src/HOL/Real/HahnBanach/Subspace.thy
author ballarin
Thu Nov 06 14:18:05 2003 +0100 (2003-11-06)
changeset 14254 342634f38451
parent 13547 bf399f3bd7dc
child 14329 ff3210fe968f
permissions -rw-r--r--
Isar/Locales: <loc>.intro and <loc>.axioms no longer intro? and elim? by
default.
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(*  Title:      HOL/Real/HahnBanach/Subspace.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 {* Subspaces *}
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theory Subspace = VectorSpace:
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subsection {* Definition *}
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text {*
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  A non-empty subset @{text U} of a vector space @{text V} is a
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  \emph{subspace} of @{text V}, iff @{text U} is closed under addition
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  and scalar multiplication.
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*}
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locale subspace = var U + var V +
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  assumes non_empty [iff, intro]: "U \<noteq> {}"
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    and subset [iff]: "U \<subseteq> V"
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    and add_closed [iff]: "x \<in> U \<Longrightarrow> y \<in> U \<Longrightarrow> x + y \<in> U"
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    and mult_closed [iff]: "x \<in> U \<Longrightarrow> a \<cdot> x \<in> U"
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declare vectorspace.intro [intro?] subspace.intro [intro?]
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syntax (symbols)
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  subspace :: "'a set \<Rightarrow> 'a set \<Rightarrow> bool"    (infix "\<unlhd>" 50)
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lemma subspace_subset [elim]: "U \<unlhd> V \<Longrightarrow> U \<subseteq> V"
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  by (rule subspace.subset)
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lemma (in subspace) subsetD [iff]: "x \<in> U \<Longrightarrow> x \<in> V"
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  using subset by blast
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lemma subspaceD [elim]: "U \<unlhd> V \<Longrightarrow> x \<in> U \<Longrightarrow> x \<in> V"
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  by (rule subspace.subsetD)
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lemma rev_subspaceD [elim?]: "x \<in> U \<Longrightarrow> U \<unlhd> V \<Longrightarrow> x \<in> V"
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  by (rule subspace.subsetD)
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lemma (in subspace) diff_closed [iff]:
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  includes vectorspace
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  shows "x \<in> U \<Longrightarrow> y \<in> U \<Longrightarrow> x - y \<in> U"
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  by (simp add: diff_eq1 negate_eq1)
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text {*
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  \medskip Similar as for linear spaces, the existence of the zero
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  element in every subspace follows from the non-emptiness of the
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  carrier set and by vector space laws.
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*}
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lemma (in subspace) zero [intro]:
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  includes vectorspace
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  shows "0 \<in> U"
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proof -
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  have "U \<noteq> {}" by (rule U_V.non_empty)
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  then obtain x where x: "x \<in> U" by blast
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  hence "x \<in> V" .. hence "0 = x - x" by simp
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  also have "... \<in> U" by (rule U_V.diff_closed)
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  finally show ?thesis .
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qed
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lemma (in subspace) neg_closed [iff]:
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  includes vectorspace
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  shows "x \<in> U \<Longrightarrow> - x \<in> U"
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  by (simp add: negate_eq1)
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text {* \medskip Further derived laws: every subspace is a vector space. *}
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lemma (in subspace) vectorspace [iff]:
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  includes vectorspace
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  shows "vectorspace U"
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proof
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  show "U \<noteq> {}" ..
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  fix x y z assume x: "x \<in> U" and y: "y \<in> U" and z: "z \<in> U"
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  fix a b :: real
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  from x y show "x + y \<in> U" by simp
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  from x show "a \<cdot> x \<in> U" by simp
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  from x y z show "(x + y) + z = x + (y + z)" by (simp add: add_ac)
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  from x y show "x + y = y + x" by (simp add: add_ac)
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  from x show "x - x = 0" by simp
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  from x show "0 + x = x" by simp
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  from x y show "a \<cdot> (x + y) = a \<cdot> x + a \<cdot> y" by (simp add: distrib)
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  from x show "(a + b) \<cdot> x = a \<cdot> x + b \<cdot> x" by (simp add: distrib)
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  from x show "(a * b) \<cdot> x = a \<cdot> b \<cdot> x" by (simp add: mult_assoc)
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  from x show "1 \<cdot> x = x" by simp
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  from x show "- x = - 1 \<cdot> x" by (simp add: negate_eq1)
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  from x y show "x - y = x + - y" by (simp add: diff_eq1)
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qed
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text {* The subspace relation is reflexive. *}
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lemma (in vectorspace) subspace_refl [intro]: "V \<unlhd> V"
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proof
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  show "V \<noteq> {}" ..
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  show "V \<subseteq> V" ..
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  fix x y assume x: "x \<in> V" and y: "y \<in> V"
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  fix a :: real
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  from x y show "x + y \<in> V" by simp
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  from x show "a \<cdot> x \<in> V" by simp
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qed
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text {* The subspace relation is transitive. *}
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lemma (in vectorspace) subspace_trans [trans]:
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  "U \<unlhd> V \<Longrightarrow> V \<unlhd> W \<Longrightarrow> U \<unlhd> W"
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proof
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  assume uv: "U \<unlhd> V" and vw: "V \<unlhd> W"
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  from uv show "U \<noteq> {}" by (rule subspace.non_empty)
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  show "U \<subseteq> W"
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  proof -
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    from uv have "U \<subseteq> V" by (rule subspace.subset)
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    also from vw have "V \<subseteq> W" by (rule subspace.subset)
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    finally show ?thesis .
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  qed
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  fix x y assume x: "x \<in> U" and y: "y \<in> U"
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  from uv and x y show "x + y \<in> U" by (rule subspace.add_closed)
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  from uv and x show "\<And>a. a \<cdot> x \<in> U" by (rule subspace.mult_closed)
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qed
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subsection {* Linear closure *}
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text {*
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  The \emph{linear closure} of a vector @{text x} is the set of all
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  scalar multiples of @{text x}.
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*}
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constdefs
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  lin :: "('a::{minus, plus, zero}) \<Rightarrow> 'a set"
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  "lin x \<equiv> {a \<cdot> x | a. True}"
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lemma linI [intro]: "y = a \<cdot> x \<Longrightarrow> y \<in> lin x"
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  by (unfold lin_def) blast
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lemma linI' [iff]: "a \<cdot> x \<in> lin x"
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  by (unfold lin_def) blast
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lemma linE [elim]:
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    "x \<in> lin v \<Longrightarrow> (\<And>a::real. x = a \<cdot> v \<Longrightarrow> C) \<Longrightarrow> C"
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  by (unfold lin_def) blast
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text {* Every vector is contained in its linear closure. *}
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lemma (in vectorspace) x_lin_x [iff]: "x \<in> V \<Longrightarrow> x \<in> lin x"
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proof -
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  assume "x \<in> V"
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  hence "x = 1 \<cdot> x" by simp
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  also have "\<dots> \<in> lin x" ..
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  finally show ?thesis .
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qed
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lemma (in vectorspace) "0_lin_x" [iff]: "x \<in> V \<Longrightarrow> 0 \<in> lin x"
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proof
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  assume "x \<in> V"
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  thus "0 = 0 \<cdot> x" by simp
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qed
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text {* Any linear closure is a subspace. *}
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lemma (in vectorspace) lin_subspace [intro]:
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  "x \<in> V \<Longrightarrow> lin x \<unlhd> V"
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proof
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  assume x: "x \<in> V"
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  thus "lin x \<noteq> {}" by (auto simp add: x_lin_x)
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  show "lin x \<subseteq> V"
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  proof
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    fix x' assume "x' \<in> lin x"
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    then obtain a where "x' = a \<cdot> x" ..
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    with x show "x' \<in> V" by simp
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  qed
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  fix x' x'' assume x': "x' \<in> lin x" and x'': "x'' \<in> lin x"
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  show "x' + x'' \<in> lin x"
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  proof -
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    from x' obtain a' where "x' = a' \<cdot> x" ..
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    moreover from x'' obtain a'' where "x'' = a'' \<cdot> x" ..
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    ultimately have "x' + x'' = (a' + a'') \<cdot> x"
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      using x by (simp add: distrib)
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    also have "\<dots> \<in> lin x" ..
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    finally show ?thesis .
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  qed
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  fix a :: real
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  show "a \<cdot> x' \<in> lin x"
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  proof -
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    from x' obtain a' where "x' = a' \<cdot> x" ..
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    with x have "a \<cdot> x' = (a * a') \<cdot> x" by (simp add: mult_assoc)
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    also have "\<dots> \<in> lin x" ..
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    finally show ?thesis .
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  qed
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qed
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text {* Any linear closure is a vector space. *}
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lemma (in vectorspace) lin_vectorspace [intro]:
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    "x \<in> V \<Longrightarrow> vectorspace (lin x)"
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  by (rule subspace.vectorspace) (rule lin_subspace)
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subsection {* Sum of two vectorspaces *}
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text {*
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  The \emph{sum} of two vectorspaces @{text U} and @{text V} is the
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  set of all sums of elements from @{text U} and @{text V}.
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*}
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instance set :: (plus) plus ..
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defs (overloaded)
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  sum_def: "U + V \<equiv> {u + v | u v. u \<in> U \<and> v \<in> V}"
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lemma sumE [elim]:
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    "x \<in> U + V \<Longrightarrow> (\<And>u v. x = u + v \<Longrightarrow> u \<in> U \<Longrightarrow> v \<in> V \<Longrightarrow> C) \<Longrightarrow> C"
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  by (unfold sum_def) blast
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lemma sumI [intro]:
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    "u \<in> U \<Longrightarrow> v \<in> V \<Longrightarrow> x = u + v \<Longrightarrow> x \<in> U + V"
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  by (unfold sum_def) blast
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lemma sumI' [intro]:
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    "u \<in> U \<Longrightarrow> v \<in> V \<Longrightarrow> u + v \<in> U + V"
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  by (unfold sum_def) blast
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text {* @{text U} is a subspace of @{text "U + V"}. *}
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lemma subspace_sum1 [iff]:
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  includes vectorspace U + vectorspace V
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  shows "U \<unlhd> U + V"
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proof
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  show "U \<noteq> {}" ..
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  show "U \<subseteq> U + V"
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  proof
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    fix x assume x: "x \<in> U"
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    moreover have "0 \<in> V" ..
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    ultimately have "x + 0 \<in> U + V" ..
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    with x show "x \<in> U + V" by simp
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  qed
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  fix x y assume x: "x \<in> U" and "y \<in> U"
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  thus "x + y \<in> U" by simp
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  from x show "\<And>a. a \<cdot> x \<in> U" by simp
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qed
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text {* The sum of two subspaces is again a subspace. *}
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lemma sum_subspace [intro?]:
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  includes subspace U E + vectorspace E + subspace V E
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  shows "U + V \<unlhd> E"
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proof
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  have "0 \<in> U + V"
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  proof
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    show "0 \<in> U" ..
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    show "0 \<in> V" ..
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    show "(0::'a) = 0 + 0" by simp
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  qed
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  thus "U + V \<noteq> {}" by blast
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  show "U + V \<subseteq> E"
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  proof
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    fix x assume "x \<in> U + V"
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    then obtain u v where x: "x = u + v" and
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      u: "u \<in> U" and v: "v \<in> V" ..
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    have "U \<unlhd> E" . with u have "u \<in> E" ..
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    moreover have "V \<unlhd> E" . with v have "v \<in> E" ..
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    ultimately show "x \<in> E" using x by simp
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  qed
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  fix x y assume x: "x \<in> U + V" and y: "y \<in> U + V"
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  show "x + y \<in> U + V"
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  proof -
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    from x obtain ux vx where "x = ux + vx" and "ux \<in> U" and "vx \<in> V" ..
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    moreover
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    from y obtain uy vy where "y = uy + vy" and "uy \<in> U" and "vy \<in> V" ..
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    ultimately
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    have "ux + uy \<in> U"
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      and "vx + vy \<in> V"
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      and "x + y = (ux + uy) + (vx + vy)"
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      using x y by (simp_all add: add_ac)
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    thus ?thesis ..
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  qed
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  fix a show "a \<cdot> x \<in> U + V"
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  proof -
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    from x obtain u v where "x = u + v" and "u \<in> U" and "v \<in> V" ..
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    hence "a \<cdot> u \<in> U" and "a \<cdot> v \<in> V"
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      and "a \<cdot> x = (a \<cdot> u) + (a \<cdot> v)" by (simp_all add: distrib)
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    thus ?thesis ..
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  qed
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qed
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text{* The sum of two subspaces is a vectorspace. *}
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lemma sum_vs [intro?]:
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    "U \<unlhd> E \<Longrightarrow> V \<unlhd> E \<Longrightarrow> vectorspace E \<Longrightarrow> vectorspace (U + V)"
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  by (rule subspace.vectorspace) (rule sum_subspace)
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subsection {* Direct sums *}
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text {*
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  The sum of @{text U} and @{text V} is called \emph{direct}, iff the
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  zero element is the only common element of @{text U} and @{text
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  V}. For every element @{text x} of the direct sum of @{text U} and
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  @{text V} the decomposition in @{text "x = u + v"} with
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  @{text "u \<in> U"} and @{text "v \<in> V"} is unique.
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*}
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lemma decomp:
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  includes vectorspace E + subspace U E + subspace V E
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  assumes direct: "U \<inter> V = {0}"
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    and u1: "u1 \<in> U" and u2: "u2 \<in> U"
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    and v1: "v1 \<in> V" and v2: "v2 \<in> V"
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    and sum: "u1 + v1 = u2 + v2"
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  shows "u1 = u2 \<and> v1 = v2"
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proof
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  have U: "vectorspace U" by (rule subspace.vectorspace)
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  have V: "vectorspace V" by (rule subspace.vectorspace)
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  from u1 u2 v1 v2 and sum have eq: "u1 - u2 = v2 - v1"
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    by (simp add: add_diff_swap)
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  from u1 u2 have u: "u1 - u2 \<in> U"
wenzelm@13515
   322
    by (rule vectorspace.diff_closed [OF U])
wenzelm@13515
   323
  with eq have v': "v2 - v1 \<in> U" by (simp only:)
wenzelm@13515
   324
  from v2 v1 have v: "v2 - v1 \<in> V"
wenzelm@13515
   325
    by (rule vectorspace.diff_closed [OF V])
wenzelm@13515
   326
  with eq have u': " u1 - u2 \<in> V" by (simp only:)
wenzelm@10687
   327
wenzelm@9035
   328
  show "u1 = u2"
wenzelm@13515
   329
  proof (rule add_minus_eq)
bauerg@9374
   330
    show "u1 \<in> E" ..
bauerg@9374
   331
    show "u2 \<in> E" ..
wenzelm@13515
   332
    from u u' and direct show "u1 - u2 = 0" by blast
wenzelm@9035
   333
  qed
wenzelm@9035
   334
  show "v1 = v2"
wenzelm@13515
   335
  proof (rule add_minus_eq [symmetric])
bauerg@9374
   336
    show "v1 \<in> E" ..
bauerg@9374
   337
    show "v2 \<in> E" ..
wenzelm@13515
   338
    from v v' and direct show "v2 - v1 = 0" by blast
wenzelm@9035
   339
  qed
wenzelm@9035
   340
qed
wenzelm@7656
   341
wenzelm@10687
   342
text {*
wenzelm@10687
   343
  An application of the previous lemma will be used in the proof of
wenzelm@10687
   344
  the Hahn-Banach Theorem (see page \pageref{decomp-H-use}): for any
wenzelm@10687
   345
  element @{text "y + a \<cdot> x\<^sub>0"} of the direct sum of a
wenzelm@10687
   346
  vectorspace @{text H} and the linear closure of @{text "x\<^sub>0"}
wenzelm@10687
   347
  the components @{text "y \<in> H"} and @{text a} are uniquely
wenzelm@10687
   348
  determined.
wenzelm@10687
   349
*}
wenzelm@7917
   350
wenzelm@10687
   351
lemma decomp_H':
wenzelm@13547
   352
  includes vectorspace E + subspace H E
wenzelm@13515
   353
  assumes y1: "y1 \<in> H" and y2: "y2 \<in> H"
wenzelm@13515
   354
    and x': "x' \<notin> H"  "x' \<in> E"  "x' \<noteq> 0"
wenzelm@13515
   355
    and eq: "y1 + a1 \<cdot> x' = y2 + a2 \<cdot> x'"
wenzelm@13515
   356
  shows "y1 = y2 \<and> a1 = a2"
wenzelm@9035
   357
proof
bauerg@9374
   358
  have c: "y1 = y2 \<and> a1 \<cdot> x' = a2 \<cdot> x'"
wenzelm@10687
   359
  proof (rule decomp)
wenzelm@10687
   360
    show "a1 \<cdot> x' \<in> lin x'" ..
bauerg@9374
   361
    show "a2 \<cdot> x' \<in> lin x'" ..
wenzelm@13515
   362
    show "H \<inter> lin x' = {0}"
wenzelm@9035
   363
    proof
wenzelm@10687
   364
      show "H \<inter> lin x' \<subseteq> {0}"
wenzelm@13515
   365
      proof
wenzelm@13515
   366
        fix x assume x: "x \<in> H \<inter> lin x'"
wenzelm@13515
   367
        then obtain a where xx': "x = a \<cdot> x'"
wenzelm@13515
   368
          by blast
wenzelm@13515
   369
        have "x = 0"
wenzelm@13515
   370
        proof cases
wenzelm@13515
   371
          assume "a = 0"
wenzelm@13515
   372
          with xx' and x' show ?thesis by simp
wenzelm@13515
   373
        next
wenzelm@13515
   374
          assume a: "a \<noteq> 0"
wenzelm@13515
   375
          from x have "x \<in> H" ..
wenzelm@13515
   376
          with xx' have "inverse a \<cdot> a \<cdot> x' \<in> H" by simp
wenzelm@13515
   377
          with a and x' have "x' \<in> H" by (simp add: mult_assoc2)
wenzelm@13515
   378
          thus ?thesis by contradiction
wenzelm@13515
   379
        qed
wenzelm@13515
   380
        thus "x \<in> {0}" ..
wenzelm@9035
   381
      qed
wenzelm@10687
   382
      show "{0} \<subseteq> H \<inter> lin x'"
wenzelm@9035
   383
      proof -
wenzelm@13515
   384
        have "0 \<in> H" ..
wenzelm@13515
   385
        moreover have "0 \<in> lin x'" ..
wenzelm@13515
   386
        ultimately show ?thesis by blast
wenzelm@9035
   387
      qed
wenzelm@9035
   388
    qed
wenzelm@13515
   389
    show "lin x' \<unlhd> E" ..
wenzelm@9035
   390
  qed
wenzelm@13515
   391
  thus "y1 = y2" ..
wenzelm@13515
   392
  from c have "a1 \<cdot> x' = a2 \<cdot> x'" ..
wenzelm@13515
   393
  with x' show "a1 = a2" by (simp add: mult_right_cancel)
wenzelm@9035
   394
qed
wenzelm@7535
   395
wenzelm@10687
   396
text {*
wenzelm@10687
   397
  Since for any element @{text "y + a \<cdot> x'"} of the direct sum of a
wenzelm@10687
   398
  vectorspace @{text H} and the linear closure of @{text x'} the
wenzelm@10687
   399
  components @{text "y \<in> H"} and @{text a} are unique, it follows from
wenzelm@10687
   400
  @{text "y \<in> H"} that @{text "a = 0"}.
wenzelm@10687
   401
*}
wenzelm@7917
   402
wenzelm@10687
   403
lemma decomp_H'_H:
wenzelm@13547
   404
  includes vectorspace E + subspace H E
wenzelm@13515
   405
  assumes t: "t \<in> H"
wenzelm@13515
   406
    and x': "x' \<notin> H"  "x' \<in> E"  "x' \<noteq> 0"
wenzelm@13515
   407
  shows "(SOME (y, a). t = y + a \<cdot> x' \<and> y \<in> H) = (t, 0)"
wenzelm@13515
   408
proof (rule, simp_all only: split_paired_all split_conv)
wenzelm@13515
   409
  from t x' show "t = t + 0 \<cdot> x' \<and> t \<in> H" by simp
wenzelm@13515
   410
  fix y and a assume ya: "t = y + a \<cdot> x' \<and> y \<in> H"
wenzelm@13515
   411
  have "y = t \<and> a = 0"
wenzelm@13515
   412
  proof (rule decomp_H')
wenzelm@13515
   413
    from ya x' show "y + a \<cdot> x' = t + 0 \<cdot> x'" by simp
wenzelm@13515
   414
    from ya show "y \<in> H" ..
wenzelm@13515
   415
  qed
wenzelm@13515
   416
  with t x' show "(y, a) = (y + a \<cdot> x', 0)" by simp
wenzelm@13515
   417
qed
wenzelm@7535
   418
wenzelm@10687
   419
text {*
wenzelm@10687
   420
  The components @{text "y \<in> H"} and @{text a} in @{text "y + a \<cdot> x'"}
wenzelm@10687
   421
  are unique, so the function @{text h'} defined by
wenzelm@10687
   422
  @{text "h' (y + a \<cdot> x') = h y + a \<cdot> \<xi>"} is definite.
wenzelm@10687
   423
*}
wenzelm@7917
   424
bauerg@9374
   425
lemma h'_definite:
wenzelm@13515
   426
  includes var H
wenzelm@13515
   427
  assumes h'_def:
wenzelm@13515
   428
    "h' \<equiv> (\<lambda>x. let (y, a) = SOME (y, a). (x = y + a \<cdot> x' \<and> y \<in> H)
wenzelm@13515
   429
                in (h y) + a * xi)"
wenzelm@13515
   430
    and x: "x = y + a \<cdot> x'"
wenzelm@13547
   431
  includes vectorspace E + subspace H E
wenzelm@13515
   432
  assumes y: "y \<in> H"
wenzelm@13515
   433
    and x': "x' \<notin> H"  "x' \<in> E"  "x' \<noteq> 0"
wenzelm@13515
   434
  shows "h' x = h y + a * xi"
wenzelm@10687
   435
proof -
wenzelm@13515
   436
  from x y x' have "x \<in> H + lin x'" by auto
wenzelm@13515
   437
  have "\<exists>!p. (\<lambda>(y, a). x = y + a \<cdot> x' \<and> y \<in> H) p" (is "\<exists>!p. ?P p")
wenzelm@9035
   438
  proof
wenzelm@13515
   439
    from x y show "\<exists>p. ?P p" by blast
wenzelm@13515
   440
    fix p q assume p: "?P p" and q: "?P q"
wenzelm@13515
   441
    show "p = q"
wenzelm@9035
   442
    proof -
wenzelm@13515
   443
      from p have xp: "x = fst p + snd p \<cdot> x' \<and> fst p \<in> H"
wenzelm@13515
   444
        by (cases p) simp
wenzelm@13515
   445
      from q have xq: "x = fst q + snd q \<cdot> x' \<and> fst q \<in> H"
wenzelm@13515
   446
        by (cases q) simp
wenzelm@13515
   447
      have "fst p = fst q \<and> snd p = snd q"
wenzelm@13515
   448
      proof (rule decomp_H')
wenzelm@13515
   449
        from xp show "fst p \<in> H" ..
wenzelm@13515
   450
        from xq show "fst q \<in> H" ..
wenzelm@13515
   451
        from xp and xq show "fst p + snd p \<cdot> x' = fst q + snd q \<cdot> x'"
wenzelm@13515
   452
          by simp
wenzelm@13515
   453
        apply_end assumption+
wenzelm@13515
   454
      qed
wenzelm@13515
   455
      thus ?thesis by (cases p, cases q) simp
wenzelm@9035
   456
    qed
wenzelm@9035
   457
  qed
wenzelm@10687
   458
  hence eq: "(SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H) = (y, a)"
wenzelm@13515
   459
    by (rule some1_equality) (simp add: x y)
wenzelm@13515
   460
  with h'_def show "h' x = h y + a * xi" by (simp add: Let_def)
wenzelm@9035
   461
qed
wenzelm@7535
   462
wenzelm@10687
   463
end