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 or the complex numbers ) to linear operators valued in topological vector spaces (TVSs).
Definitions
Throughout X and Y will be topological vector spaces (TVSs) over the field 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
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 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 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
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.
Properties
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 from Y to the completion Z of Y; that is, to the map Z → Y).[1]
Existence
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
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 Banach space over the field Then the following are equivalent:
- Y is 1-injective;
- Y has the metric extension property;
- Y has the immediate 1-extension property;
- Y has the center-radius property;
- Y has the weak intersection property;
- Y is 1-complemented in any Banach space into which it is norm embedded;
- Whenever Y in norm-embedded into a Banach space then identity map can be extended to a continuous linear map of norm to ;
- Y is linearly isometric to for some compact, Hausdorff space, extremally disconnected space T. (This space T is unique up to homeomorphism).
where if in addition, Y is a vector space over the real numbers then we may add to this list:
- Y has the binary intersection property;
- Y is linearly isometric to a complete Archimedean ordered vector lattice with order unit and endowed with the order unit norm.
Examples
Products of the underlying field
Suppose that is a vector space over , where is either or and let be any set. Let which is the product of taken times, or equivalently, the set of all -valued functions on T. Give its usual product topology, which makes it into a Hausdorff locally convex TVS. Then has the extension property.[1]
For any set the Lp space has both the extension property and the metric extension property.
See also
- 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
Citations
References
- Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. ISBN 978-1584888666. 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. ISBN 978-0-07-054236-5. 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. ISBN 978-1-4612-7155-0. OCLC 840278135.
- Trèves, François (2006) [1967]. Topological Vector Spaces, Distributions and Kernels. Mineola, N.Y.: Dover Publications. ISBN 978-0-486-45352-1. OCLC 853623322.