV f' x :> (f'' x)"

    and pos: "\<And>x. x \<in> C \<Longrightarrow> f'' x \<ge> 0"

    and "x \<in> C" "y \<in> C"

  shows "f' x * (y - x) \<le> f y - f x"

  using assms

proof -

  { fix x y :: real

    assume asm: "x \<in> C" "y \<in> C" "y > x"

    then have ge: "y - x > 0" "y - x \<ge> 0" by auto

    from asm have le: "x - y < 0" "x - y \<le> 0" by auto

    then obtain z1 where z1: "z1 > x" "z1 < y" "f y - f x = (y - x) * f' z1"

      using subsetD[OF atMostAtLeast_subset_convex[OF `convex C` `x \<in> C` `y \<in> C` `x < y`],

        THEN f', THEN MVT2[OF `x < y`, rule_format, unfolded atLeastAtMost_iff[symmetric]]]

      by auto

    then have "z1 \<in> C" using atMostAtLeast_subset_convex

      `convex C` `x \<in> C` `y \<in> C` `x < y` by fastforce

    from z1 have z1': "f x - f y = (x - y) * f' z1"

      by (simp add:field_simps)

    obtain z2 where z2: "z2 > x" "z2 < z1" "f' z1 - f' x = (z1 - x) * f'' z2"

      using subsetD[OF atMostAtLeast_subset_convex[OF `convex C` `x \<in> C` `z1 \<in> C` `x < z1`],

        THEN f'', THEN MVT2[OF `x < z1`, rule_format, unfolded atLeastAtMost_iff[symmetric]]] z1

      by auto

    obtain z3 where z3: "z3 > z1" "z3 < y" "f' y - f' z1 = (y - z1) * f'' z3"

      using subsetD[OF atMostAtLeast_subset_convex[OF `convex C` `z1 \<in> C` `y \<in> C` `z1 < y`],

        THEN f'', THEN MVT2[OF `z1 < y`, rule_format, unfolded atLeastAtMost_iff[symmetric]]] z1

      by auto

    have "f' y - (f x - f y) / (x - y) = f' y - f' z1"

      using asm z1' by auto

    also have "\<dots> = (y - z1) * f'' z3" using z3 by auto

    finally have cool': "f' y - (f x - f y) / (x - y) = (y - z1) * f'' z3" by simp

    have A': "y - z1 \<ge> 0" using z1 by auto

    have "z3 \<in> C" using z3 asm atMostAtLeast_subset_convex

      `convex C` `x \<in> C` `z1 \<in> C` `x < z1` by fastforce

    then have B': "f'' z3 \<ge> 0" using assms by auto

    from A' B' have "(y - z1) * f'' z3 \<ge> 0" by auto

    from cool' this have "f' y - (f x - f y) / (x - y) \<ge> 0" by auto

    from mult_right_mono_neg[OF this le(2)]

    have "f' y * (x - y) - (f x - f y) / (x - y) * (x - y) \<le> 0 * (x - y)"

      by (simp add: algebra_simps)

    then have "f' y * (x - y) - (f x - f y) \<le> 0" using le by auto

    then have res: "f' y * (x - y) \<le> f x - f y" by auto

    have "(f y - f x) / (y - x) - f' x = f' z1 - f' x"

      using asm z1 by auto

    also have "\<dots> = (z1 - x) * f'' z2" using z2 by auto

    finally have cool: "(f y - f x) / (y - x) - f' x = (z1 - x) * f'' z2" by simp

    have A: "z1 - x \<ge> 0" using z1 by auto

    have "z2 \<in> C" using z2 z1 asm atMostAtLeast_subset_convex

      `convex C` `z1 \<in> C` `y \<in> C` `z1 < y` by fastforce

    then have B: "f'' z2 \<ge> 0" using assms by auto

    from A B have "(z1 - x) * f'' z2 \<ge> 0" by auto

    from cool this have "(f y - f x) / (y - x) - f' x \<ge> 0" by auto

    from mult_right_mono[OF this ge(2)]

    have "(f y - f x) / (y - x) * (y - x) - f' x * (y - x) \<ge> 0 * (y - x)"

      by (simp add: algebra_simps)

    then have "f y - f x - f' x * (y - x) \<ge> 0" using ge by auto

    then have "f y - f x \<ge> f' x * (y - x)" "f' y * (x - y) \<le> f x - f y"

      using res by auto } note less_imp = this

  { fix x y :: real

    assume "x \<in> C" "y \<in> C" "x \<noteq> y"

    then have"f y - f x \<ge> f' x * (y - x)"

    unfolding neq_iff using less_imp by auto } note neq_imp = this

  moreover

  { fix x y :: real

    assume asm: "x \<in> C" "y \<in> C" "x = y"

    then have "f y - f x \<ge> f' x * (y - x)" by auto }

  ultimately show ?thesis using assms by blast

qed

lemma f''_ge0_imp_convex:

  fixes f :: "real \<Rightarrow> real"

  assumes conv: "convex C"

    and f': "\<And>x. x \<in> C \<Longrightarrow> DERIV f x :> (f' x)"

    and f'': "\<And>x. x \<in> C \<Longrightarrow> DERIV f' x :> (f'' x)"

    and pos: "\<And>x. x \<in> C \<Longrightarrow> f'' x \<ge> 0"

  shows "convex_on C f"

using f''_imp_f'[OF conv f' f'' pos] assms pos_convex_function by fastforce

lemma minus_log_convex:

  fixes b :: real

  assumes "b > 1"

  shows "convex_on {0 <..} (\<lambda> x. - log b x)"

proof -

  have "\<And>z. z > 0 \<Longrightarrow> DERIV (log b) z :> 1 / (ln b * z)" using DERIV_log by auto

  then have f': "\<And>z. z > 0 \<Longrightarrow> DERIV (\<lambda> z. - log b z) z :> - 1 / (ln b * z)"

    by (auto simp: DERIV_minus)

  have "\<And>z :: real. z > 0 \<Longrightarrow> DERIV inverse z :> - (inverse z ^ Suc (Suc 0))"

    using less_imp_neq[THEN not_sym, THEN DERIV_inverse] by auto

  from this[THEN DERIV_cmult, of _ "- 1 / ln b"]

  have "\<And>z :: real. z > 0 \<Longrightarrow>

    DERIV (\<lambda> z. (- 1 / ln b) * inverse z) z :> (- 1 / ln b) * (- (inverse z ^ Suc (Suc 0)))"

    by auto

  then have f''0: "\<And>z :: real. z > 0 \<Longrightarrow> DERIV (\<lambda> z. - 1 / (ln b * z)) z :> 1 / (ln b * z * z)"

    unfolding inverse_eq_divide by (auto simp add: mult_assoc)

  have f''_ge0: "\<And>z :: real. z > 0 \<Longrightarrow> 1 / (ln b * z * z) \<ge> 0"

    using `b > 1` by (auto intro!:less_imp_le simp add: divide_pos_pos[of 1] mult_pos_pos)

  from f''_ge0_imp_convex[OF pos_is_convex,

    unfolded greaterThan_iff, OF f' f''0 f''_ge0]

  show ?thesis by auto

qed

end