# Theory Line_Segment

```(* Title:      HOL/Analysis/Line_Segment.thy
Author:     L C Paulson, University of Cambridge
Author:     Robert Himmelmann, TU Muenchen
Author:     Bogdan Grechuk, University of Edinburgh
Author:     Armin Heller, TU Muenchen
Author:     Johannes Hoelzl, TU Muenchen
*)

section ‹Line Segment›

theory Line_Segment
imports
Convex
Topology_Euclidean_Space
begin

subsection✐‹tag unimportant› ‹Topological Properties of Convex Sets, Metric Spaces and Functions›

lemma convex_supp_sum:
assumes "convex S" and 1: "supp_sum u I = 1"
and "⋀i. i ∈ I ⟹ 0 ≤ u i ∧ (u i = 0 ∨ f i ∈ S)"
shows "supp_sum (λi. u i *⇩R f i) I ∈ S"
proof -
have fin: "finite {i ∈ I. u i ≠ 0}"
using 1 sum.infinite by (force simp: supp_sum_def support_on_def)
then have "supp_sum (λi. u i *⇩R f i) I = sum (λi. u i *⇩R f i) {i ∈ I. u i ≠ 0}"
by (force intro: sum.mono_neutral_left simp: supp_sum_def support_on_def)
also have "... ∈ S"
using 1 assms by (force simp: supp_sum_def support_on_def intro: convex_sum [OF fin ‹convex S›])
finally show ?thesis .
qed

lemma sphere_eq_empty [simp]:
fixes a :: "'a::{real_normed_vector, perfect_space}"
shows "sphere a r = {} ⟷ r < 0"
by (metis empty_iff linorder_not_less mem_sphere sphere_empty vector_choose_dist)

lemma cone_closure:
fixes S :: "'a::real_normed_vector set"
assumes "cone S"
shows "cone (closure S)"
by (metis UnCI assms closure_Un_frontier closure_eq_empty closure_scaleR cone_iff)

corollary component_complement_connected:
fixes S :: "'a::real_normed_vector set"
assumes "connected S" "C ∈ components (-S)"
shows "connected(-C)"
using component_diff_connected [of S UNIV] assms
by (auto simp: Compl_eq_Diff_UNIV)

proposition clopen:
fixes S :: "'a :: real_normed_vector set"
shows "closed S ∧ open S ⟷ S = {} ∨ S = UNIV"
using  connected_UNIV by (force simp add: connected_clopen)

corollary compact_open:
fixes S :: "'a :: euclidean_space set"
shows "compact S ∧ open S ⟷ S = {}"
by (auto simp: compact_eq_bounded_closed clopen)

corollary finite_imp_not_open:
fixes S :: "'a::{real_normed_vector, perfect_space} set"
shows "⟦finite S; open S⟧ ⟹ S={}"
using clopen [of S] finite_imp_closed not_bounded_UNIV by blast

corollary empty_interior_finite:
fixes S :: "'a::{real_normed_vector, perfect_space} set"
shows "finite S ⟹ interior S = {}"
by (metis interior_subset finite_subset open_interior [of S] finite_imp_not_open)

text ‹Balls, being convex, are connected.›

lemma convex_local_global_minimum:
fixes s :: "'a::real_normed_vector set"
assumes "e > 0"
and "convex_on s f"
and "ball x e ⊆ s"
and "∀y∈ball x e. f x ≤ f y"
shows "∀y∈s. f x ≤ f y"
proof (rule ccontr)
have "x ∈ s" using assms(1,3) by auto
assume "¬ ?thesis"
then obtain y where "y∈s" and y: "f x > f y" by auto
then have xy: "0 < dist x y"  by auto
then obtain u where "0 < u" "u ≤ 1" and u: "u < e / dist x y"
using field_lbound_gt_zero[of 1 "e / dist x y"] xy ‹e>0› by auto
then have "f ((1-u) *⇩R x + u *⇩R y) ≤ (1-u) * f x + u * f y"
using ‹x∈s› ‹y∈s› by (smt (verit) assms(2) convex_on_def)
moreover
have *: "x - ((1 - u) *⇩R x + u *⇩R y) = u *⇩R (x - y)"
by (simp add: algebra_simps)
have "(1 - u) *⇩R x + u *⇩R y ∈ ball x e"
by (smt (verit) "*" ‹0 < u› dist_norm mem_ball norm_scaleR pos_less_divide_eq u xy)
then have "f x ≤ f ((1 - u) *⇩R x + u *⇩R y)"
using assms(4) by auto
ultimately show False
using mult_strict_left_mono[OF y ‹u>0›]
unfolding left_diff_distrib
by auto
qed

lemma convex_ball [iff]:
fixes x :: "'a::real_normed_vector"
shows "convex (ball x e)"
proof (auto simp: convex_def)
fix y z
assume yz: "dist x y < e" "dist x z < e"
fix u v :: real
assume uv: "0 ≤ u" "0 ≤ v" "u + v = 1"
have "dist x (u *⇩R y + v *⇩R z) ≤ u * dist x y + v * dist x z"
using uv yz
using convex_on_dist [of "ball x e" x, unfolded convex_on_def,
THEN bspec[where x=y], THEN bspec[where x=z]]
by auto
then show "dist x (u *⇩R y + v *⇩R z) < e"
using convex_bound_lt[OF yz uv] by auto
qed

lemma convex_cball [iff]:
fixes x :: "'a::real_normed_vector"
shows "convex (cball x e)"
proof -
{
fix y z
assume yz: "dist x y ≤ e" "dist x z ≤ e"
fix u v :: real
assume uv: "0 ≤ u" "0 ≤ v" "u + v = 1"
have "dist x (u *⇩R y + v *⇩R z) ≤ u * dist x y + v * dist x z"
using uv yz
using convex_on_dist [of "cball x e" x, unfolded convex_on_def,
THEN bspec[where x=y], THEN bspec[where x=z]]
by auto
then have "dist x (u *⇩R y + v *⇩R z) ≤ e"
using convex_bound_le[OF yz uv] by auto
}
then show ?thesis by (auto simp: convex_def Ball_def)
qed

lemma connected_ball [iff]:
fixes x :: "'a::real_normed_vector"
shows "connected (ball x e)"
using convex_connected convex_ball by auto

lemma connected_cball [iff]:
fixes x :: "'a::real_normed_vector"
shows "connected (cball x e)"
using convex_connected convex_cball by auto

lemma bounded_convex_hull:
fixes s :: "'a::real_normed_vector set"
assumes "bounded s"
shows "bounded (convex hull s)"
proof -
from assms obtain B where B: "∀x∈s. norm x ≤ B"
unfolding bounded_iff by auto
show ?thesis
by (simp add: bounded_subset[OF bounded_cball, of _ 0 B] B subsetI subset_hull)
qed

lemma finite_imp_bounded_convex_hull:
fixes s :: "'a::real_normed_vector set"
shows "finite s ⟹ bounded (convex hull s)"
using bounded_convex_hull finite_imp_bounded
by auto

subsection ‹Midpoint›

definition✐‹tag important› midpoint :: "'a::real_vector ⇒ 'a ⇒ 'a"
where "midpoint a b = (inverse (2::real)) *⇩R (a + b)"

lemma midpoint_idem [simp]: "midpoint x x = x"
unfolding midpoint_def  by simp

lemma midpoint_sym: "midpoint a b = midpoint b a"
unfolding midpoint_def by (auto simp add: scaleR_right_distrib)

lemma midpoint_eq_iff: "midpoint a b = c ⟷ a + b = c + c"
proof -
have "midpoint a b = c ⟷ scaleR 2 (midpoint a b) = scaleR 2 c"
by simp
then show ?thesis
unfolding midpoint_def scaleR_2 [symmetric] by simp
qed

lemma
fixes a::real
assumes "a ≤ b" shows ge_midpoint_1: "a ≤ midpoint a b"
and le_midpoint_1: "midpoint a b ≤ b"
by (simp_all add: midpoint_def assms)

lemma dist_midpoint:
fixes a b :: "'a::real_normed_vector" shows
"dist a (midpoint a b) = (dist a b) / 2" (is ?t1)
"dist b (midpoint a b) = (dist a b) / 2" (is ?t2)
"dist (midpoint a b) a = (dist a b) / 2" (is ?t3)
"dist (midpoint a b) b = (dist a b) / 2" (is ?t4)
proof -
have *: "⋀x y::'a. 2 *⇩R x = - y ⟹ norm x = (norm y) / 2"
unfolding equation_minus_iff by auto
have **: "⋀x y::'a. 2 *⇩R x =   y ⟹ norm x = (norm y) / 2"
by auto
note scaleR_right_distrib [simp]
show ?t1
unfolding midpoint_def dist_norm
apply (rule **)
apply (simp add: scaleR_right_diff_distrib)
apply (simp add: scaleR_2)
done
show ?t2
unfolding midpoint_def dist_norm
apply (rule *)
apply (simp add: scaleR_right_diff_distrib)
apply (simp add: scaleR_2)
done
show ?t3
unfolding midpoint_def dist_norm
apply (rule *)
apply (simp add: scaleR_right_diff_distrib)
apply (simp add: scaleR_2)
done
show ?t4
unfolding midpoint_def dist_norm
apply (rule **)
apply (simp add: scaleR_right_diff_distrib)
apply (simp add: scaleR_2)
done
qed

lemma midpoint_eq_endpoint [simp]:
"midpoint a b = a ⟷ a = b"
"midpoint a b = b ⟷ a = b"
unfolding midpoint_eq_iff by auto

lemma midpoint_plus_self [simp]: "midpoint a b + midpoint a b = a + b"
using midpoint_eq_iff by metis

lemma midpoint_linear_image:
"linear f ⟹ midpoint(f a)(f b) = f(midpoint a b)"
by (simp add: linear_iff midpoint_def)

subsection ‹Open and closed segments›

definition✐‹tag important› closed_segment :: "'a::real_vector ⇒ 'a ⇒ 'a set"
where "closed_segment a b = {(1 - u) *⇩R a + u *⇩R b | u::real. 0 ≤ u ∧ u ≤ 1}"

definition✐‹tag important› open_segment :: "'a::real_vector ⇒ 'a ⇒ 'a set" where
"open_segment a b ≡ closed_segment a b - {a,b}"

lemmas segment = open_segment_def closed_segment_def

lemma in_segment:
"x ∈ closed_segment a b ⟷ (∃u. 0 ≤ u ∧ u ≤ 1 ∧ x = (1 - u) *⇩R a + u *⇩R b)"
"x ∈ open_segment a b ⟷ a ≠ b ∧ (∃u. 0 < u ∧ u < 1 ∧ x = (1 - u) *⇩R a + u *⇩R b)"
using less_eq_real_def by (auto simp: segment algebra_simps)

lemma closed_segment_linear_image:
"closed_segment (f a) (f b) = f ` (closed_segment a b)" if "linear f"
proof -
interpret linear f by fact
show ?thesis
by (force simp add: in_segment add scale)
qed

lemma open_segment_linear_image:
"⟦linear f; inj f⟧ ⟹ open_segment (f a) (f b) = f ` (open_segment a b)"
by (force simp: open_segment_def closed_segment_linear_image inj_on_def)

lemma closed_segment_translation:
"closed_segment (c + a) (c + b) = (λx. c + x) ` (closed_segment a b)" (is "?L = _ ` ?R")
proof -
have "⋀x. x ∈ ?L ⟹ x - c ∈ ?R" "⋀x. ⟦x ∈ ?R⟧ ⟹ c + x ∈ ?L"
by (auto simp: in_segment algebra_simps)
then show ?thesis by force
qed

lemma open_segment_translation:
"open_segment (c + a) (c + b) = image (λx. c + x) (open_segment a b)"
by (simp add: open_segment_def closed_segment_translation translation_diff)

lemma closed_segment_of_real:
"closed_segment (of_real x) (of_real y) = of_real ` closed_segment x y"
by (simp add: closed_segment_linear_image linearI scaleR_conv_of_real)

lemma open_segment_of_real:
"open_segment (of_real x) (of_real y) = of_real ` open_segment x y"
by (simp add: closed_segment_of_real image_set_diff inj_of_real open_segment_def)

lemma closed_segment_Reals:
"⟦x ∈ Reals; y ∈ Reals⟧ ⟹ closed_segment x y = of_real ` closed_segment (Re x) (Re y)"
by (metis closed_segment_of_real of_real_Re)

lemma open_segment_Reals:
"⟦x ∈ Reals; y ∈ Reals⟧ ⟹ open_segment x y = of_real ` open_segment (Re x) (Re y)"
by (metis open_segment_of_real of_real_Re)

lemma open_segment_PairD:
"(x, x') ∈ open_segment (a, a') (b, b')
⟹ (x ∈ open_segment a b ∨ a = b) ∧ (x' ∈ open_segment a' b' ∨ a' = b')"
by (auto simp: in_segment)

lemma closed_segment_PairD:
"(x, x') ∈ closed_segment (a, a') (b, b') ⟹ x ∈ closed_segment a b ∧ x' ∈ closed_segment a' b'"
by (auto simp: closed_segment_def)

lemma closed_segment_translation_eq [simp]:
"d + x ∈ closed_segment (d + a) (d + b) ⟷ x ∈ closed_segment a b"
proof -
have *: "⋀d x a b. x ∈ closed_segment a b ⟹ d + x ∈ closed_segment (d + a) (d + b)"
using closed_segment_translation by blast
show ?thesis
using * [where d = "-d"] * by fastforce
qed

lemma open_segment_translation_eq [simp]:
"d + x ∈ open_segment (d + a) (d + b) ⟷ x ∈ open_segment a b"
by (simp add: open_segment_def)

lemma of_real_closed_segment [simp]:
"of_real x ∈ closed_segment (of_real a) (of_real b) ⟷ x ∈ closed_segment a b"
by (simp add: closed_segment_of_real image_iff)

lemma of_real_open_segment [simp]:
"of_real x ∈ open_segment (of_real a) (of_real b) ⟷ x ∈ open_segment a b"
by (simp add: image_iff open_segment_of_real)

lemma convex_contains_segment:
"convex S ⟷ (∀a∈S. ∀b∈S. closed_segment a b ⊆ S)"
unfolding convex_alt closed_segment_def by auto

lemma closed_segment_in_Reals:
"⟦x ∈ closed_segment a b; a ∈ Reals; b ∈ Reals⟧ ⟹ x ∈ Reals"
by (meson subsetD convex_Reals convex_contains_segment)

lemma open_segment_in_Reals:
"⟦x ∈ open_segment a b; a ∈ Reals; b ∈ Reals⟧ ⟹ x ∈ Reals"
by (metis Diff_iff closed_segment_in_Reals open_segment_def)

lemma closed_segment_subset: "⟦x ∈ S; y ∈ S; convex S⟧ ⟹ closed_segment x y ⊆ S"
by (simp add: convex_contains_segment)

lemma closed_segment_subset_convex_hull:
"⟦x ∈ convex hull S; y ∈ convex hull S⟧ ⟹ closed_segment x y ⊆ convex hull S"
using convex_contains_segment by blast

lemma segment_convex_hull:
"closed_segment a b = convex hull {a,b}"
proof -
have *: "⋀x. {x} ≠ {}" by auto
show ?thesis
unfolding segment convex_hull_insert[OF *] convex_hull_singleton
by (safe; rule_tac x="1 - u" in exI; force)
qed

lemma open_closed_segment: "u ∈ open_segment w z ⟹ u ∈ closed_segment w z"
by (auto simp add: closed_segment_def open_segment_def)

lemma segment_open_subset_closed:
"open_segment a b ⊆ closed_segment a b"
by (simp add: open_closed_segment subsetI)

lemma bounded_closed_segment:
fixes a :: "'a::real_normed_vector" shows "bounded (closed_segment a b)"
by (simp add: bounded_convex_hull segment_convex_hull)

lemma bounded_open_segment:
fixes a :: "'a::real_normed_vector" shows "bounded (open_segment a b)"
by (rule bounded_subset [OF bounded_closed_segment segment_open_subset_closed])

lemmas bounded_segment = bounded_closed_segment open_closed_segment

lemma ends_in_segment [iff]: "a ∈ closed_segment a b" "b ∈ closed_segment a b"
by (simp_all add: hull_inc segment_convex_hull)

lemma eventually_closed_segment:
fixes x0::"'a::real_normed_vector"
assumes "open X0" "x0 ∈ X0"
shows "∀⇩F x in at x0 within U. closed_segment x0 x ⊆ X0"
proof -
from openE[OF assms]
obtain e where e: "0 < e" "ball x0 e ⊆ X0" .
then have "∀⇩F x in at x0 within U. x ∈ ball x0 e"
by (auto simp: dist_commute eventually_at)
then show ?thesis
proof eventually_elim
case (elim x)
have "x0 ∈ ball x0 e" using ‹e > 0› by simp
then have "closed_segment x0 x ⊆ ball x0 e"
using closed_segment_subset elim by blast
then show ?case
using e(2) by auto
qed
qed

lemma closed_segment_commute: "closed_segment a b = closed_segment b a"
proof -
have "{a, b} = {b, a}" by auto
thus ?thesis
by (simp add: segment_convex_hull)
qed

lemma segment_bound1:
assumes "x ∈ closed_segment a b"
shows "norm (x - a) ≤ norm (b - a)"
proof -
obtain u where u: "x = (1 - u) *⇩R a + u *⇩R b" "0 ≤ u" "u ≤ 1"
using assms by (auto simp add: closed_segment_def)
then have "norm (u *⇩R b - u *⇩R a) ≤ norm (b - a)"
by (simp add: mult_left_le_one_le flip: scaleR_diff_right)
with u show ?thesis
by (metis add_diff_cancel_left scaleR_collapse)
qed

lemma segment_bound:
assumes "x ∈ closed_segment a b"
shows "norm (x - a) ≤ norm (b - a)" "norm (x - b) ≤ norm (b - a)"
by (metis assms closed_segment_commute dist_commute dist_norm segment_bound1)+

lemma open_segment_bound1:
assumes "x ∈ open_segment a b"
shows "norm (x - a) < norm (b - a)"
proof -
obtain u where u: "x = (1 - u) *⇩R a + u *⇩R b" "0 < u" "u < 1"
by (meson assms in_segment)
then have "norm (u *⇩R b - u *⇩R a) < norm (b - a)"
using assms in_segment(2) less_eq_real_def by (fastforce simp flip: scaleR_diff_right)
with u show ?thesis
by (metis add_diff_cancel_left scaleR_collapse)
qed

lemma open_segment_commute: "open_segment a b = open_segment b a"
by (simp add: closed_segment_commute insert_commute open_segment_def)

lemma closed_segment_idem [simp]: "closed_segment a a = {a}"
unfolding segment by (auto simp add: algebra_simps)

lemma open_segment_idem [simp]: "open_segment a a = {}"
by (simp add: open_segment_def)

lemma closed_segment_eq_open: "closed_segment a b = open_segment a b ∪ {a,b}"
using open_segment_def by auto

lemma convex_contains_open_segment:
"convex s ⟷ (∀a∈s. ∀b∈s. open_segment a b ⊆ s)"
by (simp add: convex_contains_segment closed_segment_eq_open)

lemma closed_segment_eq_real_ivl1:
fixes a b::real
assumes "a ≤ b"
shows "closed_segment a b = {a .. b}"
proof safe
fix x
assume "x ∈ closed_segment a b"
then obtain u where u: "0 ≤ u" "u ≤ 1" and x_def: "x = (1 - u) * a + u * b"
by (auto simp: closed_segment_def)
have "u * a ≤ u * b" "(1 - u) * a ≤ (1 - u) * b"
by (auto intro!: mult_left_mono u assms)
then show "x ∈ {a .. b}"
unfolding x_def by (auto simp: algebra_simps)
next
show "⋀x. x ∈ {a..b} ⟹ x ∈ closed_segment a b"
by (force simp: closed_segment_def divide_simps algebra_simps
intro: exI[where x="(x - a) / (b - a)" for x])
qed

lemma closed_segment_eq_real_ivl:
fixes a b::real
shows "closed_segment a b = (if a ≤ b then {a .. b} else {b .. a})"
by (metis closed_segment_commute closed_segment_eq_real_ivl1 nle_le)

lemma open_segment_eq_real_ivl:
fixes a b::real
shows "open_segment a b = (if a ≤ b then {a<..<b} else {b<..<a})"
by (auto simp: closed_segment_eq_real_ivl open_segment_def split: if_split_asm)

lemma closed_segment_real_eq:
fixes u::real shows "closed_segment u v = (λx. (v - u) * x + u) ` {0..1}"
by (simp add: closed_segment_eq_real_ivl image_affinity_atLeastAtMost)

lemma closed_segment_same_Re:
assumes "Re a = Re b"
shows   "closed_segment a b = {z. Re z = Re a ∧ Im z ∈ closed_segment (Im a) (Im b)}"
proof safe
fix z assume "z ∈ closed_segment a b"
then obtain u where u: "u ∈ {0..1}" "z = a + of_real u * (b - a)"
by (auto simp: closed_segment_def scaleR_conv_of_real algebra_simps)
from assms show "Re z = Re a" by (auto simp: u)
from u(1) show "Im z ∈ closed_segment (Im a) (Im b)"
by (force simp: u closed_segment_def algebra_simps)
next
fix z assume [simp]: "Re z = Re a" and "Im z ∈ closed_segment (Im a) (Im b)"
then obtain u where u: "u ∈ {0..1}" "Im z = Im a + of_real u * (Im b - Im a)"
by (auto simp: closed_segment_def scaleR_conv_of_real algebra_simps)
from u(1) show "z ∈ closed_segment a b" using assms
by (force simp: u closed_segment_def algebra_simps scaleR_conv_of_real complex_eq_iff)
qed

lemma closed_segment_same_Im:
assumes "Im a = Im b"
shows   "closed_segment a b = {z. Im z = Im a ∧ Re z ∈ closed_segment (Re a) (Re b)}"
proof safe
fix z assume "z ∈ closed_segment a b"
then obtain u where u: "u ∈ {0..1}" "z = a + of_real u * (b - a)"
by (auto simp: closed_segment_def scaleR_conv_of_real algebra_simps)
from assms show "Im z = Im a" by (auto simp: u)
from u(1) show "Re z ∈ closed_segment (Re a) (Re b)"
by (force simp: u closed_segment_def algebra_simps)
next
fix z assume [simp]: "Im z = Im a" and "Re z ∈ closed_segment (Re a) (Re b)"
then obtain u where u: "u ∈ {0..1}" "Re z = Re a + of_real u * (Re b - Re a)"
by (auto simp: closed_segment_def scaleR_conv_of_real algebra_simps)
from u(1) show "z ∈ closed_segment a b" using assms
by (force simp: u closed_segment_def algebra_simps scaleR_conv_of_real complex_eq_iff)
qed

lemma dist_in_closed_segment:
fixes a :: "'a :: euclidean_space"
assumes "x ∈ closed_segment a b"
shows "dist x a ≤ dist a b ∧ dist x b ≤ dist a b"
by (metis assms dist_commute dist_norm segment_bound(2) segment_bound1)

lemma dist_in_open_segment:
fixes a :: "'a :: euclidean_space"
assumes "x ∈ open_segment a b"
shows "dist x a < dist a b ∧ dist x b < dist a b"
by (metis assms dist_commute dist_norm open_segment_bound1 open_segment_commute)

lemma dist_decreases_open_segment_0:
fixes x :: "'a :: euclidean_space"
assumes "x ∈ open_segment 0 b"
shows "dist c x < dist c 0 ∨ dist c x < dist c b"
proof (rule ccontr, clarsimp simp: not_less)
obtain u where u: "0 ≠ b" "0 < u" "u < 1" and x: "x = u *⇩R b"
using assms by (auto simp: in_segment)
have xb: "x ∙ b < b ∙ b"
using u x by auto
assume "norm c ≤ dist c x"
then have "c ∙ c ≤ (c - x) ∙ (c - x)"
by (simp add: dist_norm norm_le)
moreover have "0 < x ∙ b"
using u x by auto
ultimately have less: "c ∙ b < x ∙ b"
by (simp add: x algebra_simps inner_commute u)
assume "dist c b ≤ dist c x"
then have "(c - b) ∙ (c - b) ≤ (c - x) ∙ (c - x)"
by (simp add: dist_norm norm_le)
then have "(b ∙ b) * (1 - u*u) ≤ 2 * (b ∙ c) * (1-u)"
by (simp add: x algebra_simps inner_commute)
then have "(1+u) * (b ∙ b) * (1-u) ≤ 2 * (b ∙ c) * (1-u)"
by (simp add: algebra_simps)
then have "(1+u) * (b ∙ b) ≤ 2 * (b ∙ c)"
using ‹u < 1› by auto
with xb have "c ∙ b ≥ x ∙ b"
by (auto simp: x algebra_simps inner_commute)
with less show False by auto
qed

proposition dist_decreases_open_segment:
fixes a :: "'a :: euclidean_space"
assumes "x ∈ open_segment a b"
shows "dist c x < dist c a ∨ dist c x < dist c b"
proof -
have *: "x - a ∈ open_segment 0 (b - a)" using assms
by (metis diff_self open_segment_translation_eq uminus_add_conv_diff)
show ?thesis
using dist_decreases_open_segment_0 [OF *, of "c-a"] assms
by (simp add: dist_norm)
qed

corollary open_segment_furthest_le:
fixes a b x y :: "'a::euclidean_space"
assumes "x ∈ open_segment a b"
shows "norm (y - x) < norm (y - a) ∨  norm (y - x) < norm (y - b)"
by (metis assms dist_decreases_open_segment dist_norm)

corollary dist_decreases_closed_segment:
fixes a :: "'a :: euclidean_space"
assumes "x ∈ closed_segment a b"
shows "dist c x ≤ dist c a ∨ dist c x ≤ dist c b"
by (smt (verit, ccfv_threshold) Un_iff assms closed_segment_eq_open dist_norm empty_iff insertE open_segment_furthest_le)

corollary segment_furthest_le:
fixes a b x y :: "'a::euclidean_space"
assumes "x ∈ closed_segment a b"
shows "norm (y - x) ≤ norm (y - a) ∨  norm (y - x) ≤ norm (y - b)"
by (metis assms dist_decreases_closed_segment dist_norm)

lemma convex_intermediate_ball:
fixes a :: "'a :: euclidean_space"
shows "⟦ball a r ⊆ T; T ⊆ cball a r⟧ ⟹ convex T"
by (smt (verit) convex_contains_open_segment dist_decreases_open_segment mem_ball mem_cball subset_eq)

lemma csegment_midpoint_subset: "closed_segment (midpoint a b) b ⊆ closed_segment a b"
apply (clarsimp simp: midpoint_def in_segment)
apply (rule_tac x="(1 + u) / 2" in exI)
apply (simp add: algebra_simps add_divide_distrib diff_divide_distrib)
by (metis field_sum_of_halves scaleR_left.add)

lemma notin_segment_midpoint:
fixes a :: "'a :: euclidean_space"
shows "a ≠ b ⟹ a ∉ closed_segment (midpoint a b) b"
by (auto simp: dist_midpoint dest!: dist_in_closed_segment)

subsubsection‹More lemmas, especially for working with the underlying formula›

lemma segment_eq_compose:
fixes a :: "'a :: real_vector"
shows "(λu. (1 - u) *⇩R a + u *⇩R b) = (λx. a + x) o (λu. u *⇩R (b - a))"
by (simp add: o_def algebra_simps)

lemma segment_degen_1:
fixes a :: "'a :: real_vector"
shows "(1 - u) *⇩R a + u *⇩R b = b ⟷ a=b ∨ u=1"
by (smt (verit, best) add_right_cancel scaleR_cancel_left scaleR_collapse)

lemma segment_degen_0:
fixes a :: "'a :: real_vector"
shows "(1 - u) *⇩R a + u *⇩R b = a ⟷ a=b ∨ u=0"
using segment_degen_1 [of "1-u" b a] by (auto simp: algebra_simps)

lemma add_scaleR_degen:
fixes a b ::"'a::real_vector"
assumes  "(u *⇩R b + v *⇩R a) = (u *⇩R a + v *⇩R b)"  "u ≠ v"
shows "a=b"
by (smt (verit) add_diff_cancel_left' add_diff_eq assms scaleR_cancel_left scaleR_left.diff)

lemma closed_segment_image_interval:
"closed_segment a b = (λu. (1 - u) *⇩R a + u *⇩R b) ` {0..1}"
by (auto simp: set_eq_iff image_iff closed_segment_def)

lemma open_segment_image_interval:
"open_segment a b = (if a=b then {} else (λu. (1 - u) *⇩R a + u *⇩R b) ` {0<..<1})"
by (auto simp:  open_segment_def closed_segment_def segment_degen_0 segment_degen_1)

lemmas segment_image_interval = closed_segment_image_interval open_segment_image_interval

lemma closed_segment_neq_empty [simp]: "closed_segment a b ≠ {}"
by auto

lemma open_segment_eq_empty [simp]: "open_segment a b = {} ⟷ a = b"
by (simp add: segment_image_interval(2))

lemma open_segment_eq_empty' [simp]: "{} = open_segment a b ⟷ a = b"
using open_segment_eq_empty by blast

lemmas segment_eq_empty = closed_segment_neq_empty open_segment_eq_empty

lemma inj_segment:
fixes a :: "'a :: real_vector"
assumes "a ≠ b"
shows "inj_on (λu. (1 - u) *⇩R a + u *⇩R b) I"
proof
fix x y
assume "(1 - x) *⇩R a + x *⇩R b = (1 - y) *⇩R a + y *⇩R b"
then have "x *⇩R (b - a) = y *⇩R (b - a)"
by (simp add: algebra_simps)
with assms show "x = y"
by (simp add: real_vector.scale_right_imp_eq)
qed

lemma finite_closed_segment [simp]: "finite(closed_segment a b) ⟷ a = b"
using infinite_Icc [OF zero_less_one] finite_imageD [OF _ inj_segment [of a b]]
unfolding segment_image_interval
by (smt (verit, del_insts) finite.emptyI finite_insert finite_subset image_subset_iff insertCI segment_degen_0)

lemma finite_open_segment [simp]: "finite(open_segment a b) ⟷ a = b"
by (auto simp: open_segment_def)

lemmas finite_segment = finite_closed_segment finite_open_segment

lemma closed_segment_eq_sing: "closed_segment a b = {c} ⟷ a = c ∧ b = c"
by auto

lemma open_segment_eq_sing: "open_segment a b ≠ {c}"
by (metis finite_insert finite_open_segment insert_not_empty open_segment_image_interval)

lemmas segment_eq_sing = closed_segment_eq_sing open_segment_eq_sing

lemma compact_segment [simp]:
fixes a :: "'a::real_normed_vector"
shows "compact (closed_segment a b)"
by (auto simp: segment_image_interval intro!: compact_continuous_image continuous_intros)

lemma closed_segment [simp]:
fixes a :: "'a::real_normed_vector"
shows "closed (closed_segment a b)"
by (simp add: compact_imp_closed)

lemma closure_closed_segment [simp]:
fixes a :: "'a::real_normed_vector"
shows "closure(closed_segment a b) = closed_segment a b"
by simp

lemma open_segment_bound:
assumes "x ∈ open_segment a b"
shows "norm (x - a) < norm (b - a)" "norm (x - b) < norm (b - a)"
by (metis assms norm_minus_commute open_segment_bound1 open_segment_commute)+

lemma closure_open_segment [simp]:
"closure (open_segment a b) = (if a = b then {} else closed_segment a b)"
for a :: "'a::euclidean_space"
proof (cases "a = b")
case True
then show ?thesis
by simp
next
case False
have "closure ((λu. u *⇩R (b - a)) ` {0<..<1}) = (λu. u *⇩R (b - a)) ` closure {0<..<1}"
proof (rule closure_injective_linear_image [symmetric])
qed (use False in ‹auto intro!: injI›)
then have "closure
((λu. (1 - u) *⇩R a + u *⇩R b) ` {0<..<1}) = (λx. (1 - x) *⇩R a + x *⇩R b) ` closure {0<..<1}"
using closure_translation [of a "((λx. x *⇩R b - x *⇩R a) ` {0<..<1})"]
by (simp add: segment_eq_compose field_simps scaleR_diff_left scaleR_diff_right image_image)
then show ?thesis
by (simp add: segment_image_interval closure_greaterThanLessThan [symmetric] del: closure_greaterThanLessThan)
qed

lemma closed_open_segment_iff [simp]:
fixes a :: "'a::euclidean_space"  shows "closed(open_segment a b) ⟷ a = b"
by (metis open_segment_def DiffE closure_eq closure_open_segment ends_in_segment(1) insert_iff segment_image_interval(2))

lemma compact_open_segment_iff [simp]:
fixes a :: "'a::euclidean_space"  shows "compact(open_segment a b) ⟷ a = b"
by (simp add: bounded_open_segment compact_eq_bounded_closed)

lemma convex_closed_segment [iff]: "convex (closed_segment a b)"
unfolding segment_convex_hull by(rule convex_convex_hull)

lemma convex_open_segment [iff]: "convex (open_segment a b)"
proof -
have "convex ((λu. u *⇩R (b - a)) ` {0<..<1})"
by (rule convex_linear_image) auto
then have "convex ((+) a ` (λu. u *⇩R (b - a)) ` {0<..<1})"
by (rule convex_translation)
then show ?thesis
by (simp add: image_image open_segment_image_interval segment_eq_compose field_simps scaleR_diff_left scaleR_diff_right)
qed

lemmas convex_segment = convex_closed_segment convex_open_segment

lemma subset_closed_segment:
"closed_segment a b ⊆ closed_segment c d ⟷
a ∈ closed_segment c d ∧ b ∈ closed_segment c d"
using closed_segment_subset convex_closed_segment ends_in_segment in_mono by blast

lemma subset_co_segment:
"closed_segment a b ⊆ open_segment c d ⟷
a ∈ open_segment c d ∧ b ∈ open_segment c d"
using closed_segment_subset by blast

lemma subset_open_segment:
fixes a :: "'a::euclidean_space"
shows "open_segment a b ⊆ open_segment c d ⟷
a = b ∨ a ∈ closed_segment c d ∧ b ∈ closed_segment c d"
(is "?lhs = ?rhs")
proof (cases "a = b")
case True then show ?thesis by simp
next
case False show ?thesis
proof
assume rhs: ?rhs
with ‹a ≠ b› have "c ≠ d"
using closed_segment_idem singleton_iff by auto
have "∃uc. (1 - u) *⇩R ((1 - ua) *⇩R c + ua *⇩R d) + u *⇩R ((1 - ub) *⇩R c + ub *⇩R d) =
(1 - uc) *⇩R c + uc *⇩R d ∧ 0 < uc ∧ uc < 1"
if neq: "(1 - ua) *⇩R c + ua *⇩R d ≠ (1 - ub) *⇩R c + ub *⇩R d" "c ≠ d"
and "a = (1 - ua) *⇩R c + ua *⇩R d" "b = (1 - ub) *⇩R c + ub *⇩R d"
and u: "0 < u" "u < 1" and uab: "0 ≤ ua" "ua ≤ 1" "0 ≤ ub" "ub ≤ 1"
for u ua ub
proof -
have "ua ≠ ub"
using neq by auto
moreover have "(u - 1) * ua ≤ 0" using u uab
by (simp add: mult_nonpos_nonneg)
ultimately have lt: "(u - 1) * ua < u * ub" using u uab
by (metis antisym_conv diff_ge_0_iff_ge le_less_trans mult_eq_0_iff mult_le_0_iff not_less)
have "p * ua + q * ub < p+q" if p: "0 < p" and  q: "0 < q" for p q
proof -
have "¬ p ≤ 0" "¬ q ≤ 0"
using p q not_less by blast+
then show ?thesis
by (smt (verit) ‹ua ≠ ub› mult_cancel_left1 mult_left_le uab(2) uab(4))
qed
then have "(1 - u) * ua + u * ub < 1" using u ‹ua ≠ ub›
by (metis diff_add_cancel diff_gt_0_iff_gt)
with lt show ?thesis
by (rule_tac x="ua + u*(ub-ua)" in exI) (simp add: algebra_simps)
qed
with rhs ‹a ≠ b› ‹c ≠ d› show ?lhs
unfolding open_segment_image_interval closed_segment_def
by (fastforce simp add:)
next
assume lhs: ?lhs
with ‹a ≠ b› have "c ≠ d"
by (meson finite_open_segment rev_finite_subset)
have "closure (open_segment a b) ⊆ closure (open_segment c d)"
using lhs closure_mono by blast
then have "closed_segment a b ⊆ closed_segment c d"
by (simp add: ‹a ≠ b› ‹c ≠ d›)
then show ?rhs
by (force simp: ‹a ≠ b›)
qed
qed

lemma closed_segment_same_fst:
"fst a = fst b ⟹ closed_segment a b = {fst a} × closed_segment (snd a) (snd b)"
by (auto simp: closed_segment_def scaleR_prod_def)

lemma closed_segment_same_snd:
"snd a = snd b ⟹ closed_segment a b = closed_segment (fst a) (fst b) × {snd a}"
by (auto simp: closed_segment_def scaleR_prod_def)

lemma subset_oc_segment:
fixes a :: "'a::euclidean_space"
shows "open_segment a b ⊆ closed_segment c d ⟷
a = b ∨ a ∈ closed_segment c d ∧ b ∈ closed_segment c d"
(is "?lhs = ?rhs")
proof
show "?lhs ⟹ ?rhs"
by (metis closure_closed_segment closure_mono closure_open_segment subset_closed_segment)
show "?rhs ⟹ ?lhs"
by (meson dual_order.trans segment_open_subset_closed subset_open_segment)
qed

lemmas subset_segment = subset_closed_segment subset_co_segment subset_oc_segment subset_open_segment

lemma dist_half_times2:
fixes a :: "'a :: real_normed_vector"
shows "dist ((1 / 2) *⇩R (a + b)) x * 2 = dist (a+b) (2 *⇩R x)"
proof -
have "norm ((1 / 2) *⇩R (a + b) - x) * 2 = norm (2 *⇩R ((1 / 2) *⇩R (a + b) - x))"
by simp
also have "... = norm ((a + b) - 2 *⇩R x)"
by (simp add: real_vector.scale_right_diff_distrib)
finally show ?thesis
by (simp only: dist_norm)
qed

lemma closed_segment_as_ball:
"closed_segment a b = affine hull {a,b} ∩ cball(inverse 2 *⇩R (a + b))(norm(b - a) / 2)"
proof (cases "b = a")
case True then show ?thesis by (auto simp: hull_inc)
next
case False
then have *: "((∃u v. x = u *⇩R a + v *⇩R b ∧ u + v = 1) ∧
dist ((1 / 2) *⇩R (a + b)) x * 2 ≤ norm (b - a)) =
(∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ 0 ≤ u ∧ u ≤ 1)" for x
proof -
have "((∃u v. x = u *⇩R a + v *⇩R b ∧ u + v = 1) ∧
dist ((1 / 2) *⇩R (a + b)) x * 2 ≤ norm (b - a)) =
((∃u. x = (1 - u) *⇩R a + u *⇩R b) ∧
dist ((1 / 2) *⇩R (a + b)) x * 2 ≤ norm (b - a))"
unfolding eq_diff_eq [symmetric] by simp
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
norm ((a+b) - (2 *⇩R x)) ≤ norm (b - a))"
by (simp add: dist_half_times2) (simp add: dist_norm)
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
norm ((a+b) - (2 *⇩R ((1 - u) *⇩R a + u *⇩R b))) ≤ norm (b - a))"
by auto
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
norm ((1 - u * 2) *⇩R (b - a)) ≤ norm (b - a))"
by (simp add: algebra_simps scaleR_2)
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
¦1 - u * 2¦ * norm (b - a) ≤ norm (b - a))"
by simp
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ ¦1 - u * 2¦ ≤ 1)"
by (simp add: mult_le_cancel_right2 False)
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ 0 ≤ u ∧ u ≤ 1)"
by auto
finally show ?thesis .
qed
show ?thesis
by (simp add: affine_hull_2 Set.set_eq_iff closed_segment_def *)
qed

lemma open_segment_as_ball:
"open_segment a b =
affine hull {a,b} ∩ ball(inverse 2 *⇩R (a + b))(norm(b - a) / 2)"
proof (cases "b = a")
case True then show ?thesis by (auto simp: hull_inc)
next
case False
then have *: "((∃u v. x = u *⇩R a + v *⇩R b ∧ u + v = 1) ∧
dist ((1 / 2) *⇩R (a + b)) x * 2 < norm (b - a)) =
(∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ 0 < u ∧ u < 1)" for x
proof -
have "((∃u v. x = u *⇩R a + v *⇩R b ∧ u + v = 1) ∧
dist ((1 / 2) *⇩R (a + b)) x * 2 < norm (b - a)) =
((∃u. x = (1 - u) *⇩R a + u *⇩R b) ∧
dist ((1 / 2) *⇩R (a + b)) x * 2 < norm (b - a))"
unfolding eq_diff_eq [symmetric] by simp
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
norm ((a+b) - (2 *⇩R x)) < norm (b - a))"
by (simp add: dist_half_times2) (simp add: dist_norm)
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
norm ((a+b) - (2 *⇩R ((1 - u) *⇩R a + u *⇩R b))) < norm (b - a))"
by auto
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
norm ((1 - u * 2) *⇩R (b - a)) < norm (b - a))"
by (simp add: algebra_simps scaleR_2)
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧
¦1 - u * 2¦ * norm (b - a) < norm (b - a))"
by simp
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ ¦1 - u * 2¦ < 1)"
by (simp add: mult_le_cancel_right2 False)
also have "... = (∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ 0 < u ∧ u < 1)"
by auto
finally show ?thesis .
qed
show ?thesis
using False by (force simp: affine_hull_2 Set.set_eq_iff open_segment_image_interval *)
qed

lemmas segment_as_ball = closed_segment_as_ball open_segment_as_ball

lemma connected_segment [iff]:
fixes x :: "'a :: real_normed_vector"
shows "connected (closed_segment x y)"
by (simp add: convex_connected)

lemma is_interval_closed_segment_1[intro, simp]: "is_interval (closed_segment a b)" for a b::real
unfolding closed_segment_eq_real_ivl
by (auto simp: is_interval_def)

lemma IVT'_closed_segment_real:
fixes f :: "real ⇒ real"
assumes "y ∈ closed_segment (f a) (f b)"
assumes "continuous_on (closed_segment a b) f"
shows "∃x ∈ closed_segment a b. f x = y"
using IVT'[of f a y b]
IVT'[of "-f" a "-y" b]
IVT'[of f b y a]
IVT'[of "-f" b "-y" a] assms
by (cases "a ≤ b"; cases "f b ≥ f a") (auto simp: closed_segment_eq_real_ivl continuous_on_minus)

subsection ‹Betweenness›

definition✐‹tag important› "between = (λ(a,b) x. x ∈ closed_segment a b)"

lemma betweenI:
assumes "0 ≤ u" "u ≤ 1" "x = (1 - u) *⇩R a + u *⇩R b"
shows "between (a, b) x"
using assms unfolding between_def closed_segment_def by auto

lemma betweenE:
assumes "between (a, b) x"
obtains u where "0 ≤ u" "u ≤ 1" "x = (1 - u) *⇩R a + u *⇩R b"
using assms unfolding between_def closed_segment_def by auto

lemma between_implies_scaled_diff:
assumes "between (S, T) X" "between (S, T) Y" "S ≠ Y"
obtains c where "(X - Y) = c *⇩R (S - Y)"
proof -
from ‹between (S, T) X› obtain u⇩X where X: "X = u⇩X *⇩R S + (1 - u⇩X) *⇩R T"
by (metis add.commute betweenE eq_diff_eq)
from ‹between (S, T) Y› obtain u⇩Y where Y: "Y = u⇩Y *⇩R S + (1 - u⇩Y) *⇩R T"
by (metis add.commute betweenE eq_diff_eq)
have "X - Y = (u⇩X - u⇩Y) *⇩R (S - T)"
by (simp add: X Y scaleR_left.diff scaleR_right_diff_distrib)
moreover from Y have "S - Y = (1 - u⇩Y) *⇩R (S - T)"
by (simp add: real_vector.scale_left_diff_distrib real_vector.scale_right_diff_distrib)
moreover note ‹S ≠ Y›
ultimately have "(X - Y) = ((u⇩X - u⇩Y) / (1 - u⇩Y)) *⇩R (S - Y)" by auto
from this that show thesis by blast
qed

lemma between_mem_segment: "between (a,b) x ⟷ x ∈ closed_segment a b"
unfolding between_def by auto

lemma between: "between (a, b) (x::'a::euclidean_space) ⟷ dist a b = (dist a x) + (dist x b)"
proof (cases "a = b")
case True
then show ?thesis
by (auto simp add: between_def dist_commute)
next
case False
then have Fal: "norm (a - b) ≠ 0" and Fal2: "norm (a - b) > 0"
by auto
have *: "⋀u. a - ((1 - u) *⇩R a + u *⇩R b) = u *⇩R (a - b)"
by (auto simp add: algebra_simps)
have "norm (a - x) *⇩R (x - b) = norm (x - b) *⇩R (a - x)" if "x = (1 - u) *⇩R a + u *⇩R b" "0 ≤ u" "u ≤ 1" for u
proof -
have *: "a - x = u *⇩R (a - b)" "x - b = (1 - u) *⇩R (a - b)"
unfolding that(1) by (auto simp add:algebra_simps)
show "norm (a - x) *⇩R (x - b) = norm (x - b) *⇩R (a - x)"
unfolding norm_minus_commute[of x a] * using ‹0 ≤ u› ‹u ≤ 1›
by simp
qed
moreover have "∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ 0 ≤ u ∧ u ≤ 1" if "dist a b = dist a x + dist x b"
proof -
let ?β = "norm (a - x) / norm (a - b)"
show "∃u. x = (1 - u) *⇩R a + u *⇩R b ∧ 0 ≤ u ∧ u ≤ 1"
proof (intro exI conjI)
show "?β ≤ 1"
using Fal2 unfolding that[unfolded dist_norm] norm_ge_zero by auto
show "x = (1 - ?β) *⇩R a + (?β) *⇩R b"
proof (subst euclidean_eq_iff; intro ballI)
fix i :: 'a
assume i: "i ∈ Basis"
have "((1 - ?β) *⇩R a + (?β) *⇩R b) ∙ i
= ((norm (a - b) - norm (a - x)) * (a ∙ i) + norm (a - x) * (b ∙ i)) / norm (a - b)"
using Fal by (auto simp add: field_simps inner_simps)
also have "… = x∙i"
apply (rule divide_eq_imp[OF Fal])
unfolding that[unfolded dist_norm]
using that[unfolded dist_triangle_eq] i
apply (subst (asm) euclidean_eq_iff)
apply (auto simp add: field_simps inner_simps)
done
finally show "x ∙ i = ((1 - ?β) *⇩R a + (?β) *⇩R b) ∙ i"
by auto
qed
qed (use Fal2 in auto)
qed
ultimately show ?thesis
by (force simp add: between_def closed_segment_def dist_triangle_eq)
qed

lemma between_midpoint:
fixes a :: "'a::euclidean_space"
shows "between (a,b) (midpoint a b)" (is ?t1)
and "between (b,a) (midpoint a b)" (is ?t2)
proof -
have *: "⋀x y z. x = (1/2::real) *⇩R z ⟹ y = (1/2) *⇩R z ⟹ norm z = norm x + norm y"
by auto
show ?t1 ?t2
unfolding between midpoint_def dist_norm
by (auto simp add: field_simps inner_simps euclidean_eq_iff[where 'a='a] intro!: *)
qed

lemma between_mem_convex_hull:
"between (a,b) x ⟷ x ∈ convex hull {a,b}"
unfolding between_mem_segment segment_convex_hull ..

lemma between_triv_iff [simp]: "between (a,a) b ⟷ a=b"
by (auto simp: between_def)

lemma between_triv1 [simp]: "between (a,b) a"
by (auto simp: between_def)

lemma between_triv2 [simp]: "between (a,b) b"
by (auto simp: between_def)

lemma between_commute:
"between (a,b) = between (b,a)"
by (auto simp: between_def closed_segment_commute)

lemma between_antisym:
fixes a :: "'a :: euclidean_space"
shows "⟦between (b,c) a; between (a,c) b⟧ ⟹ a = b"
by (auto simp: between dist_commute)

lemma between_trans:
fixes a :: "'a :: euclidean_space"
shows "⟦between (b,c) a; between (a,c) d⟧ ⟹ between (b,c) d"
using dist_triangle2 [of b c d] dist_triangle3 [of b d a]
by (auto simp: between dist_commute)

lemma between_norm:
fixes a :: "'a :: euclidean_space"
shows "between (a,b) x ⟷ norm(x - a) *⇩R (b - x) = norm(b - x) *⇩R (x - a)"
by (auto simp: between dist_triangle_eq norm_minus_commute algebra_simps)

lemma between_swap:
fixes A B X Y :: "'a::euclidean_space"
assumes "between (A, B) X"
assumes "between (A, B) Y"
shows "between (X, B) Y ⟷ between (A, Y) X"
using assms by (auto simp add: between)

lemma between_translation [simp]: "between (a + y,a + z) (a + x) ⟷ between (y,z) x"
by (auto simp: between_def)

lemma between_trans_2:
fixes a :: "'a :: euclidean_space"
shows "⟦between (b,c) a; between (a,b) d⟧ ⟹ between (c,d) a"
by (metis between_commute between_swap between_trans)

lemma between_scaleR_lift [simp]:
fixes v :: "'a::euclidean_space"
shows "between (a *⇩R v, b *⇩R v) (c *⇩R v) ⟷ v = 0 ∨ between (a, b) c"
by (simp add: between dist_norm flip: scaleR_left_diff_distrib distrib_right)

lemma between_1:
fixes x::real
shows "between (a,b) x ⟷ (a ≤ x ∧ x ≤ b) ∨ (b ≤ x ∧ x ≤ a)"
by (auto simp: between_mem_segment closed_segment_eq_real_ivl)

end```