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In functional analysis, a topological vector space (TVS) ${\displaystyle X}$ is called ultrabornological if every bounded linear operator from ${\displaystyle X}$ into another TVS is necessarily continuous. A general version of the closed graph theorem holds for ultrabornological spaces. Ultrabornological spaces were introduced by Alexander Grothendieck (Grothendieck "espace du type (β)").

Definitions

Let ${\displaystyle X}$ be a topological vector space (TVS).

Preliminaries

A disk is a convex and balanced set. A disk in a TVS ${\displaystyle X}$ is called bornivorous if it absorbs every bounded subset of ${\displaystyle X.}$

A linear map between two TVSs is called infrabounded if it maps Banach disks to bounded disks.

A disk ${\displaystyle D}$ in a TVS ${\displaystyle X}$ is called infrabornivorous if it satisfies any of the following equivalent conditions:

1. ${\displaystyle D}$ absorbs every Banach disks in ${\displaystyle X.}$

while if ${\displaystyle X}$ locally convex then we may add to this list:

2. the gauge of ${\displaystyle D}$ is an infrabounded map;

while if ${\displaystyle X}$ locally convex and Hausdorff then we may add to this list:

3. ${\displaystyle D}$ absorbs all compact disks; that is, ${\displaystyle D}$ is "compactivorious".

Ultrabornological space

A TVS ${\displaystyle X}$ is ultrabornological if it satisfies any of the following equivalent conditions:

1. every infrabornivorous disk in ${\displaystyle X}$ is a neighborhood of the origin;

while if ${\displaystyle X}$ is a locally convex space then we may add to this list:

2. every bounded linear operator from ${\displaystyle X}$ into a complete metrizable TVS is necessarily continuous;
3. every infrabornivorous disk is a neighborhood of 0;
4. ${\displaystyle X}$ be the inductive limit of the spaces ${\displaystyle X_{D}}$ as D varies over all compact disks in ${\displaystyle X}$;
5. a seminorm on ${\displaystyle X}$ that is bounded on each Banach disk is necessarily continuous;
6. for every locally convex space ${\displaystyle Y}$ and every linear map ${\displaystyle u:X\to Y,}$ if ${\displaystyle u}$ is bounded on each Banach disk then ${\displaystyle u}$ is continuous;
7. for every Banach space ${\displaystyle Y}$ and every linear map ${\displaystyle u:X\to Y,}$ if ${\displaystyle u}$ is bounded on each Banach disk then ${\displaystyle u}$ is continuous.

while if ${\displaystyle X}$ is a Hausdorff locally convex space then we may add to this list:

8. ${\displaystyle X}$ is an inductive limit of Banach spaces;

Properties

Every locally convex ultrabornological space is barrelled, quasi-ultrabarrelled space, and a bornological space but there exist bornological spaces that are not ultrabornological.

• Every ultrabornological space ${\displaystyle X}$ is the inductive limit of a family of nuclear Fréchet spaces, spanning ${\displaystyle X.}$
• Every ultrabornological space ${\displaystyle X}$ is the inductive limit of a family of nuclear DF-spaces, spanning ${\displaystyle X.}$

Examples and sufficient conditions

The finite product of locally convex ultrabornological spaces is ultrabornological. Inductive limits of ultrabornological spaces are ultrabornological.

Every Hausdorff sequentially complete bornological space is ultrabornological. Thus every complete Hausdorff bornological space is ultrabornological. In particular, every Fréchet space is ultrabornological.

The strong dual space of a complete Schwartz space is ultrabornological.

Every Hausdorff bornological space that is quasi-complete is ultrabornological.

Counter-examples

There exist ultrabarrelled spaces that are not ultrabornological. There exist ultrabornological spaces that are not ultrabarrelled.

See also

• Bounded linear operator – Linear transformation between topological vector spacesPages displaying short descriptions of redirect targets
• Bounded set (topological vector space) – Generalization of boundedness
• Bornological space – Space where bounded operators are continuous
• Bornology – Mathematical generalization of boundedness
• Locally convex topological vector space – A vector space with a topology defined by convex open sets
• Space of linear maps
• Topological vector space – Vector space with a notion of nearness
• Vector bornology

External links

• Some characterizations of ultrabornological spaces

References

1. Narici & Beckenstein 2011, p. 441.
2. ^ Narici & Beckenstein 2011, pp. 441–457.

• Hogbe-Nlend, Henri (1977). Bornologies and functional analysis. Amsterdam: North-Holland Publishing Co. pp. xii+144. ISBN 0-7204-0712-5. MR 0500064.
• Edwards, Robert E. (1995). Functional Analysis: Theory and Applications. New York: Dover Publications. ISBN 978-0-486-68143-6. OCLC 30593138.
• Grothendieck, Alexander (1955). "Produits Tensoriels Topologiques et Espaces Nucléaires" [Topological Tensor Products and Nuclear Spaces]. Memoirs of the American Mathematical Society Series (in French). 16. Providence: American Mathematical Society. ISBN 978-0-8218-1216-7. MR 0075539. OCLC 1315788.
• Grothendieck, Alexander (1973). Topological Vector Spaces. Translated by Chaljub, Orlando. New York: Gordon and Breach Science Publishers. ISBN 978-0-677-30020-7. OCLC 886098.
• Khaleelulla, S. M. (1982). Counterexamples in Topological Vector Spaces. Lecture Notes in Mathematics. Vol. 936. Berlin, Heidelberg, New York: Springer-Verlag. ISBN 978-3-540-11565-6. OCLC 8588370.
• Kriegl, Andreas; Michor, Peter W. (1997). The Convenient Setting of Global Analysis (PDF). Mathematical Surveys and Monographs. Vol. 53. Providence, R.I: American Mathematical Society. ISBN 978-0-8218-0780-4. OCLC 37141279.
• Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. ISBN 978-1584888666. OCLC 144216834.
• 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.
• Wilansky, Albert (2013). Modern Methods in Topological Vector Spaces. Mineola, New York: Dover Publications, Inc. ISBN 978-0-486-49353-4. OCLC 849801114.

• Topological vector spaces