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Espacios de producto interno (II).

Palaniappan Kannappan (1995)

Mathware and Soft Computing

Among normal linear spaces, the inner product spaces (i.p.s.) are particularly interesting. Many characterizations of i.p.s. among linear spaces are known using various functional equations. Three functional equations characterizations of i.p.s. are based on the Frchet condition, the Jordan and von Neumann identity and the Ptolemaic inequality respectively. The object of this paper is to solve generalizations of these functional equations.

Espacios semi-LpB.

Carmen Fernández (1989)

Collectanea Mathematica

This paper is devoted to the study of semi-LpB spaces, which coincide with the semi-LB spaces defined by Valdivia when p = 1. We give new results in localization and lifting. We study the relation between the class of semi-LpB spaces and the class of webbed spaces. Finally we obtain localization theorems without any convexity assumptions.

Essential norm of the difference of composition operators on Bloch space

Ke-Ben Yang, Ze-Hua Zhou (2010)

Czechoslovak Mathematical Journal

Let ϕ and ψ be holomorphic self-maps of the unit disk, and denote by C ϕ , C ψ the induced composition operators. This paper gives some simple estimates of the essential norm for the difference of composition operators C ϕ - C ψ from Bloch spaces to Bloch spaces in the unit disk. Compactness of the difference is also characterized.

Essential normality for certain finite linear combinations of linear-fractional composition operators on the Hardy space H 2

Mahsa Fatehi, Bahram Khani Robati (2012)

Czechoslovak Mathematical Journal

In 1999 Nina Zorboska and in 2003 P. S. Bourdon, D. Levi, S. K. Narayan and J. H. Shapiro investigated the essentially normal composition operator C ϕ , when ϕ is a linear-fractional self-map of 𝔻 . In this paper first, we investigate the essential normality problem for the operator T w C ϕ on the Hardy space H 2 , where w is a bounded measurable function on 𝔻 which is continuous at each point of F ( ϕ ) , ϕ 𝒮 ( 2 ) , and T w is the Toeplitz operator with symbol w . Then we use these results and characterize the essentially normal...

Essential norms of weighted composition operators on the space of Dirichlet series

Pascal Lefèvre (2009)

Studia Mathematica

We estimate the essential norm of a weighted composition operator relative to the class of Dunford-Pettis operators or the class of weakly compact operators, on the space of Dirichlet series. As particular cases, we obtain the precise value of the generalized essential norm of a composition operator and of a multiplication operator.

Essentially Incomparable Banach Spaces of Continuous Functions

Rogério Augusto dos Santos Fajardo (2010)

Bulletin of the Polish Academy of Sciences. Mathematics

We construct, under Axiom ♢, a family ( C ( K ξ ) ) ξ < 2 ( 2 ω ) of indecomposable Banach spaces with few operators such that every operator from C ( K ξ ) into C ( K η ) is weakly compact, for all ξ ≠ η. In particular, these spaces are pairwise essentially incomparable. Assuming no additional set-theoretic axiom, we obtain this result with size 2 ω instead of 2 ( 2 ω ) .

Essentially-Euclidean convex bodies

Alexander E. Litvak, Vitali D. Milman, Nicole Tomczak-Jaegermann (2010)

Studia Mathematica

In this note we introduce a notion of essentially-Euclidean normed spaces (and convex bodies). Roughly speaking, an n-dimensional space is λ-essentially-Euclidean (with 0 < λ < 1) if it has a [λn]-dimensional subspace which has further proportional-dimensional Euclidean subspaces of any proportion. We consider a space X₁ = (ℝⁿ,||·||₁) with the property that if a space X₂ = (ℝⁿ,||·||₂) is "not too far" from X₁ then there exists a [λn]-dimensional subspace E⊂ ℝⁿ such that E₁ = (E,||·||₁) and...

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