Bornological space

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In mathematics, particularly in functional analysis, a bornological space is a type of space which, in some sense, possesses the minimum amount of structure needed to address questions of boundedness of sets and linear maps, in the same way that a topological space possesses the minimum amount of structure needed to address questions of continuity. Bornological spaces are distinguished by the property that a linear map from a bornological space into any locally convex spaces is continuous if and only if it is a bounded linear operator. Bornological spaces were first studied by George Mackey.[citation needed] The name was coined by Bourbaki[citation needed] after borné, the French word for "bounded".

Bornologies and bounded maps

A bornology on a set X is a collection of subsets of X that satisfy all the following conditions:

  1. covers X; that is, X=;
  2. is stable under inclusions; that is, if B and AB, then A;
  3. is stable under finite unions; that is, if B1,,Bn then B1Bn;

Elements of the collection are called -bounded or simply bounded sets if is understood.[1] The pair (X,) is called a bounded structure or a bornological set.[1] A base or fundamental system of a bornology is a subset 0 of such that each element of is a subset of some element of 0. Given a collection 𝒮 of subsets of X, the smallest bornology containing 𝒮 is called the bornology generated by 𝒮.[2] If (X,) and (Y,𝒞) are bornological sets then their product bornology on X×Y is the bornology having as a base the collection of all sets of the form B×C, where B and C𝒞.[2] A subset of X×Y is bounded in the product bornology if and only if its image under the canonical projections onto X and Y are both bounded.

Bounded maps

If (X,) and (Y,𝒞) are bornological sets then a function f:XY is said to be a locally bounded map or a bounded map (with respect to these bornologies) if it maps -bounded subsets of X to 𝒞-bounded subsets of Y; that is, if f()𝒞.[2] If in addition f is a bijection and f1 is also bounded then f is called a bornological isomorphism.

Vector bornologies

Let X be a vector space over a field 𝕂 where 𝕂 has a bornology 𝕂. A bornology on X is called a vector bornology on X if it is stable under vector addition, scalar multiplication, and the formation of balanced hulls (i.e. if the sum of two bounded sets is bounded, etc.). If X is a topological vector space (TVS) and is a bornology on X, then the following are equivalent:

  1. is a vector bornology;
  2. Finite sums and balanced hulls of -bounded sets are -bounded;[2]
  3. The scalar multiplication map 𝕂×XX defined by (s,x)sx and the addition map X×XX defined by (x,y)x+y, are both bounded when their domains carry their product bornologies (i.e. they map bounded subsets to bounded subsets).[2]

A vector bornology is called a convex vector bornology if it is stable under the formation of convex hulls (i.e. the convex hull of a bounded set is bounded) then . And a vector bornology is called separated if the only bounded vector subspace of X is the 0-dimensional trivial space {0}. Usually, 𝕂 is either the real or complex numbers, in which case a vector bornology on X will be called a convex vector bornology if has a base consisting of convex sets.

Bornivorous subsets

A subset A of X is called bornivorous and a bornivore if it absorbs every bounded set. In a vector bornology, A is bornivorous if it absorbs every bounded balanced set and in a convex vector bornology A is bornivorous if it absorbs every bounded disk. Two TVS topologies on the same vector space have that same bounded subsets if and only if they have the same bornivores.[3] Every bornivorous subset of a locally convex metrizable topological vector space is a neighborhood of the origin.[4]

Mackey convergence

A sequence x=(xi)i=1 in a TVS X is said to be Mackey convergent to 0 if there exists a sequence of positive real numbers r=(ri)i=1 diverging to such that (rixi)i=1 converges to 0 in X.[5]

Bornology of a topological vector space

Every topological vector space X, at least on a non discrete valued field gives a bornology on X by defining a subset BX to be bounded (or von-Neumann bounded), if and only if for all open sets UX containing zero there exists a r>0 with BrU. If X is a locally convex topological vector space then BX is bounded if and only if all continuous semi-norms on X are bounded on B. The set of all bounded subsets of a topological vector space X is called the bornology or the von Neumann bornology of X. If X is a locally convex topological vector space, then an absorbing disk D in X is bornivorous (resp. infrabornivorous) if and only if its Minkowski functional is locally bounded (resp. infrabounded).[4]

Induced topology

If is a convex vector bornology on a vector space X, then the collection 𝒩(0) of all convex balanced subsets of X that are bornivorous forms a neighborhood basis at the origin for a locally convex topology on X called the topology induced by .[4] If (X,τ) is a TVS then the bornological space associated with X is the vector space X endowed with the locally convex topology induced by the von Neumann bornology of (X,τ).[4]

Theorem[4] — Let X and Y be locally convex TVS and let Xb denote X endowed with the topology induced by von Neumann bornology of X. Define Yb similarly. Then a linear map L:XY is a bounded linear operator if and only if L:XbY is continuous. Moreover, if X is bornological, Y is Hausdorff, and L:XY is continuous linear map then so is L:XYb. If in addition X is also ultrabornological, then the continuity of L:XY implies the continuity of L:XYub, where Yub is the ultrabornological space associated with Y.

Quasi-bornological spaces

Quasi-bornological spaces where introduced by S. Iyahen in 1968.[6] A topological vector space (TVS) (X,τ) with a continuous dual X is called a quasi-bornological space[6] if any of the following equivalent conditions holds:

  1. Every bounded linear operator from X into another TVS is continuous.[6]
  2. Every bounded linear operator from X into a complete metrizable TVS is continuous.[6][7]
  3. Every knot in a bornivorous string is a neighborhood of the origin.[6]

Every pseudometrizable TVS is quasi-bornological. [6] A TVS (X,τ) in which every bornivorous set is a neighborhood of the origin is a quasi-bornological space.[8] If X is a quasi-bornological TVS then the finest locally convex topology on X that is coarser than τ makes X into a locally convex bornological space.

Bornological space

In functional analysis, a locally convex topological vector space is a bornological space if its topology can be recovered from its bornology in a natural way. Every locally convex quasi-bornological space is bornological but there exist bornological spaces that are not quasi-bornological.[6] A topological vector space (TVS) (X,τ) with a continuous dual X is called a bornological space if it is locally convex and any of the following equivalent conditions holds:

  1. Every convex, balanced, and bornivorous set in X is a neighborhood of zero.[4]
  2. Every bounded linear operator from X into a locally convex TVS is continuous.[4]
    • Recall that a linear map is bounded if and only if it maps any sequence converging to 0 in the domain to a bounded subset of the codomain.[4] In particular, any linear map that is sequentially continuous at the origin is bounded.
  3. Every bounded linear operator from X into a seminormed space is continuous.[4]
  4. Every bounded linear operator from X into a Banach space is continuous.[4]

If X is a Hausdorff locally convex space then we may add to this list:[7]

  1. The locally convex topology induced by the von Neumann bornology on X is the same as τ, X's given topology.
  2. Every bounded seminorm on X is continuous.[4]
  3. Any other Hausdorff locally convex topological vector space topology on X that has the same (von Neumann) bornology as (X,τ) is necessarily coarser than τ.
  4. X is the inductive limit of normed spaces.[4]
  5. X is the inductive limit of the normed spaces XD as D varies over the closed and bounded disks of X (or as D varies over the bounded disks of X).[4]
  6. X carries the Mackey topology τ(X,X) and all bounded linear functionals on X are continuous.[4]
  7. X has both of the following properties:
    • X is convex-sequential or C-sequential, which means that every convex sequentially open subset of X is open,
    • X is sequentially bornological or S-bornological, which means that every convex and bornivorous subset of X is sequentially open.
    where a subset A of X is called sequentially open if every sequence converging to 0 eventually belongs to A.

Every sequentially continuous linear operator from a locally convex bornological space into a locally convex TVS is continuous,[4] where recall that a linear operator is sequentially continuous if and only if it is sequentially continuous at the origin. Thus for linear maps from a bornological space into a locally convex space, continuity is equivalent to sequential continuity at the origin. More generally, we even have the following:

  • Any linear map F:XY from a locally convex bornological space into a locally convex space Y that maps null sequences in X to bounded subsets of Y is necessarily continuous.

Sufficient conditions

Mackey–Ulam theorem[9] — The product of a collection X=(Xi)iI locally convex bornological spaces is bornological if and only if I does not admit an Ulam measure.

As a consequent of the Mackey–Ulam theorem, "for all practical purposes, the product of bornological spaces is bornological."[9] The following topological vector spaces are all bornological:

  • Any locally convex pseudometrizable TVS is bornological.[4][10]
  • Any strict inductive limit of bornological spaces, in particular any strict LF-space, is bornological.
    • This shows that there are bornological spaces that are not metrizable.
  • A countable product of locally convex bornological spaces is bornological.[11][10]
  • Quotients of Hausdorff locally convex bornological spaces are bornological.[10]
  • The direct sum and inductive limit of Hausdorff locally convex bornological spaces is bornological.[10]
  • Fréchet Montel spaces have bornological strong duals.
  • The strong dual of every reflexive Fréchet space is bornological.[12]
  • If the strong dual of a metrizable locally convex space is separable, then it is bornological.[12]
  • A vector subspace of a Hausdorff locally convex bornological space X that has finite codimension in X is bornological.[4][10]
  • The finest locally convex topology on a vector space is bornological.[4]
Counterexamples

There exists a bornological LB-space whose strong bidual is not bornological.[13] A closed vector subspace of a locally convex bornological space is not necessarily bornological.[4][14] There exists a closed vector subspace of a locally convex bornological space that is complete (and so sequentially complete) but neither barrelled nor bornological.[4] Bornological spaces need not be barrelled and barrelled spaces need not be bornological.[4] Because every locally convex ultrabornological space is barrelled,[4] it follows that a bornological space is not necessarily ultrabornological.

Properties

  • The strong dual space of a locally convex bornological space is complete.[4]
  • Every locally convex bornological space is infrabarrelled.[4]
  • Every Hausdorff sequentially complete bornological TVS is ultrabornological.[4]
    • Thus every complete Hausdorff bornological space is ultrabornological.
    • In particular, every Fréchet space is ultrabornological.[4]
  • The finite product of locally convex ultrabornological spaces is ultrabornological.[4]
  • Every Hausdorff bornological space is quasi-barrelled.[15]
  • Given a bornological space X with continuous dual X, the topology of X coincides with the Mackey topology τ(X,X).
  • Every quasi-complete (i.e. all closed and bounded subsets are complete) bornological space is barrelled. There exist, however, bornological spaces that are not barrelled.
  • Every bornological space is the inductive limit of normed spaces (and Banach spaces if the space is also quasi-complete).
  • Let X be a metrizable locally convex space with continuous dual X. Then the following are equivalent:
    1. β(X,X) is bornological.
    2. β(X,X) is quasi-barrelled.
    3. β(X,X) is barrelled.
    4. X is a distinguished space.
  • If L:XY is a linear map between locally convex spaces and if X is bornological, then the following are equivalent:
    1. L:XY is continuous.
    2. L:XY is sequentially continuous.[4]
    3. For every set BX that's bounded in X, L(B) is bounded.
    4. If x=(xi)i=1 is a null sequence in X then Lx=(L(xi))i=1 is a null sequence in Y.
    5. If x=(xi)i=1 is a Mackey convergent null sequence in X then Lx=(L(xi))i=1 is a bounded subset of Y.
  • Suppose that X and Y are locally convex TVSs and that the space of continuous linear maps Lb(X;Y) is endowed with the topology of uniform convergence on bounded subsets of X. If X is a bornological space and if Y is complete then Lb(X;Y) is a complete TVS.[4]
    • In particular, the strong dual of a locally convex bornological space is complete.[4] However, it need not be bornological.
Subsets
  • In a locally convex bornological space, every convex bornivorous set B is a neighborhood of 0 (B is not required to be a disk).[4]
  • Every bornivorous subset of a locally convex metrizable topological vector space is a neighborhood of the origin.[4]
  • Closed vector subspaces of bornological space need not be bornological.[4]

Ultrabornological spaces

A disk in a topological vector space X is called infrabornivorous if it absorbs all Banach disks. If X is locally convex and Hausdorff, then a disk is infrabornivorous if and only if it absorbs all compact disks. A locally convex space is called ultrabornological if any of the following equivalent conditions hold:

  1. Every infrabornivorous disk is a neighborhood of the origin.
  2. X is the inductive limit of the spaces XD as D varies over all compact disks in X.
  3. A seminorm on X that is bounded on each Banach disk is necessarily continuous.
  4. For every locally convex space Y and every linear map u:XY, if u is bounded on each Banach disk then u is continuous.
  5. For every Banach space Y and every linear map u:XY, if u is bounded on each Banach disk then u is continuous.

Properties

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

See also

References

Bibliography

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