(* Title: HOL/Real/HahnBanach/Bounds.thy
ID: $Id$
Author: Gertrud Bauer, TU Munich
*)
header {* Bounds *}
theory Bounds = Main + Real:
locale lub =
fixes A and x
assumes least [intro?]: "(\<And>a. a \<in> A \<Longrightarrow> a \<le> b) \<Longrightarrow> x \<le> b"
and upper [intro?]: "a \<in> A \<Longrightarrow> a \<le> x"
lemmas [elim?] = lub.least lub.upper
constdefs
the_lub :: "'a::order set \<Rightarrow> 'a"
"the_lub A \<equiv> The (lub A)"
syntax (xsymbols)
the_lub :: "'a::order set \<Rightarrow> 'a" ("\<Squnion>_" [90] 90)
lemma the_lub_equality [elim?]:
includes lub
shows "\<Squnion>A = (x::'a::order)"
proof (unfold the_lub_def)
from lub_axioms show "The (lub A) = x"
proof
fix x' assume lub': "lub A x'"
show "x' = x"
proof (rule order_antisym)
from lub' show "x' \<le> x"
proof
fix a assume "a \<in> A"
then show "a \<le> x" ..
qed
show "x \<le> x'"
proof
fix a assume "a \<in> A"
with lub' show "a \<le> x'" ..
qed
qed
qed
qed
lemma the_lubI_ex:
assumes ex: "\<exists>x. lub A x"
shows "lub A (\<Squnion>A)"
proof -
from ex obtain x where x: "lub A x" ..
also from x have [symmetric]: "\<Squnion>A = x" ..
finally show ?thesis .
qed
lemma lub_compat: "lub A x = isLub UNIV A x"
proof -
have "isUb UNIV A = (\<lambda>x. A *<= x \<and> x \<in> UNIV)"
by (rule ext) (simp only: isUb_def)
then show ?thesis
by (simp only: lub_def isLub_def leastP_def setge_def setle_def) blast
qed
lemma real_complete:
fixes A :: "real set"
assumes nonempty: "\<exists>a. a \<in> A"
and ex_upper: "\<exists>y. \<forall>a \<in> A. a \<le> y"
shows "\<exists>x. lub A x"
proof -
from ex_upper have "\<exists>y. isUb UNIV A y"
by (unfold isUb_def setle_def) blast
with nonempty have "\<exists>x. isLub UNIV A x"
by (rule reals_complete)
then show ?thesis by (simp only: lub_compat)
qed
end