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3 Studia Mathematica

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Ideals of finite rank operators, intersection properties of balls, and the approximation property

100%

Lima Å. , Oja E.

nr 2

175-186

We characterize the approximation property of Banach spaces and their dual spaces by the position of finite rank operators in the space of compact operators. In particular, we show that a Banach space E has the approximation property if and only if for all closed subspaces F of $c_0$, the space ℱ(F,E) of finite rank operators from F to E has the n-intersection property in the corresponding space K(F,E) of compact operators for all n, or equivalently, ℱ(F,E) is an ideal in K(F,E).

Strict u-ideals in Banach spaces

100%

Lima V. , Lima Å.

Studia Mathematica

2009

tom 195

nr 3

275-285

We study strict u-ideals in Banach spaces. A Banach space X is a strict u-ideal in its bidual when the canonical decomposition $X*** = X* ⊕ X^{⊥}$ is unconditional. We characterize Banach spaces which are strict u-ideals in their bidual and show that if X is a strict u-ideal in a Banach space Y then X contains c₀. We also show that $ℓ_{∞}$ is not a u-ideal.

On the compact approximation property

80%

Lima V. , Lima Å. , Nygaard O.

Studia Mathematica

2004

tom 160

nr 2

185-200

We show that a Banach space X has the compact approximation property if and only if for every Banach space Y and every weakly compact operator T: Y → X, the space 𝔈 = {S ∘ T: S compact operator on X} is an ideal in 𝔉 = span(𝔈,T) if and only if for every Banach space Y and every weakly compact operator T: Y → X, there is a net $(S_γ)$ of compact operators on X such that $sup_{γ}||S_{γ}T|| ≤ ||T||$ and $S_{γ} → I_{X}$ in the strong operator topology. Similar results for dual spaces are also proved.