Some remarks on ultrapowers and superproperties of the sum and interpolation spaces of Banach spaces
On duals of Calderón-Lozanovskiĭ intermediate spaces
We give a description of the dual of a Calderón-Lozanovskiĭ intermediate space φ(X,Y) of a couple of Banach Köthe function spaces as an intermediate space ψ(X*,Y*) of the duals, associated with a "variable" function ψ.
Ultrapowers of Köthe function spaces.
The ultrapowers, relative to a fixed ultrafilter, of all the Köthe function spaces with non trivial concavity over the same measure space can be represented as Köthe function spaces over the same (enlarged) measure space. The existence of a uniform homeomorphism between the unit spheres of two such Köthe function spaces is reproved.
The range of a contractive projection in L(H).
We show that the range of a contractive projection on a Lebesgue-Bochner space of Hilbert valued functions L(H) is isometric to a l-direct sum of Hilbert-valued L-spaces. We explicit the structure of contractive projections. As a consequence for every 1 < p < ∞ the class C of l-direct sums of Hilbert-valued L-spaces is axiomatizable (in the class of all Banach spaces).
On complemented subspaces of rearrangement invariant function spaces.
A necessary and sufficient condition is given for a rearrangement invariant function space to contain a complemented isomorphic copy of l1(l2).
Dual spaces to Orlicz-Lorentz spaces
For an Orlicz function φ and a decreasing weight w, two intrinsic exact descriptions are presented for the norm in the Köthe dual of the Orlicz-Lorentz function space or the sequence space , equipped with either the Luxemburg or Amemiya norms. The first description is via the modular , where f* is the decreasing rearrangement of f, ≺ denotes submajorization, and φ⁎ is the complementary function to φ. The second description is in terms of the modular ,where (f*)⁰ is Halperin’s level function...
Asymptotically hilbertian modular Banach spaces: examples of uncountable categoricity
We give a criterion ensuring that the elementary class of a modular Banach space (that is, the class of Banach spaces, some ultrapower of which is linearly isometric to an ultrapower of ) consists of all direct sums , where is an arbitrary Hilbert space and denotes the modular direct sum. Also, we give several families of examples in the class of Nakano direct sums of finite dimensional normed spaces that satisfy this criterion. This yields many new examples of uncountably categorical Banach...
An explicit expression for the K functionals of interpolation between L spaces.
When dealing with interpolation spaces by real methods one is lead to compute (or at least to estimate) the K-functional associated to the couple of interpolation spaces. This concept was first introduced by J. Peetre (see [8], [9]) and some efforts have been done to find explicit expressions of it for the case of Lebesgue spaces. It is well known that for the couple consisting of L1 and L∞ on [0, ∞) K is given by K (t; f, L1, L∞...
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