6,417 research outputs found
Combined Field Integral Equation Based Theory of Characteristic Mode
Conventional electric field integral equation based theory is susceptible to
the spurious internal resonance problem when the characteristic modes of closed
perfectly conducting objects are computed iteratively. In this paper, we
present a combined field integral equation based theory to remove the
difficulty of internal resonances in characteristic mode analysis. The electric
and magnetic field integral operators are shown to share a common set of
non-trivial characteristic pairs (values and modes), leading to a generalized
eigenvalue problem which is immune to the internal resonance corruption.
Numerical results are presented to validate the proposed formulation. This work
may offer efficient solutions to characteristic mode analysis which involves
electrically large closed surfaces
A decidable weakening of Compass Logic based on cone-shaped cardinal directions
We introduce a modal logic, called Cone Logic, whose formulas describe
properties of points in the plane and spatial relationships between them.
Points are labelled by proposition letters and spatial relations are induced by
the four cone-shaped cardinal directions. Cone Logic can be seen as a weakening
of Venema's Compass Logic. We prove that, unlike Compass Logic and other
projection-based spatial logics, its satisfiability problem is decidable
(precisely, PSPACE-complete). We also show that it is expressive enough to
capture meaningful interval temporal logics - in particular, the interval
temporal logic of Allen's relations "Begins", "During", and "Later", and their
transposes
Invisibility in non-Hermitian tight-binding lattices
Reflectionless defects in Hermitian tight-binding lattices, synthesized by
the intertwining operator technique of supersymmetric quantum mechanics, are
generally not invisible and time-of-flight measurements could reveal the
existence of the defects. Here it is shown that, in a certain class of
non-Hermitian tight-binding lattices with complex hopping amplitudes, defects
in the lattice can appear fully invisible to an outside observer. The
synthesized non-Hermitian lattices with invisible defects possess a real-valued
energy spectrum, however they lack of parity-time (PT) symmetry, which does not
play any role in the present work.Comment: to appear in Phys. Rev.
Emerging Linguistic Functions in Early Infancy
This paper presents results from experimental
studies on early language acquisition in infants and
attempts to interpret the experimental results within
the framework of the Ecological Theory of
Language Acquisition (ETLA) recently proposed
by (Lacerda et al., 2004a). From this perspective,
the infant’s first steps in the acquisition of the
ambient language are seen as a consequence of the
infant’s general capacity to represent sensory input
and the infant’s interaction with other actors in its
immediate ecological environment. On the basis of
available experimental evidence, it will be argued
that ETLA offers a productive alternative to
traditional descriptive views of the language
acquisition process by presenting an operative
model of how early linguistic function may emerge
through interaction
Reduction of the radar cross section of arbitrarily shaped cavity structures
The problem of the reduction of the radar cross section (RCS) of open-ended cavities was studied. The issues investigated were reduction through lossy coating materials on the inner cavity wall and reduction through shaping of the cavity. A method was presented to calculate the RCS of any arbitrarily shaped structure in order to study the shaping problem. The limitations of this method were also addressed. The modal attenuation was studied in a multilayered coated waveguide. It was shown that by employing two layers of coating, it was possible to achieve an increase in both the magnitude of attenuation and the frequency band of effectiveness. The numerical method used in finding the roots of the characteristic equation breaks down when the coating thickness is very lossy and large in terms of wavelength. A new method of computing the RCS of an arbitrary cavity was applied to study the effects of longitudinal bending on RCS reduction. The ray and modal descriptions for the fields in a parallel plate waveguide were compared. To extend the range of validity of the Shooting and Bouncing Ray (SBR) method, the simple ray picture must be modified to account for the beam blurring
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