Continuity of Hahn Banach Extension Operators

In mathematics, specifically in functional analysis and Hilbert space theory, vector-valued Hahn–Banach theorems are generalizations of the Hahn–Banach theorems from linear functionals (which are always valued in the real numbers ${\displaystyle \mathbb {R} }$ or the complex numbers ${\displaystyle \mathbb {C} }$ ) to linear operators valued in topological vector spaces (TVSs).

Definitions [edit]

Throughout X and Y will be topological vector spaces (TVSs) over the field ${\displaystyle \mathbb {K} }$ and L(X; Y) will denote the vector space of all continuous linear maps from X to Y, where if X and Y are normed spaces then we endow L(X; Y) with its canonical operator norm.

Extensions [edit]

If M is a vector subspace of a TVS X then Y has the extension property from M to X if every continuous linear map f : M → Y has a continuous linear extension to all of X. If X and Y are normed spaces, then we say that Y has the metric extension property from M to X if this continuous linear extension can be chosen to have norm equal to || f ||.

A TVS Y has the extension property from all subspaces of X (to X) if for every vector subspace M of X, Y has the extension property from M to X. If X and Y are normed spaces then Y has the metric extension property from all subspace of X (to X) if for every vector subspace M of X, Y has the metric extension property from M to X.

A TVS Y has the extension property ^[1] if for every locally convex space X and every vector subspace M of X, Y has the extension property from M to X.

A Banach space Y has the metric extension property ^[1] if for every Banach space X and every vector subspace M of X, Y has the metric extension property from M to X.

1-extensions

If M is a vector subspace of normed space X over the field ${\displaystyle \mathbb {K} }$ then a normed space Y has the immediate 1-extension property from M to X if for every x ∉ M , every continuous linear map f : M → Y has a continuous linear extension ${\displaystyle F:M\oplus (\mathbb {K} x)\to Y}$ such that || f || = || F ||. We say that Y has the immediate 1-extension property if Y has the immediate 1-extension property from M to X for every Banach space X and every vector subspace M of X.

Injective spaces [edit]

A locally convex topological vector space Y is injective ^[1] if for every locally convex space Z containing Y as a topological vector subspace, there exists a continuous projection from Z onto Y.

A Banach space Y is 1-injective ^[1] or a P ₁ -space if for every Banach space Z containing Y as a normed vector subspace (i.e. the norm of Y is identical to the usual restriction to Y of Z's norm), there exists a continuous projection from Z onto Y having norm 1.

Properties [edit]

In order for a TVS Y to have the extension property, it must be complete (since it must be possible to extend the identity map ${\displaystyle \mathbf {1} :Y\to Y}$ from Y to the completion Z of Y; that is, to the map Z → Y ).^[1]

Existence [edit]

If f : M → Y is a continuous linear map from a vector subspace M of X into a complete Hausdorff space Y then there always exists a unique continuous linear extension of f from M to the closure of M in X.^{[1] [2]} Consequently, it suffices to only consider maps from closed vector subspaces into complete Hausdorff spaces.^[1]

Results [edit]

Any locally convex space having the extension property is injective.^[1] If Y is an injective Banach space, then for every Banach space X, every continuous linear operator from a vector subspace of X into Y has a continuous linear extension to all of X.^[1]

In 1953, Alexander Grothendieck showed that any Banach space with the extension property is either finite-dimensional or else not separable.^[1]

Theorem^[1]  —Suppose that Y is a real Banach space with the metric extension property. Then the following are equivalent:

1. Y is reflexive;
2. Y is separable;
3. Y is finite-dimensional;
4. Y is linearly isometric to ${\displaystyle C\left(T,\mathbb {K} ,\|\cdot \|_{\infty }\right),}$ for some discrete finite space ${\displaystyle T.}$

Examples [edit]

Products of the underlying field

Suppose that ${\displaystyle X}$ is a vector space over ${\displaystyle \mathbb {K} }$ , where ${\displaystyle \mathbb {K} }$ is either ${\displaystyle \mathbb {R} }$ or ${\displaystyle \mathbb {C} }$ and let ${\displaystyle T}$ be any set. Let ${\displaystyle Y:=\mathbb {K} ^{T},}$ which is the product of ${\displaystyle \mathbb {K} }$ taken ${\displaystyle |\mathbb {K} |}$ times, or equivalently, the set of all ${\displaystyle \mathbb {K} }$ -valued functions on T. Give ${\displaystyle Y}$ its usual product topology, which makes it into a Hausdorff locally convex TVS. Then ${\displaystyle Y}$ has the extension property.^[1] l ^∞

For any set ${\displaystyle T,}$ the Lp space ${\displaystyle \ell ^{\infty }(T)}$ has both the extension property and the metric extension property.

See also [edit]

• Hahn–Banach theorem – Theorem on extension of bounded linear functionals
• Continuous linear extension – Mathematical method in functional analysis
• Continuous linear operator

Citations [edit]

1. ^ ^{a b c d e f g h i j k l m} Narici & Beckenstein 2011, pp. 341–370.
2. ^ Rudin 1991, p. 40 Stated for linear maps into F-spaces only; outlines proof.

References [edit]

• Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. ISBN978-1584888666. OCLC 144216834.
• Rudin, Walter (1991). Functional Analysis. International Series in Pure and Applied Mathematics. Vol. 8 (Second ed.). New York, NY: McGraw-Hill Science/Engineering/Math. ISBN978-0-07-054236-5. OCLC 21163277.
• Schaefer, Helmut H.; Wolff, Manfred P. (1999). Topological Vector Spaces. GTM. Vol. 8 (Second ed.). New York, NY: Springer New York Imprint Springer. ISBN978-1-4612-7155-0. OCLC 840278135.
• Trèves, François (2006) [1967]. Topological Vector Spaces, Distributions and Kernels. Mineola, N.Y.: Dover Publications. ISBN978-0-486-45352-1. OCLC 853623322.

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Source: https://en.wikipedia.org/wiki/Vector-valued_Hahn%E2%80%93Banach_theorems

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