89 research outputs found
Corona-type theorems and division in some function algebras on planar domains
Let be an algebra of bounded smooth functions on the interior of a
compact set in the plane. We study the following problem: if
satisfy , does there exist
and a constant such that ? A
prominent role in our proofs is played by a new space, C_{\dbar, 1}(K), which
we call the algebra of \dbar-smooth functions.
In the case , a complete solution is given for the algebras of
functions holomorphic in and whose first -derivatives extend
continuously to \ov{K^\circ}. This necessitates the introduction of a special
class of compacta, the so-called locally L-connected sets.
We also present another constructive proof of the Nullstellensatz for ,
that is only based on elementary \dbar-calculus and Wolff's method.Comment: 23 pages, 6 figure
Siciak-Zahariuta extremal functions, analytic discs and polynomial hulls
We prove two disc formulas for the Siciak-Zahariuta extremal function of an
arbitrary open subset of complex affine space. We use these formulas to
characterize the polynomial hull of an arbitrary compact subset of complex
affine space in terms of analytic discs. Similar results in previous work of
ours required the subsets to be connected
Interface modes of two-dimensional composite structures
The surface modes of a composite consisting of aligned metallic wires with
square cross sections are investigated, on the basis of photonic band structure
calculations. The effective long-wavelength dielectric response function is
computed, as a function of the filling fraction. The dependence of the optical
absorption on the shape of the wires and the polarization of light is
discussed, and the effect of sharp corners analyzed. The effect of the
interaction between the wires on the localization of surface plasmons is also
addressed.Comment: 12 pages, 4 figures, to appear in Surf. Sc
Reconstruction of Bandlimited Functions from Unsigned Samples
We consider the recovery of real-valued bandlimited functions from the
absolute values of their samples, possibly spaced nonuniformly. We show that
such a reconstruction is always possible if the function is sampled at more
than twice its Nyquist rate, and may not necessarily be possible if the samples
are taken at less than twice the Nyquist rate. In the case of uniform samples,
we also describe an FFT-based algorithm to perform the reconstruction. We prove
that it converges exponentially rapidly in the number of samples used and
examine its numerical behavior on some test cases
A Weighted Estimate for the Square Function on the Unit Ball in \C^n
We show that the Lusin area integral or the square function on the unit ball
of \C^n, regarded as an operator in weighted space has a linear
bound in terms of the invariant characteristic of the weight. We show a
dimension-free estimate for the ``area-integral'' associated to the weighted
norm of the square function. We prove the equivalence of the classical
and the invariant classes.Comment: 11 pages, to appear in Arkiv for Matemati
Electron energy loss and induced photon emission in photonic crystals
The interaction of a fast electron with a photonic crystal is investigated by
solving the Maxwell equations exactly for the external field provided by the
electron in the presence of the crystal. The energy loss is obtained from the
retarding force exerted on the electron by the induced electric field. The
features of the energy loss spectra are shown to be related to the photonic
band structure of the crystal. Two different regimes are discussed: for small
lattice constants relative to the wavelength of the associated electron
excitations , an effective medium theory can be used to describe the
material; however, for the photonic band structure plays an
important role. Special attention is paid to the frequency gap regions in the
latter case.Comment: 12 pages, 7 figure
Causality, particle localization and positivity of the energy
Positivity of the Hamiltonian alone is used to show that particles, if
initially localized in a finite region, immediately develop infinite tails.Comment: To appear in: Irreversibility and Causality in Quantum Theory --
Semigroups and Rigged Hilbert Spaces, edited by A. Bohm, H.-D. Doebner and P.
Kielanowski, Springer Lecture Notes in Physics, Vol. 504 (1998
Boundedness, compactness and Schatten-class membership of weighted composition operators
The boundedness and compactness of weighted composition operators on the
Hardy space of the unit disc is analysed. Particular reference
is made to the case when the self-map of the disc is an inner function.
Schatten-class membership is also considered; as a result, stronger forms of
the two main results of a recent paper of Gunatillake are derived. Finally,
weighted composition operators on weighted Bergman spaces are considered, and the results of Harper and Smith,
linking their properties to those of Carleson embeddings, are extended to this
situation.Comment: 12 page
Recurrence for discrete time unitary evolutions
We consider quantum dynamical systems specified by a unitary operator U and
an initial state vector \phi. In each step the unitary is followed by a
projective measurement checking whether the system has returned to the initial
state. We call the system recurrent if this eventually happens with probability
one. We show that recurrence is equivalent to the absence of an absolutely
continuous part from the spectral measure of U with respect to \phi. We also
show that in the recurrent case the expected first return time is an integer or
infinite, for which we give a topological interpretation. A key role in our
theory is played by the first arrival amplitudes, which turn out to be the
(complex conjugated) Taylor coefficients of the Schur function of the spectral
measure. On the one hand, this provides a direct dynamical interpretation of
these coefficients; on the other hand it links our definition of first return
times to a large body of mathematical literature.Comment: 27 pages, 5 figures, typos correcte
Zeros of analytic functions, with or without multiplicities
The classical Mason-Stothers theorem deals with nontrivial polynomial
solutions to the equation . It provides a lower bound on the number of
distinct zeros of the polynomial in terms of the degrees of , and
. We extend this to general analytic functions living on a reasonable
bounded domain , rather than on the whole of . The estimates obtained are sharp, for any , and a generalization of
the original result on polynomials can be recovered from them by a limiting
argument.Comment: This is a retitled and slightly revised version of my paper
arXiv:1004.359
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