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Extension of compact bilinear operators

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Title: Extension of compact bilinear operators
Author(s): Koutsompinas, Ioannis Jr
Contributor: University of Helsinki, Faculty of Science
Degree program: Master 's Programme in Mathematics and Statistics
Specialisation: Mathematics
Language: English
Acceptance year: 2021
In this thesis we study extension results related to compact bilinear operators in the setting of interpolation theory and more specifically the complex interpolation method, as introduced by Calderón. We say that: 1. the bilinear operator T is compact if it maps bounded sets to sets of compact closure. 2.\bar{ A} = (A_0,A_1) is a Banach couple if A_0,A_1 are Banach spaces that are continuously embedded in the same Hausdorff topological vector space. Moreover, if (Ω,\mathcal{A}, μ) is a σ-finite measure space, we say that: 3. E is a Banach function space if E is a Banach space of scalar-valued functions defined on Ω that are finite μ-a.e. and so that the norm of E is related to the measure μ in an appropriate way. 4. the Banach function space E has absolutely continuous norm if for any function f ∈ E and for any sequence (Γ_n)_{n=1}^{+∞}⊂ \mathcal{A} satisfying χ_{Γn} → 0 μ-a.e. we have that ∥f · χ_{Γ_n}∥_E → 0. Assume that \bar{A} and \bar{B} are Banach couples, \bar{E} is a couple of Banach function spaces on Ω, θ ∈ (0, 1) and E_0 has absolutely continuous norm. If the bilinear operator T : (A_0 ∩ A_1) × (B_0 ∩ B_1) → E_0 ∩ E_1 satisfies a certain boundedness assumption and T : \tilde{A_0} × \tilde{B_0} → E_0 compactly, we show that T may be uniquely extended to a compact bilinear operator T : [A_0,A_1]_θ × [B_0,B_1]_θ → [E_0,E_1]_θ where \tilde{A_j} denotes the closure of A_0 ∩ A_1 in A_j and [A_0,A_1]_θ denotes the complex interpolation space generated by \bar{A}. The proof of this result comes after we study the case where the couple of Banach function spaces is replaced by a single Banach space.
Keyword(s): Compact bilinear operators Interpolation spaces Complex interpolation Banach function spaces

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