78 research outputs found
Heat transfer in a complex medium
The heat equation is considered in the complex medium consisting of many
small bodies (particles) embedded in a given material. On the surfaces of the
small bodies an impedance boundary condition is imposed. An equation for the
limiting field is derived when the characteristic size of the small bodies
tends to zero, their total number tends to infinity at a
suitable rate, and the distance between neighboring small bodies
tends to zero: , . No periodicity is
assumed about the distribution of the small bodies. These results are basic for
a method of creating a medium in which heat signals are transmitted along a
given line. The technical part for this method is based on an inverse problem
of finding potential with prescribed eigenvalues.Comment: arXiv admin note: text overlap with arXiv:1207.056
Vortex structure in exponentially shaped Josephson junctions
We report the numerical calculations of the static vortex structure and
critical curves in exponentially shaped long Josephson junctions for in-line
and overlap geometries. Each solution of the corresponding boundary value
problem is associated with the Sturm-Liouville problem whose minimal eigenvalue
allows to make a conclusion about the stability of the vortex. The change in
width of the junction leads to the renormalization of the magnetic flux in
comparison to the case of a linear one-dimensional model. We study the
influence of the model's parameters and, particularly, the shape parameter on
the stability of the states of the magnetic flux. We compare the vortex
structure and critical curves for the in-line and overlap geometries. Our
numerically constructed critical curve of the Josephson junction matches well
with the experimental one.Comment: 8 pages, 10 figures, NATO Advanced Research Workshop on "Vortex
dynamics in superconductors and other complex systems" Yalta, Crimea,
Ukraine, 13-17 September 200
On the Two Spectra Inverse Problem for Semi-Infinite Jacobi Matrices
We present results on the unique reconstruction of a semi-infinite Jacobi
operator from the spectra of the operator with two different boundary
conditions. This is the discrete analogue of the Borg-Marchenko theorem for
Schr{\"o}dinger operators in the half-line. Furthermore, we give necessary and
sufficient conditions for two real sequences to be the spectra of a Jacobi
operator with different boundary conditions.Comment: In this slightly revised version we have reworded some of the
theorems, and we updated two reference
The Two-Spectra Inverse Problem for Semi-Infinite Jacobi Matrices in The Limit-Circle Case
We present a technique for reconstructing a semi-infinite Jacobi operator in
the limit circle case from the spectra of two different self-adjoint
extensions. Moreover, we give necessary and sufficient conditions for two real
sequences to be the spectra of two different self-adjoint extensions of a
Jacobi operator in the limit circle case.Comment: 26 pages. Changes in the presentation of some result
Inverse spectral problems for Dirac operators with summable matrix-valued potentials
We consider the direct and inverse spectral problems for Dirac operators on
with matrix-valued potentials whose entries belong to ,
. We give a complete description of the spectral data
(eigenvalues and suitably introduced norming matrices) for the operators under
consideration and suggest a method for reconstructing the potential from the
corresponding spectral data.Comment: 32 page
Localization on quantum graphs with random vertex couplings
We consider Schr\"odinger operators on a class of periodic quantum graphs
with randomly distributed Kirchhoff coupling constants at all vertices. Using
the technique of self-adjoint extensions we obtain conditions for localization
on quantum graphs in terms of finite volume criteria for some energy-dependent
discrete Hamiltonians. These conditions hold in the strong disorder limit and
at the spectral edges
L^{2}-restriction bounds for eigenfunctions along curves in the quantum completely integrable case
We show that for a quantum completely integrable system in two dimensions,the
-normalized joint eigenfunctions of the commuting semiclassical
pseudodifferential operators satisfy restriction bounds ofthe form for generic
curves on the surface. We also prove that the maximal restriction
bounds of Burq-Gerard-Tzvetkov are always attained for certain exceptional
subsequences of eigenfunctions.Comment: Correct some typos and added some more detail in section
Cantor and band spectra for periodic quantum graphs with magnetic fields
We provide an exhaustive spectral analysis of the two-dimensional periodic
square graph lattice with a magnetic field. We show that the spectrum consists
of the Dirichlet eigenvalues of the edges and of the preimage of the spectrum
of a certain discrete operator under the discriminant (Lyapunov function) of a
suitable Kronig-Penney Hamiltonian. In particular, between any two Dirichlet
eigenvalues the spectrum is a Cantor set for an irrational flux, and is
absolutely continuous and has a band structure for a rational flux. The
Dirichlet eigenvalues can be isolated or embedded, subject to the choice of
parameters. Conditions for both possibilities are given. We show that
generically there are infinitely many gaps in the spectrum, and the
Bethe-Sommerfeld conjecture fails in this case.Comment: Misprints correcte
A functional model, eigenvalues, and finite singular critical points for indefinite Sturm-Liouville operators
Eigenvalues in the essential spectrum of a weighted Sturm-Liouville operator
are studied under the assumption that the weight function has one turning
point. An abstract approach to the problem is given via a functional model for
indefinite Sturm-Liouville operators. Algebraic multiplicities of eigenvalues
are obtained. Also, operators with finite singular critical points are
considered.Comment: 38 pages, Proposition 2.2 and its proof corrected, Remarks 2.5, 3.4,
and 3.12 extended, details added in subsections 2.3 and 4.2, section 6
rearranged, typos corrected, references adde
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