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In functional analysis, a subset of a real or complex vector space that has an associated vector bornology is called bornivorous and a bornivore if it absorbs every element of If is a topological vector space (TVS) then a subset of is bornivorous if it is bornivorous with respect to the von-Neumann bornology of .

Bornivorous sets play an important role in the definitions of many classes of topological vector spaces, particularly bornological spaces.

Definitions[edit]

If is a TVS then a subset of is called bornivorous[1] and a bornivore if absorbs every bounded subset of

An absorbing disk in a locally convex space is bornivorous if and only if its Minkowski functional is locally bounded (i.e. maps bounded sets to bounded sets).[1]

Infrabornivorous sets and infrabounded maps[edit]

A linear map between two TVSs is called infrabounded if it maps Banach disks to bounded disks.[2]

A disk in is called infrabornivorous if it absorbs every Banach disk.[3]

An absorbing disk in a locally convex space is infrabornivorous if and only if its Minkowski functional is infrabounded.[1] A disk in a Hausdorff locally convex space is infrabornivorous if and only if it absorbs all compact disks (that is, if it is "compactivorous").[1]

Properties[edit]

Every bornivorous and infrabornivorous subset of a TVS is absorbing. In a pseudometrizable TVS, every bornivore is a neighborhood of the origin.[4]

Two TVS topologies on the same vector space have that same bounded subsets if and only if they have the same bornivores.[5]

Suppose is a vector subspace of finite codimension in a locally convex space and If is a barrel (resp. bornivorous barrel, bornivorous disk) in then there exists a barrel (resp. bornivorous barrel, bornivorous disk) in such that [6]

Examples and sufficient conditions[edit]

Every neighborhood of the origin in a TVS is bornivorous. The convex hull, closed convex hull, and balanced hull of a bornivorous set is again bornivorous. The preimage of a bornivore under a bounded linear map is a bornivore.[7]

If is a TVS in which every bounded subset is contained in a finite dimensional vector subspace, then every absorbing set is a bornivore.[5]

Counter-examples[edit]

Let be as a vector space over the reals. If is the balanced hull of the closed line segment between and then is not bornivorous but the convex hull of is bornivorous. If is the closed and "filled" triangle with vertices and then is a convex set that is not bornivorous but its balanced hull is bornivorous.

See also[edit]

References[edit]

  1. ^ a b c d Narici & Beckenstein 2011, pp. 441–457.
  2. ^ Narici & Beckenstein 2011, p. 442.
  3. ^ Narici & Beckenstein 2011, p. 443.
  4. ^ Narici & Beckenstein 2011, pp. 172–173.
  5. ^ a b Wilansky 2013, p. 50.
  6. ^ Narici & Beckenstein 2011, pp. 371–423.
  7. ^ Wilansky 2013, p. 48.

Bibliography[edit]