Minimal projections on hyperplanes in sequence spaces

Summary The projection constants of hyperplanes in the classical sequence spaces (c ₀ ) and (l ₁ ) are determined, together with the projections of minimum norm. Entrata in Redazione I'll dicembre 1972.     

Minimal projections in spaces of functions of N variables

We will construct a minimal and co-minimal projection from L^p([0,1]ⁿ) onto L^p([0,1]^n₁)++L^p([0,1]^n_k), where n=n₁++n_k (see Theorem 2.9). This is a generalization of a result of Cheney, Halton and Light from (Approximation Theory in Tensor Product Spaces, Lecture Notes in Mathematics, Springer, Berlin, 1985; Math. Proc. Cambridge Philos. Soc. 97 (1985) 127; Math. Z. 191 (1986) 633) where they proved the minimality in the case n=2. We provide also some further generalizations (see Theorems 2.10 and 2.11 (Orlicz spaces) and Theorem 2.8). Also a discrete case (Theorem 2.2) is considered. Our approach differs from methods used in [8,13,20].     

Minimal linear spaces

A decent linear space DLS(k) is a linear space with minimal line size at least three and with maximal line size exactly k. Denote by v_k (resp. b_k) the minimum number of points (resp. lines) in a DLS(k). We determine the numbers v_k and b_k for all k and prove that each DLS(k) with b_k lines has v_k points. Thus the DLS(k)'s with b_k lines are the minimal linear spaces.     

Oblique projections in Hilbert spaces

In this study we define some classes of projections in a Hilbert space and extend results obtained in a former paper, [2]. The theory allows better insight into examples described in [2], and it is demonstrated how a certain family of projections can be expressed by means of familiar operators.     

s-numbers of projections in Banach spaces

Given ans-number sequences te {h, x, y, c, d, a, Γ}, we find a characterization of the following property of a Banach spaceX:(P _s). There is a constantC>0 such that, for anyn-dimensional subspaceE ofX, we can find a projectionP fromX ontoE with sup k ks _k(P)≦Cn. As an application, we prove thatX has weak type 2 if and only ifX is finite dimensionally norming, thus answering a question of Casazza and Shura. Weak Hilbert spaces are also characterized in a new way, the main tool in the proof being a characterization of weak cotype 2 by means of projections. The latter is applied to the study of U.A.P., too.     

Minimal vectors in arbitrary Banach spaces

We extend the method of minimal vectors to arbitrary Banach spaces. It is proved, by a variant of the method, that certain quasinilpotent operators on arbitrary Banach spaces have hyperinvariant subspaces.

     

Contractive projections and operator spaces

Parallel to the study of finite-dimensional Banach spaces, there is a growing interest in the corresponding local theory of operator spaces. We define a family of Hilbertian operator spaces , , generalizing the row and column Hilbert spaces and , and we show that an atomic subspace that is the range of a contractive projection on is isometrically completely contractive to an -sum of the and Cartan factors of types 1 to 4. In particular, for finite-dimensional , this answers a question posed by Oikhberg and Rosenthal. Explicit in the proof is a classification up to complete isometry of atomic w-closed -triples without an infinite-dimensional rank 1 w-closed ideal.

     

Minimal widths of metric spaces

The minimal width of an arbitrary metric space is defined as the greatest lower bound of its Kolmogorov widths under all isometric embeddings in all possible Banach spaces and is computed or estimated in a number of examples. Supported by RFBR grant No. 96-15-96249. Human Moscow State Technical University. Translated from Funktsional’nyi Analiz i Ego Prilozheniya, Vol. 33, No. 4, 49, October–December, 1999. Translated by R. S. Ismagilov     

BERGMAN TYPE PROJECTIONS ON L^p SPACES

The paper is a survey on the action of Bergman type projections on various Lp on three types of holomorphic function spaces： weighted Bergman spaces, the Bloch spaces. The focus is space, and diagonal Besov spaces.     

Bicircular projections on some Banach spaces

In this paper we show that the bicircular projections are precisely the Hermitian projections and prove some immediate consequences of this result.