author | wenzelm |
Sat, 16 Dec 2000 21:41:51 +0100 | |
changeset 10687 | c186279eecea |
parent 9969 | 4753185f1dd2 |
child 11472 | d08d4e17a5f6 |
permissions | -rw-r--r-- |
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(* Title: HOL/Real/HahnBanach/FunctionOrder.thy |
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ID: $Id$ |
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Author: Gertrud Bauer, TU Munich |
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*) |
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header {* An order on functions *} |
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theory FunctionOrder = Subspace + Linearform: |
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subsection {* The graph of a function *} |
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text {* |
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We define the \emph{graph} of a (real) function @{text f} with |
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domain @{text F} as the set |
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\begin{center} |
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@{text "{(x, f x). x \<in> F}"} |
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\end{center} |
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So we are modeling partial functions by specifying the domain and |
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the mapping function. We use the term ``function'' also for its |
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graph. |
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*} |
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types 'a graph = "('a * real) set" |
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constdefs |
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graph :: "'a set \<Rightarrow> ('a \<Rightarrow> real) \<Rightarrow> 'a graph " |
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"graph F f \<equiv> {(x, f x) | x. x \<in> F}" |
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lemma graphI [intro?]: "x \<in> F \<Longrightarrow> (x, f x) \<in> graph F f" |
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by (unfold graph_def, intro CollectI exI) blast |
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lemma graphI2 [intro?]: "x \<in> F \<Longrightarrow> \<exists>t\<in> (graph F f). t = (x, f x)" |
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by (unfold graph_def) blast |
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lemma graphD1 [intro?]: "(x, y) \<in> graph F f \<Longrightarrow> x \<in> F" |
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by (unfold graph_def) blast |
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lemma graphD2 [intro?]: "(x, y) \<in> graph H h \<Longrightarrow> y = h x" |
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by (unfold graph_def) blast |
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subsection {* Functions ordered by domain extension *} |
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text {* A function @{text h'} is an extension of @{text h}, iff the |
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graph of @{text h} is a subset of the graph of @{text h'}. *} |
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lemma graph_extI: |
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"(\<And>x. x \<in> H \<Longrightarrow> h x = h' x) \<Longrightarrow> H \<subseteq> H' |
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\<Longrightarrow> graph H h \<subseteq> graph H' h'" |
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by (unfold graph_def) blast |
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lemma graph_extD1 [intro?]: |
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"graph H h \<subseteq> graph H' h' \<Longrightarrow> x \<in> H \<Longrightarrow> h x = h' x" |
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by (unfold graph_def) blast |
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lemma graph_extD2 [intro?]: |
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"graph H h \<subseteq> graph H' h' \<Longrightarrow> H \<subseteq> H'" |
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by (unfold graph_def) blast |
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subsection {* Domain and function of a graph *} |
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text {* |
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The inverse functions to @{text graph} are @{text domain} and |
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@{text funct}. |
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*} |
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constdefs |
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domain :: "'a graph \<Rightarrow> 'a set" |
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"domain g \<equiv> {x. \<exists>y. (x, y) \<in> g}" |
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funct :: "'a graph \<Rightarrow> ('a \<Rightarrow> real)" |
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"funct g \<equiv> \<lambda>x. (SOME y. (x, y) \<in> g)" |
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text {* |
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The following lemma states that @{text g} is the graph of a function |
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if the relation induced by @{text g} is unique. |
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*} |
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lemma graph_domain_funct: |
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"(\<And>x y z. (x, y) \<in> g \<Longrightarrow> (x, z) \<in> g \<Longrightarrow> z = y) |
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\<Longrightarrow> graph (domain g) (funct g) = g" |
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proof (unfold domain_def funct_def graph_def, auto) |
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fix a b assume "(a, b) \<in> g" |
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show "(a, SOME y. (a, y) \<in> g) \<in> g" by (rule someI2) |
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show "\<exists>y. (a, y) \<in> g" .. |
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assume uniq: "\<And>x y z. (x, y) \<in> g \<Longrightarrow> (x, z) \<in> g \<Longrightarrow> z = y" |
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show "b = (SOME y. (a, y) \<in> g)" |
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proof (rule some_equality [symmetric]) |
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fix y assume "(a, y) \<in> g" show "y = b" by (rule uniq) |
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qed |
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qed |
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subsection {* Norm-preserving extensions of a function *} |
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text {* |
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Given a linear form @{text f} on the space @{text F} and a seminorm |
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@{text p} on @{text E}. The set of all linear extensions of @{text |
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f}, to superspaces @{text H} of @{text F}, which are bounded by |
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@{text p}, is defined as follows. |
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*} |
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constdefs |
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norm_pres_extensions :: |
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"'a::{plus, minus, zero} set \<Rightarrow> ('a \<Rightarrow> real) \<Rightarrow> 'a set \<Rightarrow> ('a \<Rightarrow> real) |
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\<Rightarrow> 'a graph set" |
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"norm_pres_extensions E p F f |
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\<equiv> {g. \<exists>H h. graph H h = g |
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\<and> is_linearform H h |
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\<and> is_subspace H E |
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\<and> is_subspace F H |
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\<and> graph F f \<subseteq> graph H h |
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\<and> (\<forall>x \<in> H. h x \<le> p x)}" |
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lemma norm_pres_extension_D: |
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"g \<in> norm_pres_extensions E p F f |
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\<Longrightarrow> \<exists>H h. graph H h = g |
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\<and> is_linearform H h |
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\<and> is_subspace H E |
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\<and> is_subspace F H |
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\<and> graph F f \<subseteq> graph H h |
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\<and> (\<forall>x \<in> H. h x \<le> p x)" |
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by (unfold norm_pres_extensions_def) blast |
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lemma norm_pres_extensionI2 [intro]: |
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"is_linearform H h \<Longrightarrow> is_subspace H E \<Longrightarrow> is_subspace F H \<Longrightarrow> |
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graph F f \<subseteq> graph H h \<Longrightarrow> \<forall>x \<in> H. h x \<le> p x |
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\<Longrightarrow> (graph H h \<in> norm_pres_extensions E p F f)" |
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by (unfold norm_pres_extensions_def) blast |
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lemma norm_pres_extensionI [intro]: |
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"\<exists>H h. graph H h = g |
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\<and> is_linearform H h |
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\<and> is_subspace H E |
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\<and> is_subspace F H |
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\<and> graph F f \<subseteq> graph H h |
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\<and> (\<forall>x \<in> H. h x \<le> p x) |
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\<Longrightarrow> g \<in> norm_pres_extensions E p F f" |
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by (unfold norm_pres_extensions_def) blast |
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end |