In mathematics, an LB-space, also written (LB)-space, is a topological vector space that is a locally convex inductive limit of a countable inductive system of Banach spaces. This means that is a direct limit of a direct system in the category of locally convex topological vector spaces and each is a Banach space.
If each of the bonding maps is an embedding of TVSs then the LB-space is called a strict LB-space. This means that the topology induced on by is identical to the original topology on [1] Some authors (e.g. Schaefer) define the term "LB-space" to mean "strict LB-space," so when reading mathematical literature, its recommended to always check how LB-space is defined.
The topology on can be described by specifying that an absolutely convex subset is a neighborhood of if and only if is an absolutely convex neighborhood of in for every
A strict LB-space is complete, [2] barrelled, [2] and bornological [2] (and thus ultrabornological).
If is a locally compact topological space that is countable at infinity (that is, it is equal to a countable union of compact subspaces) then the space of all continuous, complex-valued functions on with compact support is a strict LB-space. [3] For any compact subset let denote the Banach space of complex-valued functions that are supported by with the uniform norm and order the family of compact subsets of by inclusion. [3]
Let
denote the space of finite sequences , where denotes the space of all real sequences. For every natural number let denote the usual Euclidean space endowed with the Euclidean topology and let denote the canonical inclusion defined by so that its image is
and consequently,
Endow the set with the final topology induced by the family of all canonical inclusions. With this topology, becomes a complete Hausdorff locally convex sequential topological vector space that is not a Fréchet–Urysohn space. The topology is strictly finer than the subspace topology induced on by where is endowed with its usual product topology. Endow the image with the final topology induced on it by the bijection that is, it is endowed with the Euclidean topology transferred to it from via This topology on is equal to the subspace topology induced on it by A subset is open (resp. closed) in if and only if for every the set is an open (resp. closed) subset of The topology is coherent with family of subspaces This makes into an LB-space. Consequently, if and is a sequence in then in if and only if there exists some such that both and are contained in and in
Often, for every the canonical inclusion is used to identify with its image in explicitly, the elements and are identified together. Under this identification, becomes a direct limit of the direct system where for every the map is the canonical inclusion defined by where there are trailing zeros.
There exists a bornological LB-space whose strong bidual is not bornological. [4] There exists an LB-space that is not quasi-complete. [4]
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