9,388 research outputs found
Lie groupoids and algebroids applied to the study of uniformity and homogeneity of material bodies
A Lie groupoid, called \textit{material Lie groupoid}, is associated in a
natural way to any elastic material. The corresponding Lie algebroid, called
\textit{material algebroid}, is used to characterize the uniformity and the
homogeneity properties of the material. The relation to previous results in
terms of structures is discussed in detail. An illustrative example is
presented as an application of the theory
On the use of bianisotropic huygens' metasurfaces to build leaky-wave antennas
The Electromagnetics AcademyHuygens' metasurfaces are considered a powerful tool to achieve anomalous electromagnetic field transformations. They consist of an artifcial surface built of pairs of collocated electric and magetic dipoles that force the boundary conditions for the desired transformation to be ful lled [1]. Despite their possibilities, the achievable transformations must ful l some conditions. In [2] it was
shown that Huygens' metasurfaces with passive and lossless particles can achieve an arbitrary field transformation provided that the power is conserved at each point of the metasurface and there is wave impedance matching. However, it was shown in [3], that by introducing bianisotropy of the omega-type, the matching condition can be suppressed, which allows the control of both the transmission and rejection coe cients on the metasurface.Universidad de MĂĄlaga. Campus de Excelencia Internacional AndalucĂa Tech
Fermat Principle in Finsler Spacetimes
It is shown that, on a manifold with a Finsler metric of Lorentzian
signature, the lightlike geodesics satisfy the following variational principle.
Among all lightlike curves from a point (emission event) to a timelike curve
(worldline of receiver), the lightlike geodesics make the arrival time
stationary. Here ``arrival time'' refers to a parametrization of the timelike
curve. This variational principle can be applied (i) to the vacuum light rays
in an alternative spacetime theory, based on Finsler geometry, and (ii) to
light rays in an anisotropic non-dispersive medium with a general-relativistic
spacetime as background.Comment: 18 pages, submitted to Gen. Rel. Gra
QED theory of the nuclear recoil effect in atoms
The quantum electrodynamic theory of the nuclear recoil effect in atoms to
all orders in \alpha Z is formulated. The nuclear recoil corrections for atoms
with one and two electrons over closed shells are considered in detail. The
problem of the composite nuclear structure in the theory of the nuclear recoil
effect is discussed.Comment: 20 pages, 6 figures, Late
Existential questions in (relatively) hyperbolic groups {\it and} Finding relative hyperbolic structures
This arXived paper has two independant parts, that are improved and corrected
versions of different parts of a single paper once named "On equations in
relatively hyperbolic groups".
The first part is entitled "Existential questions in (relatively) hyperbolic
groups". We study there the existential theory of torsion free hyperbolic and
relatively hyperbolic groups, in particular those with virtually abelian
parabolic subgroups. We show that the satisfiability of systems of equations
and inequations is decidable in these groups.
In the second part, called "Finding relative hyperbolic structures", we
provide a general algorithm that recognizes the class of groups that are
hyperbolic relative to abelian subgroups.Comment: Two independant parts 23p + 9p, revised. To appear separately in
Israel J. Math, and Bull. London Math. Soc. respectivel
Hydromagnetic instabilities in protoneutron stars
The stability properties of newly born neutron stars, or proto--neutron
stars, are considered. We take into account dissipative processes, such as
neutrino transport and viscosity, in the presence of a magnetic field. In order
to find the regions of the star subject to different sorts of instability, we
derive the general instability criteria and apply it to evolutionary models of
PNSs. The influence of the magnetic field on instabilities is analyzed and the
critical magnetic field stabilizing the star is obtained. In the light of our
results, we estimate of the maximum poloidal magnetic field that might be
present in young pulsars or magnetars.Comment: 18 pages, 4 figures, to appear in Astrophysical Journa
Online unit clustering in higher dimensions
We revisit the online Unit Clustering and Unit Covering problems in higher
dimensions: Given a set of points in a metric space, that arrive one by
one, Unit Clustering asks to partition the points into the minimum number of
clusters (subsets) of diameter at most one; while Unit Covering asks to cover
all points by the minimum number of balls of unit radius. In this paper, we
work in using the norm.
We show that the competitive ratio of any online algorithm (deterministic or
randomized) for Unit Clustering must depend on the dimension . We also give
a randomized online algorithm with competitive ratio for Unit
Clustering}of integer points (i.e., points in , , under norm). We show that the competitive ratio of
any deterministic online algorithm for Unit Covering is at least . This
ratio is the best possible, as it can be attained by a simple deterministic
algorithm that assigns points to a predefined set of unit cubes. We complement
these results with some additional lower bounds for related problems in higher
dimensions.Comment: 15 pages, 4 figures. A preliminary version appeared in the
Proceedings of the 15th Workshop on Approximation and Online Algorithms (WAOA
2017
Distribution of time-constants for tunneling through a 1D Disordered Chain
The dynamics of electronic tunneling through a disordered 1D chain of finite
length is considered. We calculate distributions of the transmission
coefficient T, Wigner delay time and, and the transport time,
. The central bodies of these distributions have a power-law
form, what can be understood in terms of the resonant tunneling through
localised states.Comment: 5 pages, 3 figures, submitted to PR
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