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A Panel Test of Purchasing Power Parity Under the Null of Stationarity
Purchasing Power Parity (PPP) is tested using a sample of real exchange rate data for
twelve European countries. Acknowledging that Augmented Dickey Fuller tests have
low power, we apply a Panel test that considers the null of stationarity and corrects for
serial dependence using a non-parametric kernel based method
Ontological aspects of the Casimir Effect
The role of the vacuum, in the Casimir Effect, is a matter of some dispute:
the Casimir force has been variously described as a phenomenon resulting "from
the alteration, by the boundaries, of the zero-point electromagnetic energy",
or a "Van der Waals force between the metal plates" that can be "computed
without reference to zero point energies". Neither of these descriptions are
grounded in a consistently quantum mechanical treatment of matter interacting
with the electromagnetic field. However, the Casimir Effect has been
canonically described within the framework of macroscopic quantum
electrodynamics. On this account, the force is seen to arise due to the
coupling of fluctuating currents to the zero-point radiation, and it is in this
restricted sense that the phenomenon requires the existence of zero-point
fields. The conflicting descriptions of the Casimir Effect, on the other hand,
appear to arise from inadequate ontologies in which an unwarranted metaphysical
priority is assigned either to the matter or the fields. Such ontological
errors may have a direct bearing on the problem of the cosmological constant
and the correct prediction of the Casimir force in a state of thermal
equilibrium.Comment: 6 page
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The specification of cross exchange rate equations used to test Purchasing Power Parity
The Article considers the speciÞcation of models used to test Pur-
chasing Power Parity when applied to cross exchange rates. SpeciÞcally,
conventional dynamic models used to test stationarity of the real exchange
rate are likely to be misspeciÞed, except when the parameters of each ex-
change rate equation are the sam
The paradox of the Casimir force in inhomogeneous transformation media
It has recently been argued that Casimir-Lifshitz forces depend in detail on
the microphysics of a system; calculations of the Casimir force in
inhomogeneous media yield results that are cutoff-dependent. This result has
been shown to hold generally. But suppose we introduce an inhomogeneous
metamaterial into a cavity that effectively implements a simple distortion of
the coordinate system. Considered in its 'virtual space', the optical
properties of such a material are homogeneous and consequently free from the
cutoff-dependency associated with inhomogeneous media. This conclusion should
be reconciled with recent advances in our understanding of Casimir-Lifshitz
forces. We consider an example of such a system here and demonstrate that,
whilst the size of the Casimir force is modified by the inhomogeneous medium,
the force is cutoff-independent and can be stated exactly. The apparent paradox
dissolves when we recognise that an idealised metamaterial that could implement
a virtual geometry for all frequencies would be devoid of internal scattering,
and would not give rise to a cutoff-dependency in the Casimir force for that
reason.Comment: 7 page
Magmatic focusing to mid-ocean ridges: the role of grain size variability and non-Newtonian viscosity
Melting beneath mid-ocean ridges occurs over a region that is much broader
than the zone of magmatic emplacement to form the oceanic crust. Magma is
focused into this zone by lateral transport. This focusing has typically been
explained by dynamic pressure gradients associated with corner flow, or by a
sub-lithospheric channel sloping upward toward the ridge axis. Here we discuss
a novel mechanism for magmatic focusing: lateral transport driven by gradients
in compaction pressure within the asthenosphere. These gradients arise from the
co-variation of melting rate and compaction viscosity. The compaction
viscosity, in previous models, was given as a function of melt fraction and
temperature. In contrast, we show that the viscosity variations relevant to
melt focusing arise from grain-size variability and non-Newtonian creep. The
asthenospheric distribution of melt fraction predicted by our models provides
an improved ex- planation of the electrical resistivity structure beneath one
location on the East Pacific Rise. More generally, although grain size and
non-Newtonian viscosity are properties of the solid phase, we find that in the
context of mid-ocean ridges, their effect on melt transport is more profound
than their effect on the mantle corner-flow.Comment: 20 pages, 4 figures, 1 tabl
Correcting mean-field approximations for spatially-dependent advection-diffusion-reaction processes
On the microscale, migration, proliferation and death are crucial in the development, homeostasis and repair of an organism; on the macroscale, such effects are important in the sustainability of a population in its environment. Dependent on the relative rates of migration, proliferation and death, spatial heterogeneity may arise within an initially uniform field; this leads to the formation of spatial correlations and can have a negative impact upon population growth. Usually, such effects are neglected in modeling studies and simple phenomenological descriptions, such as the logistic model, are used to model population growth. In this work we outline some methods for analyzing exclusion processes which include agent proliferation, death and motility in two and three spatial dimensions with spatially homogeneous initial conditions. The mean-field description for these types of processes is of logistic form; we show that, under certain parameter conditions, such systems may display large deviations from the mean field, and suggest computationally tractable methods to correct the logistic-type description
Space environmental work simulator Patent
Space environmental work simulator with portions of space suit mounted to vacuum chamber wal
Models of collective cell motion for cell populations with different aspect ratio: diffusion, proliferation & travelling waves
Continuum, partial differential equation models are often used to describe the collective motion of cell populations, with various types of motility represented by the choice of diffusion coefficient, and cell proliferation captured by the source terms. Previously, the choice of diffusion coefficient has been largely arbitrary, with the decision to choose a particular linear or nonlinear form generally based on calibration arguments rather than making any physical connection with the underlying individual-level properties of the cell motility mechanism. In this work we provide a new link between individual-level models, which account for important cell properties such as varying cell shape and volume exclusion, and population-level partial differential equation models. We work in an exclusion process framework, considering aligned, elongated cells that may occupy more than one lattice site, in order to represent populations of agents with different sizes. Three different idealisations of the individual-level mechanism are proposed, and these are connected to three different partial differential equations, each with a different diffusion coefficient; one linear, one nonlinear and degenerate and one nonlinear and nondegenerate. We test the ability of these three models to predict the population-level response of a cell spreading problem for both proliferative and nonproliferative cases. We also explore the potential of our models to predict long time travelling wave invasion rates and extend our results to two-dimensional spreading and invasion. Our results show that each model can accurately predict density data for nonproliferative systems, but that only one does so for proliferative systems. Hence great care must be taken to predict density data with varying cell shape
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