23,640 research outputs found

    A spectral radius type formula for approximation numbers of composition operators

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    For approximation numbers an(Cϕ)a_n (C_\phi) of composition operators CϕC_\phi on weighted analytic Hilbert spaces, including the Hardy, Bergman and Dirichlet cases, with symbol ϕ\phi of uniform norm <1< 1, we prove that \lim_{n \to \infty} [a_n (C_\phi)]^{1/n} = \e^{- 1/ \capa [\phi (\D)]}, where \capa [\phi (\D)] is the Green capacity of \phi (\D) in \D. This formula holds also for HpH^p with 1p<1 \leq p < \infty.Comment: 25 page

    Estimates for approximation numbers of some classes of composition operators on the Hardy space

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    We give estimates for the approximation numbers of composition operators on H2H^2, in terms of some modulus of continuity. For symbols whose image is contained in a polygon, we get that these approximation numbers are dominated by \e^{- c \sqrt n}. When the symbol is continuous on the closed unit disk and has a domain touching the boundary non-tangentially at a finite number of points, with a good behavior at the boundary around those points, we can improve this upper estimate. A lower estimate is given when this symbol has a good radial behavior at some point. As an application we get that, for the cusp map, the approximation numbers are equivalent, up to constants, to \e^{- c \, n / \log n}, very near to the minimal value \e^{- c \, n}. We also see the limitations of our methods. To finish, we improve a result of O. El-Fallah, K. Kellay, M. Shabankhah and H. Youssfi, in showing that for every compact set KK of the unit circle \T with Lebesgue measure 0, there exists a compact composition operator Cϕ ⁣:H2H2C_\phi \colon H^2 \to H^2, which is in all Schatten classes, and such that ϕ=1\phi = 1 on KK and ϕ<1|\phi | < 1 outside KK

    Infinitesimal Carleson property for weighted measures induced by analytic self-maps of the unit disk

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    We prove that, for every α>1\alpha > -1, the pull-back measure ϕ(Aα)\phi ({\cal A}_\alpha) of the measure dAα(z)=(α+1)(1z2)αdA(z)d{\cal A}_\alpha (z) = (\alpha + 1) (1 - |z|^2)^\alpha \, d{\cal A} (z), where A{\cal A} is the normalized area measure on the unit disk \D, by every analytic self-map \phi \colon \D \to \D is not only an (α+2)(\alpha + 2)-Carleson measure, but that the measure of the Carleson windows of size \eps h is controlled by \eps^{\alpha + 2} times the measure of the corresponding window of size hh. This means that the property of being an (α+2)(\alpha + 2)-Carleson measure is true at all infinitesimal scales. We give an application by characterizing the compactness of composition operators on weighted Bergman-Orlicz spaces
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