Vector-valued Hahn–Banach theorems

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 ${\displaystyle \mathbb {R} }${\displaystyle \mathbb {R} } or the complex numbers ${\displaystyle \mathbb {C} }${\displaystyle \mathbb {C} }) to linear operators valued in topological vector spaces (TVSs).

Throughout X and Y will be topological vector spaces (TVSs) over the field ${\displaystyle \mathbb {K} }${\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.

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 ${\displaystyle \mathbb {K} }${\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 ${\displaystyle F:M\oplus (\mathbb {K} x)\to Y}${\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.

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 P₁-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.

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 ${\displaystyle \mathbf {1} :Y\to Y}${\displaystyle \mathbf {1} :Y\to Y} from Y to the completion Z of Y; that is, to the map Z → Y).^[1]

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]

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]

Products of the underlying field

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

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

• Continuous linear extension – Mathematical method in functional analysis
• Continuous linear operator
• Hahn–Banach theorem – Theorem on extension of bounded linear functionals
• Hyperplane separation theorem – On the existence of hyperplanes separating disjoint convex sets

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• Topological vector spaces
• Theorems in functional analysis