Theory Convex

theory Convex
imports Product_Vector
(*  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 => bool"
where "convex s <-> (∀x∈s. ∀y∈s. ∀u≥0. ∀v≥0. u + v = 1 --> u *R x + v *R y ∈ s)"

lemma convexI:
assumes "!!x y u v. x ∈ s ==> y ∈ s ==> 0 ≤ u ==> 0 ≤ v ==> u + v = 1 ==> u *R x + v *R y ∈ s"
shows "convex s"
using assms unfolding convex_def by fast

lemma convexD:
assumes "convex s" and "x ∈ s" and "y ∈ s" and "0 ≤ u" and "0 ≤ v" and "u + v = 1"
shows "u *R x + v *R y ∈ s"
using assms unfolding convex_def by fast

lemma convex_alt:
"convex s <-> (∀x∈s. ∀y∈s. ∀u. 0 ≤ u ∧ u ≤ 1 --> ((1 - u) *R x + u *R y) ∈ s)"
(is "_ <-> ?alt")
proof
assume alt[rule_format]: ?alt
{ fix x y and u v :: real assume mem: "x ∈ s" "y ∈ s"
assume "0 ≤ u" "0 ≤ v"
moreover assume "u + v = 1" then have "u = 1 - v" by auto
ultimately have "u *R x + v *R y ∈ 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 ∈ s" "b ∈ s" "0 ≤ u" "u ≤ 1"
shows "((1 - u) *R a + u *R b) ∈ 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: "(∀s∈f. convex s) ==> convex(\<Inter> f)"
unfolding convex_def by auto

lemma convex_Int: "convex s ==> convex t ==> convex (s ∩ t)"
unfolding convex_def by auto

lemma convex_INT: "∀i∈A. convex (B i) ==> convex (\<Inter>i∈A. B i)"
unfolding convex_def by auto

lemma convex_Times: "convex s ==> convex t ==> convex (s × t)"
unfolding convex_def by auto

lemma convex_halfspace_le: "convex {x. inner a x ≤ b}"
unfolding convex_def
by (auto simp: inner_add intro!: convex_bound_le)

lemma convex_halfspace_ge: "convex {x. inner a x ≥ b}"
proof -
have *: "{x. inner a x ≥ b} = {x. inner (-a) x ≤ -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 ≤ b} ∩ {x. inner a x ≥ 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 ≤ inner 1 x}" by auto
then show 1: "convex {a..}" by (simp only: convex_halfspace_ge)
have "{..b} = {x. inner 1 x ≤ 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..} ∩ {..b}" by auto
then show "convex {a..b}" by (simp only: convex_Int 1 2)
have "{a<..b} = {a<..} ∩ {..b}" by auto
then show "convex {a<..b}" by (simp only: convex_Int 3 2)
have "{a..<b} = {a..} ∩ {..<b}" by auto
then show "convex {a..<b}" by (simp only: convex_Int 1 4)
have "{a<..<b} = {a<..} ∩ {..<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 "(∑ i ∈ s. a i) = 1"
assumes "!!i. i ∈ s ==> a i ≥ 0" and "!!i. i ∈ s ==> y i ∈ C"
shows "(∑ j ∈ s. a j *R y j) ∈ C"
using assms
proof (induct s arbitrary:a rule: finite_induct)
case empty
then show ?case by auto
next
case (insert i s) note asms = this
{ assume "a i = 1"
then have "(∑ j ∈ s. a j) = 0"
using asms by auto
then have "!!j. j ∈ s ==> 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 ≠ 1"
from asms have yai: "y i ∈ C" "a i ≥ 0" by auto
have fis: "finite (insert i s)" using asms by auto
then have ai1: "a i ≤ 1" using setsum_nonneg_leq_bound[of "insert i s" a 1] asms by simp
then have "a i < 1" using asm by auto
then have i0: "1 - a i > 0" by auto
let ?a = "λj. a j / (1 - a i)"
{ fix j assume "j ∈ s"
then have "?a j ≥ 0"
using i0 asms divide_nonneg_pos
by fastforce
} note a_nonneg = this
have "(∑ j ∈ insert i s. a j) = 1" using asms by auto
then have "(∑ j ∈ s. a j) = 1 - a i" using setsum.insert asms by fastforce
then have "(∑ j ∈ s. a j) / (1 - a i) = 1" using i0 by auto
then have a1: "(∑ j ∈ s. ?a j) = 1" unfolding setsum_divide_distrib by simp
with asms have "(∑j∈s. ?a j *R y j) ∈ C" using a_nonneg by fastforce
then have "a i *R y i + (1 - a i) *R (∑ j ∈ s. ?a j *R y j) ∈ C"
using asms yai ai1 by (auto intro: convexD)
then have "a i *R y i + (∑ j ∈ s. (1 - a i) *R (?a j *R y j)) ∈ C"
using scaleR_right.setsum[of "(1 - a i)" "λ j. ?a j *R y j" s] by auto
then have "a i *R y i + (∑ j ∈ s. a j *R y j) ∈ C" using i0 by auto
then have ?case using setsum.insert asms by auto
}
ultimately show ?case by auto
qed

lemma convex:
"convex s <-> (∀(k::nat) u x. (∀i. 1≤i ∧ i≤k --> 0 ≤ u i ∧ x i ∈s) ∧ (setsum u {1..k} = 1)
--> setsum (λi. u i *R x i) {1..k} ∈ s)"

proof safe
fix k :: nat
fix u :: "nat => real"
fix x
assume "convex s"
"∀i. 1 ≤ i ∧ i ≤ k --> 0 ≤ u i ∧ x i ∈ s"
"setsum u {1..k} = 1"
from this convex_setsum[of "{1 .. k}" s]
show "(∑j∈{1 .. k}. u j *R x j) ∈ s" by auto
next
assume asm: "∀k u x. (∀ i :: nat. 1 ≤ i ∧ i ≤ k --> 0 ≤ u i ∧ x i ∈ s) ∧ setsum u {1..k} = 1
--> (∑i = 1..k. u i *R (x i :: 'a)) ∈ s"

{ fix μ :: real
fix x y :: 'a
assume xy: "x ∈ s" "y ∈ s"
assume mu: "μ ≥ 0" "μ ≤ 1"
let ?u = "λi. if (i :: nat) = 1 then μ else 1 - μ"
let ?x = "λi. if (i :: nat) = 1 then x else y"
have "{1 :: nat .. 2} ∩ - {x. x = 1} = {2}" by auto
then have card: "card ({1 :: nat .. 2} ∩ - {x. x = 1}) = 1" by simp
then have "setsum ?u {1 .. 2} = 1"
using setsum_cases[of "{(1 :: nat) .. 2}" "λ x. x = 1" "λ x. μ" "λ x. 1 - μ"]
by auto
with asm[rule_format, of "2" ?u ?x] have s: "(∑j ∈ {1..2}. ?u j *R ?x j) ∈ s"
using mu xy by auto
have grarr: "(∑j ∈ {Suc (Suc 0)..2}. ?u j *R ?x j) = (1 - μ) *R y"
using setsum_head_Suc[of "Suc (Suc 0)" 2 "λ j. (1 - μ) *R y"] by auto
from setsum_head_Suc[of "Suc 0" 2 "λ j. ?u j *R ?x j", simplified this]
have "(∑j ∈ {1..2}. ?u j *R ?x j) = μ *R x + (1 - μ) *R y" by auto
then have "(1 - μ) *R y + μ *R x ∈ 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 <->
(∀t u. finite t ∧ t ⊆ s ∧ (∀x∈t. 0 ≤ u x) ∧ setsum u t = 1 --> setsum (λx. u x *R x) t ∈ s)"

proof safe
fix t
fix u :: "'a => real"
assume "convex s" "finite t"
"t ⊆ s" "∀x∈t. 0 ≤ u x" "setsum u t = 1"
then show "(∑x∈t. u x *R x) ∈ s"
using convex_setsum[of t s u "λ x. x"] by auto
next
assume asm0: "∀t. ∀ u. finite t ∧ t ⊆ s ∧ (∀x∈t. 0 ≤ u x)
∧ setsum u t = 1 --> (∑x∈t. u x *R x) ∈ s"

show "convex s"
unfolding convex_alt
proof safe
fix x y
fix μ :: real
assume asm: "x ∈ s" "y ∈ s" "0 ≤ μ" "μ ≤ 1"
{ assume "x ≠ y"
then have "(1 - μ) *R x + μ *R y ∈ s"
using asm0[rule_format, of "{x, y}" "λ z. if z = x then 1 - μ else μ"]
asm by auto }
moreover
{ assume "x = y"
then have "(1 - μ) *R x + μ *R y ∈ s"
using asm0[rule_format, of "{x, y}" "λ z. 1"]
asm by (auto simp:field_simps real_vector.scale_left_diff_distrib) }
ultimately show "(1 - μ) *R x + μ *R y ∈ s" by blast
qed
qed

lemma convex_finite:
assumes "finite s"
shows "convex s <-> (∀u. (∀x∈s. 0 ≤ u x) ∧ setsum u s = 1
--> setsum (λx. u x *R x) s ∈ s)"

unfolding convex_explicit
proof safe
fix t u
assume sum: "∀u. (∀x∈s. 0 ≤ u x) ∧ setsum u s = 1 --> (∑x∈s. u x *R x) ∈ s"
and as: "finite t" "t ⊆ s" "∀x∈t. 0 ≤ u x" "setsum u t = (1::real)"
have *: "s ∩ t = t" using as(2) by auto
have if_distrib_arg: "!!P f g x. (if P then f else g) x = (if P then f x else g x)"
by simp
show "(∑x∈t. u x *R x) ∈ s"
using sum[THEN spec[where x="λx. if x∈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)

definition convex_on :: "'a::real_vector set => ('a => real) => bool"
where "convex_on s f <->
(∀x∈s. ∀y∈s. ∀u≥0. ∀v≥0. u + v = 1 --> f (u *R x + v *R y) ≤ u * f x + v * f y)"


lemma convex_on_subset: "convex_on t f ==> s ⊆ t ==> 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 (λx. f x + g x)"
proof -
{ fix x y
assume "x∈s" "y∈s"
moreover
fix u v :: real
assume "0 ≤ u" "0 ≤ v" "u + v = 1"
ultimately
have "f (u *R x + v *R y) + g (u *R x + v *R y) ≤ (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 *R x + v *R y) + g (u *R x + v *R y) ≤ 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 ≤ (c::real)" "convex_on s f"
shows "convex_on s (λx. c * f x)"
proof-
have *: "!!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∈s" "y ∈ s" "0 ≤ u" "0 ≤ v" "u + v = 1"
shows "f (u *R x + v *R y) ≤ max (f x) (f y)"
proof-
let ?m = "max (f x) (f y)"
have "u * f x + v * f y ≤ 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 "… = 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 (λx. dist a x)"
proof (auto simp add: convex_on_def dist_norm)
fix x y
assume "x∈s" "y∈s"
fix u v :: real
assume "0 ≤ u" "0 ≤ v" "u + v = 1"
have "a = u *R a + v *R a"
unfolding scaleR_left_distrib[symmetric] and `u+v=1` by simp
then have *: "a - (u *R x + v *R y) = (u *R (a - x)) + (v *R (a - y))"
by (auto simp add: algebra_simps)
show "norm (a - (u *R x + v *R y)) ≤ u * norm (a - x) + v * norm (a - y)"
unfolding * using norm_triangle_ineq[of "u *R (a - x)" "v *R (a - y)"]
using `0 ≤ u` `0 ≤ 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 ((λx. c *R x) ` s)"
proof -
have "linear (λx. c *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 ((λx. - x) ` s)"
proof -
have "linear (λx. - 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 ∈ s ∧ y ∈ t}"
proof -
have "linear (λ(x, y). x + y)"
by (auto intro: linearI simp add: scaleR_add_right)
with assms have "convex ((λ(x, y). x + y) ` (s × t))"
by (intro convex_linear_image convex_Times)
also have "((λ(x, y). x + y) ` (s × t)) = {x + y| x y. x ∈ s ∧ y ∈ t}"
by auto
finally show ?thesis .
qed

lemma convex_differences:
assumes "convex s" "convex t"
shows "convex {x - y| x y. x ∈ s ∧ y ∈ t}"
proof -
have "{x - y| x y. x ∈ s ∧ y ∈ t} = {x + y |x y. x ∈ s ∧ y ∈ uminus ` t}"
unfolding diff_def by auto
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 ((λx. a + x) ` s)"
proof -
have "{a + y |y. y ∈ s} = (λx. 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 ((λx. a + c *R x) ` s)"
proof -
have "(λx. a + c *R x) ` s = op + a ` op *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 μ :: real
assume asms: "y > 0" "x > 0" "μ ≥ 0" "μ ≤ 1"
{ assume "μ = 0"
then have "μ *R x + (1 - μ) *R y = y" by simp
then have "μ *R x + (1 - μ) *R y > 0" using asms by simp }
moreover
{ assume "μ = 1"
then have "μ *R x + (1 - μ) *R y > 0" using asms by simp }
moreover
{ assume "μ ≠ 1" "μ ≠ 0"
then have "μ > 0" "(1 - μ) > 0" using asms by auto
then have "μ *R x + (1 - μ) *R y > 0" using asms
by (auto simp add: add_pos_pos mult_pos_pos) }
ultimately show "(1 - μ) *R y + μ *R x > 0" using assms by fastforce
qed

lemma convex_on_setsum:
fixes a :: "'a => real"
and y :: "'a => 'b::real_vector"
and f :: "'b => real"
assumes "finite s" "s ≠ {}"
and "convex_on C f"
and "convex C"
and "(∑ i ∈ s. a i) = 1"
and "!!i. i ∈ s ==> a i ≥ 0"
and "!!i. i ∈ s ==> y i ∈ C"
shows "f (∑ i ∈ s. a i *R y i) ≤ (∑ i ∈ 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: "!!x y μ. x ∈ C ==> y ∈ C ==> 0 ≤ μ ==> μ ≤ 1
==> f (μ *R x + (1 - μ) *R y) ≤ μ * f x + (1 - μ) * f y"

by simp
{ assume "a i = 1"
then have "(∑ j ∈ s. a j) = 0"
using asms by auto
then have "!!j. j ∈ s ==> 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 ≠ 1"
from asms have yai: "y i ∈ C" "a i ≥ 0" by auto
have fis: "finite (insert i s)" using asms by auto
then have ai1: "a i ≤ 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 = "λj. a j / (1 - a i)"
{ fix j assume "j ∈ s"
then have "?a j ≥ 0"
using i0 asms divide_nonneg_pos
by fastforce }
note a_nonneg = this
have "(∑ j ∈ insert i s. a j) = 1" using asms by auto
then have "(∑ j ∈ s. a j) = 1 - a i" using setsum.insert asms by fastforce
then have "(∑ j ∈ s. a j) / (1 - a i) = 1" using i0 by auto
then have a1: "(∑ j ∈ s. ?a j) = 1" unfolding setsum_divide_distrib by simp
have "convex C" using asms by auto
then have asum: "(∑ j ∈ s. ?a j *R y j) ∈ C"
using asms convex_setsum[OF `finite s`
`convex C` a1 a_nonneg] by auto
have asum_le: "f (∑ j ∈ s. ?a j *R y j) ≤ (∑ j ∈ s. ?a j * f (y j))"
using a_nonneg a1 asms by blast
have "f (∑ j ∈ insert i s. a j *R y j) = f ((∑ j ∈ s. a j *R y j) + a i *R y i)"
using setsum.insert[of s i "λ j. a j *R y j", OF `finite s` `i ∉ s`] asms
by (auto simp only:add_commute)
also have "… = f (((1 - a i) * inverse (1 - a i)) *R (∑ j ∈ s. a j *R y j) + a i *R y i)"
using i0 by auto
also have "… = f ((1 - a i) *R (∑ j ∈ s. (a j * inverse (1 - a i)) *R y j) + a i *R y i)"
using scaleR_right.setsum[of "inverse (1 - a i)" "λ j. a j *R y j" s, symmetric]
by (auto simp:algebra_simps)
also have "… = f ((1 - a i) *R (∑ j ∈ s. ?a j *R y j) + a i *R y i)"
by (auto simp: divide_inverse)
also have "… ≤ (1 - a i) *R f ((∑ j ∈ s. ?a j *R y j)) + a i * f (y i)"
using conv[of "y i" "(∑ j ∈ s. ?a j *R y j)" "a i", OF yai(1) asum yai(2) ai1]
by (auto simp add:add_commute)
also have "… ≤ (1 - a i) * (∑ j ∈ 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 "… = (∑ j ∈ s. (1 - a i) * ?a j * f (y j)) + a i * f (y i)"
unfolding setsum_right_distrib[of "1 - a i" "λ j. ?a j * f (y j)"] using i0 by auto
also have "… = (∑ j ∈ s. a j * f (y j)) + a i * f (y i)" using i0 by auto
also have "… = (∑ j ∈ insert i s. a j * f (y j))" using asms by auto
finally have "f (∑ j ∈ insert i s. a j *R y j) ≤ (∑ j ∈ 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 =
(∀ x ∈ C. ∀ y ∈ C. ∀ μ :: real. μ ≥ 0 ∧ μ ≤ 1
--> f (μ *R x + (1 - μ) *R y) ≤ μ * f x + (1 - μ) * f y)"

proof safe
fix x y
fix μ :: real
assume asms: "convex_on C f" "x ∈ C" "y ∈ C" "0 ≤ μ" "μ ≤ 1"
from this[unfolded convex_on_def, rule_format]
have "!!u v. [|0 ≤ u; 0 ≤ v; u + v = 1|] ==> f (u *R x + v *R y) ≤ u * f x + v * f y" by auto
from this[of "μ" "1 - μ", simplified] asms
show "f (μ *R x + (1 - μ) *R y) ≤ μ * f x + (1 - μ) * f y" by auto
next
assume asm: "∀x∈C. ∀y∈C. ∀μ. 0 ≤ μ ∧ μ ≤ 1 --> f (μ *R x + (1 - μ) *R y) ≤ μ * f x + (1 - μ) * f y"
{ fix x y
fix u v :: real
assume lasm: "x ∈ C" "y ∈ C" "u ≥ 0" "v ≥ 0" "u + v = 1"
then have[simp]: "1 - u = v" by auto
from asm[rule_format, of x y u]
have "f (u *R x + v *R y) ≤ 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 => real"
assumes f: "convex_on I f" and I: "x∈I" "y∈I" and t: "x < t" "t < y"
shows "(f x - f t) / (x - t) ≤ (f x - f y) / (x - y)"
"(f x - f y) / (x - y) ≤ (f t - f y) / (t - y)"
proof -
def a "(t - y) / (x - y)"
with t have "0 ≤ a" "0 ≤ 1 - a" by (auto simp: field_simps)
with f `x ∈ I` `y ∈ I` have cvx: "f (a * x + (1 - a) * y) ≤ 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 "… = t" unfolding a_def using `x < t` `t < y` by simp
finally have "f t ≤ a * f x + (1 - a) * f y" using cvx by simp
also have "… = a * (f x - f y) + f y" by (simp add: field_simps)
finally have "f t - f y ≤ a * (f x - f y)" by simp
with t show "(f x - f t) / (x - t) ≤ (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) ≤ (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 => real"
assumes "convex C"
and leq: "!!x y. [|x ∈ C ; y ∈ C|] ==> f' x * (y - x) ≤ f y - f x"
shows "convex_on C f"
unfolding convex_on_alt[OF assms(1)]
using assms
proof safe
fix x y μ :: real
let ?x = "μ *R x + (1 - μ) *R y"
assume asm: "convex C" "x ∈ C" "y ∈ C" "μ ≥ 0" "μ ≤ 1"
then have "1 - μ ≥ 0" by auto
then have xpos: "?x ∈ C" using asm unfolding convex_alt by fastforce
have geq: "μ * (f x - f ?x) + (1 - μ) * (f y - f ?x)
≥ μ * f' ?x * (x - ?x) + (1 - μ) * f' ?x * (y - ?x)"

using add_mono[OF mult_left_mono[OF leq[OF xpos asm(2)] `μ ≥ 0`]
mult_left_mono[OF leq[OF xpos asm(3)] `1 - μ ≥ 0`]] by auto
then have "μ * f x + (1 - μ) * f y - f ?x ≥ 0"
by (auto simp add: field_simps)
then show "f (μ *R x + (1 - μ) *R y) ≤ μ * f x + (1 - μ) * f y"
using convex_on_alt by auto
qed

lemma atMostAtLeast_subset_convex:
fixes C :: "real set"
assumes "convex C"
and "x ∈ C" "y ∈ C" "x < y"
shows "{x .. y} ⊆ C"
proof safe
fix z assume zasm: "z ∈ {x .. y}"
{ assume asm: "x < z" "z < y"
let = "(y - z) / (y - x)"
have "0 ≤ ?μ" "?μ ≤ 1" using assms asm by (auto simp add: field_simps)
then have comb: "?μ * x + (1 - ?μ) * y ∈ C"
using assms iffD1[OF convex_alt, rule_format, of C y x ]
by (simp add: algebra_simps)
have "?μ * x + (1 - ?μ) * y = (y - z) * x / (y - x) + (1 - (y - z) / (y - x)) * y"
by (auto simp add: field_simps)
also have "… = ((y - z) * x + (y - x - (y - z)) * y) / (y - x)"
using assms unfolding add_divide_distrib by (auto simp: field_simps)
also have "… = z"
using assms by (auto simp: field_simps)
finally have "z ∈ C"
using comb by auto }
note less = this
show "z ∈ C" using zasm less assms
unfolding atLeastAtMost_iff le_less by auto
qed

lemma f''_imp_f':
fixes f :: "real => real"
assumes "convex C"
and f': "!!x. x ∈ C ==> DERIV f x :> (f' x)"
and f'': "!!x. x ∈ C ==> DERIV f' x :> (f'' x)"
and pos: "!!x. x ∈ C ==> f'' x ≥ 0"
and "x ∈ C" "y ∈ C"
shows "f' x * (y - x) ≤ f y - f x"
using assms
proof -
{ fix x y :: real
assume asm: "x ∈ C" "y ∈ C" "y > x"
then have ge: "y - x > 0" "y - x ≥ 0" by auto
from asm have le: "x - y < 0" "x - y ≤ 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 ∈ C` `y ∈ C` `x < y`],
THEN f', THEN MVT2[OF `x < y`, rule_format, unfolded atLeastAtMost_iff[symmetric]]]
by auto
then have "z1 ∈ C" using atMostAtLeast_subset_convex
`convex C` `x ∈ C` `y ∈ 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 ∈ C` `z1 ∈ 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 ∈ C` `y ∈ 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 "… = (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 ≥ 0" using z1 by auto
have "z3 ∈ C" using z3 asm atMostAtLeast_subset_convex
`convex C` `x ∈ C` `z1 ∈ C` `x < z1` by fastforce
then have B': "f'' z3 ≥ 0" using assms by auto
from A' B' have "(y - z1) * f'' z3 ≥ 0" using mult_nonneg_nonneg by auto
from cool' this have "f' y - (f x - f y) / (x - y) ≥ 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) ≤ 0 * (x - y)"
by (simp add: algebra_simps)
then have "f' y * (x - y) - (f x - f y) ≤ 0" using le by auto
then have res: "f' y * (x - y) ≤ 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 "… = (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 ≥ 0" using z1 by auto
have "z2 ∈ C" using z2 z1 asm atMostAtLeast_subset_convex
`convex C` `z1 ∈ C` `y ∈ C` `z1 < y` by fastforce
then have B: "f'' z2 ≥ 0" using assms by auto
from A B have "(z1 - x) * f'' z2 ≥ 0" using mult_nonneg_nonneg by auto
from cool this have "(f y - f x) / (y - x) - f' x ≥ 0" by auto
from mult_right_mono[OF this ge(2)]
have "(f y - f x) / (y - x) * (y - x) - f' x * (y - x) ≥ 0 * (y - x)"
by (simp add: algebra_simps)
then have "f y - f x - f' x * (y - x) ≥ 0" using ge by auto
then have "f y - f x ≥ f' x * (y - x)" "f' y * (x - y) ≤ f x - f y"
using res by auto } note less_imp = this
{ fix x y :: real
assume "x ∈ C" "y ∈ C" "x ≠ y"
then have"f y - f x ≥ f' x * (y - x)"
unfolding neq_iff using less_imp by auto } note neq_imp = this
moreover
{ fix x y :: real
assume asm: "x ∈ C" "y ∈ C" "x = y"
then have "f y - f x ≥ f' x * (y - x)" by auto }
ultimately show ?thesis using assms by blast
qed

lemma f''_ge0_imp_convex:
fixes f :: "real => real"
assumes conv: "convex C"
and f': "!!x. x ∈ C ==> DERIV f x :> (f' x)"
and f'': "!!x. x ∈ C ==> DERIV f' x :> (f'' x)"
and pos: "!!x. x ∈ C ==> f'' x ≥ 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 <..} (λ x. - log b x)"
proof -
have "!!z. z > 0 ==> DERIV (log b) z :> 1 / (ln b * z)" using DERIV_log by auto
then have f': "!!z. z > 0 ==> DERIV (λ z. - log b z) z :> - 1 / (ln b * z)"
by (auto simp: DERIV_minus)
have "!!z :: real. z > 0 ==> 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 "!!z :: real. z > 0 ==>
DERIV (λ z. (- 1 / ln b) * inverse z) z :> (- 1 / ln b) * (- (inverse z ^ Suc (Suc 0)))"

by auto
then have f''0: "!!z :: real. z > 0 ==> DERIV (λ z. - 1 / (ln b * z)) z :> 1 / (ln b * z * z)"
unfolding inverse_eq_divide by (auto simp add: mult_assoc)
have f''_ge0: "!!z :: real. z > 0 ==> 1 / (ln b * z * z) ≥ 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