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We study the composition operators on an algebra of Dirichlet series, the analogue of the Wiener algebra of absolutely convergent Taylor series, which we call the Wiener-Dirichlet algebra, namely the connection between the properties of the operator and of its symbol, with special emphasis on the compact, automorphic, or isometric character of this(More)
We study the numerical index of a Banach space from the isomorphic point of view, that is, we investigate the values of the numerical index which can be obtained by renorming the space. The set of these values is always an interval which contains [0, 1/3[ in the real case and [e−1, 1/2[ in the complex case. Moreover, for “most” Banach spaces the least upper(More)
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