On the sets of sequences that are strongly -bounded and -convergent to naught with index .
On the size of approximately convex sets in normed spaces
Let X be a normed space. A set A ⊆ X is approximately convexif d(ta+(1-t)b,A)≤1 for all a,b ∈ A and t ∈ [0,1]. We prove that every n-dimensional normed space contains approximately convex sets A with and , where ℋ denotes the Hausdorff distance. These estimates are reasonably sharp. For every D>0, we construct worst possible approximately convex sets in C[0,1] such that ℋ(A,Co(A))=(A)=D. Several results pertaining to the Hyers-Ulam stability theorem are also proved.
On the size of the sets of gradients of bump functions and starlike bodies on the Hilbert space
We study the size of the sets of gradients of bump functions on the Hilbert space , and the related question as to how small the set of tangent hyperplanes to a smooth bounded starlike body in can be. We find that those sets can be quite small. On the one hand, the usual norm of the Hilbert space can be uniformly approximated by smooth Lipschitz functions so that the cones generated by the ranges of its derivatives have empty interior. This implies that there are smooth Lipschitz bumps...
On the smoothness of Orlicz sequence spaces equipped with Orlicz norm.
We give a criterion of smoothness of Orlicz sequence spaces with Orlicz norm.
On the space of -summable sequences
On the stability of a generalized quadratic and quartic type functional equation in quasi-Banach spaces.
On the stability of the fixed point property in spaces.
On the stability of the unit circle with minimal self-perimeter in normed planes
We prove a stability result on the minimal self-perimeter L(B) of the unit disk B of a normed plane: if L(B) = 6 + ε for a sufficiently small ε, then there exists an affinely regular hexagon S such that S ⊂ B ⊂ (1 + 6∛ε) S.
On the strict convexity of the Besicovitch-Orlicz space of almost periodic functions with Orlicz norm.
The problem of strict convexity of the Besicovitch-Orlicz space of almost periodic functions is considered here in connection with the Orlicz norm. We give necessary and sufficient conditions in terms of the function f generating the space.
On the structure of Banach spaces with an unconditional basic sequence
For a Banach space X with an unconditional basic sequence, one of the following regular-irregular alternatives holds: either X contains a subspace isomorphic to ℓ₂, or X contains a subspace which has an unconditional finite-dimensional decomposition, but does not admit such a decomposition with a uniform bound for the dimensions of the decomposition. This result can be viewed in the context of Gowers' dichotomy theorem.
On the structure of Banach spaces with Mazur's intersection property.
On the structure of derivations on certain nonamenable nuclear Banach algebras.
On the structure of non-dentable subsets of
It is shown that there is no closed convex bounded non-dentable subset K of such that on subsets of K the PCP and the RNP are equivalent properties. Then applying the Schachermayer-Rosenthal theorem, we conclude that every non-dentable K contains a non-dentable subset L so that on L the weak topology coincides with the norm topology. It follows from known results that the RNP and the KMP are equivalent on subsets of .
On the Structure of Non-Weakly Compact Operators on Banach Lattices.
On the structure of separable spaces (1 < p < ∞)
On the structure of tensor norms related to (p,σ)-absolutely continuous operators.
We define an interpolation norm on tensor products of p-integrable function spaces and Banach spaces which satisfies intermediate properties between the Bochner norm and the injective norm. We obtain substitutes of the Chevet-Persson-Saphar inequalities for this case. We also use the calculus of traced tensor norms in order to obtain a tensor product description of the tensor norm associated to the interpolated ideal of (p, sigma)-absolutely continuous operators defined by Jarchow and Matter. As...
On the structure of the set of higher order spreading models
We generalize some results concerning the classical notion of a spreading model to spreading models of order ξ. Among other results, we prove that the set of ξ-order spreading models of a Banach space X generated by subordinated weakly null ℱ-sequences endowed with the pre-partial order of domination is a semilattice. Moreover, if contains an increasing sequence of length ω then it contains an increasing sequence of length ω₁. Finally, if is uncountable, then it contains an antichain of size...
On the structure of ultraproducts of real interpolation spaces.
On the sublinear operators factoring through .
On the Surjective Dunford-Pettis Property.