Relative boundedness and relative compactness for linear operators in Banach spaces

If and are linear operators acting between Banach spaces, we show that compactness of relative to does not in general imply that has -bound zero. We do, however, give conditions under which the above implication is valid.

     

A compactness criterion for translation invariant Banach spaces of functions

Установлено, что для и нвариантных относит ельно сдвига банаховых про странствВ (классов) измеримых фу нкций на локально ком пактной группе, удовлетворяю щих некоторым дополнительным усло виям, справедлив след ующий критерий компактнос ти. Замкнутое подмножествоМ?В яв ляется компактным вВ тогда и только тогда, к огда оно удовлетворя ет условиям:

1. М ограничено вВ;
2. для каждого ?>0 сущест вуетkε?(G) такое, что ∥k*f-f∥_BfεМ;
3. для каждого ?0 существ уетh??(G) такое, что ∥hf - f ∥_BfεМ.

     

Optimal recovery from a finite set in Banach spaces of entire functions

We develop an approach to the problem of optimal recovery of continuous linear functionals in Banach spaces through information on a finite number of given functionals. The results obtained are applied to the problem of the best analytic continuation from a finite set in the complex space Cn, n?1, for classes of entire functions of exponential type which belong to the space Lp, 1<p<∞, on the real subspace of Cn. These latter are known as Wiener classes.     

A note on compactness in Banach spaces

Two simple concepts are presented for constructing necessary and sufficient compactness criteria in Banach spaces. The concepts are applied to the spaces lip(T), C(T), and L_p(µ), where T is a compact metric space.     

Generalized sequential normal compactness in Banach spaces

Normal compactness conditions are important properties of sets, set-valued mappings in variational analysis, which are generalized versions of the classical Lipschitzian property and are essential for the calculus of generalized differentiation theory. In this paper we propose the notion called the generalized sequential normal compactness, and establish its basic properties and calculus in general Banach spaces.     

On weak compactness of bounded sets in Banach and locally convex spaces

We investigate the compactness of one class of bounded subsets in Banach and locally convex spaces. We obtain a generalization of the Banach-Alaoglu theorem to a class of subsets that are not polars of convex balanced neighborhoods of zero. Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 52, No. 6, pp 731–739, June. 2000.     

Boundaries for Banach spaces determine weak compactness

A boundary for a real Banach space is a subset of the dual unit sphere with the property that each element of the Banach space attains its norm on an element of that subset. Trivially, the pointwise convergence with respect to such a boundary is coarser than the weak topology on the Banach space. The boundary problem asks whether nevertheless both topologies have the same norm bounded compact sets.The main theorem of this paper states the equivalence of countable and sequential compactness of norm bounded sets with respect to an appropriate topology. This result contains, as a special case, the positive answer to the boundary problem and it carries James’ sup-characterization as a corollary.     

Weak compactness in projective tensor products of Banach spaces

Contrary to a recent conjecture, it is shown that weakly compact subsets of the projective tensor product of Banach spaces X and Y in general are not contained in the closed absolutely convex hull of a tensor product A ⊗ B of weakly compact subsets A of X and B of Y.     

Cauchy problem in a scale of Banach spaces

The concept of quasidifferential operator in a scale of Banach spaces is formulated. A theorem of existence and uniqueness of a solution to the Cauchy problem for the equation with a nonlinear quasidifferential operator is proved. As an example of application of the theorem, the correctness of the nonlinear nonlocal problem of plane-parallel unsteady potential motion of a liquid with free boundary is proved.     

On relative Hausdorff measures of noncompactness and relative Chebyshev radii in Banach spaces

In this paper we prove some formulae and evaluations on relative Hausdorff measures of noncompactness and relative Chebyshev radii in various Banach spaces. We generalize the Lifschitz constant and introduce a function .

     

On composition operators on Banach spaces of holomorphic functions

We discuss some spectral properties of composition operators in Banach spaces of holomorphic functions over infinite-dimensional domains. Our results generalize known properties of such operators in the finite-dimensional case.     

A weak Grothendieck compactness principle for Banach spaces with a symmetric basis

The Grothendieck compactness principle states that every norm compact subset of a Banach space is contained in the closed convex hull of a norm null sequence. In Dowling et al. (J Funct Anal 263(5):1378–1381, 2012), an analogue of the Grothendieck compactness principle for the weak topology was used to characterize Banach spaces with the Schur property. Using a different analogue of the Grothendieck compactness principle for the weak topology, a characterization of the Banach spaces with a symmetric basis that are not isomorphic to $\ell ^1$ and do not contain a subspace isomorphic to $c_0$ is given. As a corollary, it is shown that, in the Lorentz space $d(w,1)$ , every weakly compact set is contained in the closed convex hull of the rearrangement invariant hull of a norm null sequence.     

Near convexity,near smoothness and approximative compactness of half spaces in Banach spaces

The authors discuss the dual relation of nearly very convexity and property WS. By two kinds of near convexities and two kinds of near smoothness, the authors prove a series of characterizations such that every half-space in Banach space X and every weak* half-space in the dual space X* are approximatively weakly compact and approximatively compact. They show a sufficient condition such that a Banach space X is a Asplund space. Using upper semi-continuity of duality mapping, the authors also give two characterizations of property WS and property S.     

On the Sobolev problem in the complete scale of Banach spaces

In a bounded domainG with boundary ∂G that consists of components of different dimensions, we consider an elliptic boundary-value problem in complete scales of Banach spaces. The orders of boundary expressions are arbitrary; they are pseudodifferential along ∂G. We prove the theorem on a complete set of isomorphisms and generalize its application. The results obtained are true for elliptic Sobolev problems with a parameter and parabolic Sobolev problems as well as for systems with the Douglis-Nirenberg structure. Chernigov Pedagogical Institute, Chernigov. Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 51, No. 9, pp. 1181–1192, September, 1999.     

On the weak compactness in Banach lattices

The Grothendieck's criterion of weak compactness in spaceC(S) has been extended by C.P.Niculescu for weakly sequentially complete Banach lattices. This paper first provides some sufficient and necessary conditions for weakly sequentially complete Banach lattices, with the result that the criterion C.P.Niculescu obtained of the weak compactness has actually characterized the weakly sequentially Banach lattices. At the same time we extend C.P.Niculescu's important results. Then we solve negatively Problem 1.11 which was posed by C.P.Niculescu, and a sufficient condition for this is given which generalizes Pelczyński's result.