152 research outputs found
Scattering by magnetic fields
Consider the scattering amplitude ,
, , corresponding to an
arbitrary short-range magnetic field , . This is a smooth
function of and away from the diagonal
but it may be singular on the diagonal. If , then
the singular part of the scattering amplitude (for example, in the transversal
gauge) is a linear combination of the Dirac function and of a singular
denominator. Such structure is typical for long-range scattering. We refer to
this phenomenon as to the long-range Aharonov-Bohm effect. On the contrary, for
scattering is essentially of short-range nature although, for example,
the magnetic potential such that
and decays at infinity as only. To be more
precise, we show that, up to the diagonal Dirac function (times an explicit
function of ), the scattering amplitude has only a weak singularity in
the forward direction .
Our approach relies on a construction in the dimension of a short-range
magnetic potential corresponding to a given short-range magnetic field
The semiclassical limit of eigenfunctions of the Schr\"odinger equation and the Bohr-Sommerfeld quantization condition, revisited
Consider the semiclassical limit, as the Planck constant \hbar\ri 0, of
bound states of a quantum particle in a one-dimensional potential well. We
justify the semiclassical asymptotics of eigenfunctions and recover the
Bohr-Sommerfeld quantization condition
Spectral and scattering theory for perturbations of the Carleman operator
We study spectral properties of the Carleman operator (the Hankel operator
with kernel ) and, in particular, find an explicit formula for
its resolvent. Then we consider perturbations of the Carleman operator
by Hankel operators with kernels decaying sufficiently rapidly as
and not too singular at t=0. Our goal is to develop scattering
theory for the pair , and to construct an expansion in
eigenfunctions of the continuous spectrum of the Hankel operator . We also
prove that under general assumptions the singular continuous spectrum of the
operator is empty and that its eigenvalues may accumulate only to the edge
points 0 and in the spectrum of . We find simple conditions for
the finiteness of the total number of eigenvalues of the operator lying
above the (continuous) spectrum of the Carleman operator and obtain an
explicit estimate of this number. The theory constructed is somewhat analogous
to the theory of one-dimensional differential operators.Comment: dedicated to the memory of Vladimir Savel'evich Buslaev there are
some minor and editorial changes compared to 1210.5709 of 21 oc
On semibounded Toeplitz operators
We show that a semibounded Toeplitz quadratic form is closable in the space
if and only if its matrix elemens are Fourier
coefficients of an absolutely continuous measure. We also describe the domain
of the corresponding closed form. This allows us to define semibounded Toeplitz
operators under minimal assumptions on their matrix elements.Comment: This is a slightly revised version of the article,
arXiv:1603.06229v1, with the same tittle. Some misprints has been removed and
some arguments has been made more clear. The results are unchaged. To appear
in J. Operator theor
Quasi-Carleman operators and their spectral properties
The Carleman operator is defined as integral operator with kernel
in the space . This is the simplest example
of a Hankel operator which can be explicitly diagonalized. Here we study a
class of self-adjoint Hankel operators (we call them quasi-Carleman operators)
generalizing the Carleman operator in various directions. We find explicit
formulas for the total number of negative eigenvalues of quasi-Carleman
operators and, in particular, necessary and sufficient conditions for their
positivity. Our approach relies on the concepts of the sigma-function and of
the quasi-diagonalization of Hankel operators introduced in the preceding paper
of the author
- …