DF-space
In the field of functional analysis, DF-spaces, also written (DF)-spaces are locally convex topological vector space having a property that is shared by locally convex metrizable topological vector spaces. They play a considerable part in the theory of topological tensor products.[1]
DF-spaces were first defined by Alexander Grothendieck and studied in detail by him in (Grothendieck 1954) . Grothendieck was led to introduce these spaces by the following property of strong duals of metrizable spaces: If X is a metrizable locally convex space and is a sequence of convex 0-neighborhoods in such that absorbs every strongly bounded set, then V is a 0-neighborhood in (where is the continuous dual space of X endowed with the strong dual topology).[2]
Definition
A locally convex topological vector space (TVS) X is a DF-space, also written (DF)-space, if[1]
- X is a countably quasi-barrelled space (i.e. every strongly bounded countable union of equicontinuous subsets of is equicontinuous), and
- X possesses a fundamental sequence of bounded (i.e. there exists a countable sequence of bounded subsets such that every bounded subset of X is contained in some [3]).
Properties
- Let X be a DF-space and let V be a convex balanced subset of X. Then V is a neighborhood of the origin if and only if for every convex, balanced, bounded subset B ⊆ X, B ∩ V is a 0-neighborhood in B.[1] Thus, a linear map from a DF-space into a locally convex space is continuous if its restriction to each bounded subset of the domain is continuous.[1]
- The strong dual of a DF-space is a Fréchet space.[4]
- Every infinite-dimensional Montel DF-space is a sequential space but not a Fréchet–Urysohn space.
- Suppose X is either a DF-space or an LM-space. If X is a sequential space then it is either metrizable or else a Montel space DF-space.
- Every quasi-complete DF-space is complete.[5]
- If X is a complete nuclear DF-space then X is a Montel space.[6]
Sufficient conditions
- The strong dual of a metrizable locally convex space is a DF-space (but not conversely, in general).[1] Hence:
- Every normed space is a DF-space.[7]
- Every Banach space is a DF-space.[1]
- Every infrabarreled space possessing a fundamental sequence of bounded sets is a DF-space.
- Every Hausdorff quotient of a DF-space is a DF-space.[4]
- The completion of a DF-space is a DF-space.[4]
- The locally convex sum of a sequence of DF-spaces is a DF-space.[4]
- An inductive limit of a sequence of DF-spaces is a DF-space.[4]
- Suppose that X and Y are DF-spaces. Then the projective tensor product, as well as its completion, of these spaces is a DF-space.[6]
However,
Examples
There exist complete DF-spaces that are not TVS-isomorphic with the strong dual of a metrizable locally convex space.[4] There exist DF-spaces spaces having closed vector subspaces that are not DF-spaces.[8]
See also
- Barreled space
- Countably quasi-barrelled space
- F-space – Topological vector space with a complete translation-invariant metric
- LB-space
- LF-space
- Nuclear space – Type of topological vector space
- Projective tensor product
Citations
- Schaefer & Wolff 1999, pp. 154-155.
- Schaefer & Wolff 1999, pp. 152,154.
- Schaefer & Wolff 1999, p. 25.
- Schaefer & Wolff 1999, pp. 196-197.
- Schaefer & Wolff 1999, pp. 190-202.
- Schaefer & Wolff 1999, pp. 199-202.
- Khaleelulla 1982, p. 33.
- Khaleelulla 1982, pp. 103-110.
Bibliography
- Grothendieck, Alexander (1954). "Sur les espaces (F) et (DF)". Summa Brasil. Math. (in French). 3: 57–123. MR 0075542.
- Grothendieck, Alexander (1955). "Produits Tensoriels Topologiques et Espaces Nucléaires" [Topological Tensor Products and Nuclear Spaces]. Memoirs of the American Mathematical Society Series (in French). Providence: American Mathematical Society. 16. ISBN 978-0-8218-1216-7. MR 0075539. OCLC 1315788.
- Khaleelulla, S. M. (1982). Counterexamples in Topological Vector Spaces. Lecture Notes in Mathematics. 936. Berlin, Heidelberg, New York: Springer-Verlag. ISBN 978-3-540-11565-6. OCLC 8588370.
- Pietsch, Albrecht (1979). Nuclear Locally Convex Spaces. Ergebnisse der Mathematik und ihrer Grenzgebiete. 66 (Second ed.). Berlin, New York: Springer-Verlag. ISBN 978-0-387-05644-9. OCLC 539541.
- Pietsch, Albrecht (1972). Nuclear locally convex spaces. Berlin,New York: Springer-Verlag. ISBN 0-387-05644-0. OCLC 539541.
- Schaefer, Helmut H.; Wolff, Manfred P. (1999). Topological Vector Spaces. GTM. 8 (Second ed.). New York, NY: Springer New York Imprint Springer. ISBN 978-1-4612-7155-0. OCLC 840278135.
- Wong, Yau-Chuen (1979). Schwartz Spaces, Nuclear Spaces, and Tensor Products. Lecture Notes in Mathematics. 726. Berlin New York: Springer-Verlag. ISBN 978-3-540-09513-2. OCLC 5126158.