• The Complex-Scaled Half-Space Matching Method 

      BONNET-BEN DHIA, Anne-Sophie; Chandler-Wilde, Simon; Fliss, Sonia; Hazard, Christophe; Perfekt, Karl-Mikael; TJANDRAWIDJAJA, Yohanes (Peer reviewed; Journal article, 2022)
      The half-space matching (HSM) method has recently been developed as a new method for the solution of two-dimensional scattering problems with complex backgrounds, providing an alternative to perfectly matched layers or ...
    • Contractive inequalities for Bergman spaces and multiplicative Hankel forms 

      Bayart, Frédéric; Brevig, Ole Fredrik; Haimi, Antti; Ortega-Cerdà, Joaquim; Perfekt, Karl-Mikael (Journal article; Peer reviewed, 2019)
      We consider sharp inequalities for Bergman spaces of the unit disc, establishing analogues of the inequality in Carleman's proof of the isoperimetric inequality and of Weissler's inequality for dilations. By contractivity ...
    • Infinitely Many Embedded Eigenvalues for the Neumann-Poincaré Operator in 3D 

      Li, Wei; Perfekt, Karl-Mikael; Shipman, Stephen (Peer reviewed; Journal article, 2022)
      This article constructs a surface whose Neumann--Poincaré (NP) integral operator has infinitely many eigenvalues embedded in its essential spectrum. The surface is a sphere perturbed by smoothly attaching a conical ...
    • Nehari’s theorem for convex domain Hankel and Toeplitz operators in several variables 

      Carlsson, Marcus; Perfekt, Karl-Mikael (Peer reviewed; Journal article, 2021)
      We prove Nehari’s theorem for integral Hankel and Toeplitz operators on simple convex polytopes in several variables. A special case of the theorem, generalizing the boundedness criterion of the Hankel and Toeplitz operators ...
    • Norms of composition operators on the $H^2$ space of Dirichlet series 

      Brevig, Ole Fredrik; Perfekt, Karl-Mikael (Journal article; Peer reviewed, 2020)
      , generated by Dirichlet series symbols φ. We prove two different subordination principles for such operators. One concerns affine symbols only, and is based on an arithmetical condition on the coefficients of φ. The other ...
    • Orthogonal decomposition of composition operators on the $H^2$ space of Dirichlet series 

      Brevig, Ole Fredrik; Perfekt, Karl-Mikael (Peer reviewed; Journal article, 2022)
      Let denote the Hilbert space of Dirichlet series with square-summable coefficients. We study composition operators on which are generated by symbols of the form , in the case that . If only a subset of prime numbers ...
    • Plasmonic eigenvalue problem for corners: limiting absorption principle and absolute continuity in the essential spectrum 

      Perfekt, Karl-Mikael (Peer reviewed; Journal article, 2021)
      We consider the plasmonic eigenvalue problem for a general 2D domain with a curvilinear corner, studying the spectral theory of the Neumann–Poincaré operator of the boundary. A limiting absorption principle is proved, valid ...
    • Rectangular Summation of Multiple Fourier Series and Multi-parametric Capacity 

      Perfekt, Karl-Mikael (Peer reviewed; Journal article, 2021)
      We consider the class of multiple Fourier series associated with functions in the Dirichlet space of the polydisc. We prove that every such series is summable with respect to unrestricted rectangular partial sums, everywhere ...
    • The multiplicative Hilbert matrix 

      Brevig, Ole Fredrik; Perfekt, Karl-Mikael; Seip, Kristian; Siskakis, Aristomenis; Vukotic, Dragan (Journal article; Peer reviewed, 2016)
    • Volterra operators on Hardy spaces of Dirichlet series 

      Brevig, Ole Fredrik; Perfekt, Karl-Mikael; Seip, Kristian (Journal article; Peer reviewed, 2019)
      For a Dirichlet series symbol g.s/ D P n 1 bnn s , the associated Volterra operator Tg acting on a Dirichlet series f .s/ D P n 1 ann s is defined by the integral f 7! Z C1 s f .w/g0 .w/ dw: We show that Tg is a bounded ...