quasi-complete space

{{Short description|A topological vector space in which every closed and bounded subset is complete}}

In functional analysis, a topological vector space (TVS) is said to be quasi-complete or boundedly complete{{sfn | Wilansky | 2013 | p=73}} if every closed and bounded subset is complete.{{sfn | Schaefer | Wolff | 1999 | p=27}}

This concept is of considerable importance for non-metrizable TVSs.{{sfn | Schaefer | Wolff | 1999 | p=27}}

Properties

  • Every quasi-complete TVS is sequentially complete.{{sfn | Schaefer | Wolff | 1999 | p=27}}
  • In a quasi-complete locally convex space, the closure of the convex hull of a compact subset is again compact.{{sfn | Schaefer |Wolff| 1999 | p=201}}
  • In a quasi-complete Hausdorff TVS, every precompact subset is relatively compact.{{sfn | Schaefer | Wolff | 1999 | p=27}}
  • If {{mvar|X}} is a normed space and {{mvar|Y}} is a quasi-complete locally convex TVS then the set of all compact linear maps of {{mvar|X}} into {{mvar|Y}} is a closed vector subspace of L_b(X;Y).{{sfn | Schaefer | Wolff | 1999 | p=110}}
  • Every quasi-complete infrabarrelled space is barreled.{{sfn | Schaefer | Wolff | 1999 | p=142}}
  • If {{mvar|X}} is a quasi-complete locally convex space then every weakly bounded subset of the continuous dual space is strongly bounded.{{sfn | Schaefer | Wolff | 1999 | p=142}}
  • A quasi-complete nuclear space then {{mvar|X}} has the Heine–Borel property.{{sfn | Trèves | 2006 | p=520}}

Examples and sufficient conditions

Every complete TVS is quasi-complete.{{sfn | Narici | Beckenstein | 2011 | pp=156-175}}

The product of any collection of quasi-complete spaces is again quasi-complete.{{sfn | Schaefer | Wolff | 1999 | p=27}}

The projective limit of any collection of quasi-complete spaces is again quasi-complete.{{sfn | Schaefer | Wolff | 1999 | p=52}}

Every semi-reflexive space is quasi-complete.{{sfn | Schaefer | Wolff | 1999 | p=144}}

The quotient of a quasi-complete space by a closed vector subspace may fail to be quasi-complete.

= Counter-examples =

There exists an LB-space that is not quasi-complete.{{sfn | Khaleelulla | 1982 | pp=28-63}}

See also

  • {{annotated link|Complete topological vector space}}
  • {{annotated link|Complete uniform space}}

References

{{reflist|group=note}}

{{reflist|30em}}

Bibliography

  • {{Khaleelulla Counterexamples in Topological Vector Spaces}}
  • {{Narici Beckenstein Topological Vector Spaces|edition=2}}
  • {{Schaefer Wolff Topological Vector Spaces|edition=2}}
  • {{Trèves François Topological vector spaces, distributions and kernels}}
  • {{Wilansky Modern Methods in Topological Vector Spaces|edition=1}}
  • {{Wong Schwartz Spaces, Nuclear Spaces, and Tensor Products}}

{{Topological vector spaces}}

Category:Functional analysis