From 28407333ffceca9b99fae721c30e8ae146a863da Mon Sep 17 00:00:00 2001 From: Justin Gassner Date: Wed, 14 Feb 2024 07:24:38 +0100 Subject: Update --- .../functional-analysis-basics/reflexive-spaces.md | 86 +++++++++++----------- 1 file changed, 44 insertions(+), 42 deletions(-) (limited to 'pages/functional-analysis-basics/reflexive-spaces.md') diff --git a/pages/functional-analysis-basics/reflexive-spaces.md b/pages/functional-analysis-basics/reflexive-spaces.md index dee0e55..781fb1f 100644 --- a/pages/functional-analysis-basics/reflexive-spaces.md +++ b/pages/functional-analysis-basics/reflexive-spaces.md @@ -2,59 +2,59 @@ title: Reflexive Spaces parent: Functional Analysis Basics nav_order: 2 -# cspell:words --- # {{ page.title }} -{: .definition-title } -> Definition (Canonical Embedding) -> -> Let $X$ be a normed space. -> The mapping -> -> $$ -> C : X \longrightarrow X'', \quad x \mapsto g_x, -> $$ -> -> where the functional $g_x$ on $X'$ is defined by -> -> $$ -> g_x(f) = f(x) \quad \text{for $f \in X'$,} -> $$ -> -> is called the *canonical embedding* of $X$ into its bidual $X''$. - -{: .lemma } -> The canonical embedding $C : X \to X''$ of a normed space into its bidual -> is well-defined and an embedding of normed spaces. +{% definition Canonical Embedding %} +Let $X$ be a normed space. +The mapping + +$$ +C : X \longrightarrow X'', \quad x \mapsto g_x, +$$ + +where the functional $g_x$ on $X'$ is defined by + +$$ +g_x(f) = f(x) \quad \text{for $f \in X'$,} +$$ + +is called the *canonical embedding* of $X$ into its bidual $X''$. +{% enddefinition %} + +{% lemma %} +The canonical embedding $C : X \to X''$ of a normed space into its bidual +is well-defined and an embedding of normed spaces. +{% endlemma %} {% proof %} {% endproof %} In particular, $C$ is isometric, hence injective. -{: .definition-title } -> Definition (Reflexivity) -> -> A normed space is said to be *reflexive* -> if the canonical embedding into its bidual -> is surjective. +{% definition Reflexivity %} +A normed space is said to be *reflexive* +if the canonical embedding into its bidual +is surjective. +{% enddefinition %} If a normed space $X$ is reflexive, then $X$ is isomorphic with $X''$, its bidual. James gives a counterexample for the converse statement. -{: .theorem } -> If a normed space is reflexive, -> then it is complete; hence a Banach space. +{% theorem %} +If a normed space is reflexive, +then it is complete; hence a Banach space. +{% endtheorem %} {% proof %} {% endproof %} -{: .theorem } -> If a normed space $X$ is reflexive, -> then the weak and weak$^*$ topologies on $X'$ agree. +{% theorem %} +If a normed space $X$ is reflexive, +then the weak and weak$^*$ topologies on $X'$ agree. +{% endtheorem %} {% proof %} By definition, the weak and weak$^*$ topologies on $X'$ @@ -65,9 +65,10 @@ Since $X$ is reflexive, those sets are equal. The converse is true as well. Proof: TODO -{: .theorem } -> If a normed space $X$ is reflexive, -> then its dual $X'$ is reflexive. +{% theorem %} +If a normed space $X$ is reflexive, +then its dual $X'$ is reflexive. +{% endtheorem %} {% proof %} Since $X$ is reflexive, @@ -115,9 +116,10 @@ This shows that $D$ is surjective, hence $X'$ is reflexive. In fact, we have shown more: $D = (C')^{-1}$. {% endproof %} -{: .theorem } -> Every finite-dimensional normed space is reflexive. -> +{% theorem %} +Every finite-dimensional normed space is reflexive. +{% endtheorem %} -{: .theorem } -> Every Hilbert space is reflexive. +{% theorem %} +Every Hilbert space is reflexive. +{% endtheorem %} -- cgit v1.2.3-54-g00ecf