104 research outputs found
Modeling Spatial Dependence with Cauchy Convolution Processes
We study the class of dependence models for spatial data obtained from Cauchy
convolution processes based on different types of kernel functions. We show
that the resulting spatial processes have appealing tail dependence properties,
such as tail dependence at short distances and independence at long distances
with suitable kernel functions. We derive the extreme-value limits of these
processes, study their smoothness properties, and detail some interesting
special cases. To get higher flexibility at sub-asymptotic levels and
separately control the bulk and the tail dependence properties, we further
propose spatial models constructed by mixing a Cauchy convolution process with
a Gaussian process. We demonstrate that this framework indeed provides a rich
class of models for the joint modeling of the bulk and the tail behaviors. Our
proposed inference approach relies on matching model-based and empirical
summary statistics, and an extensive simulation study shows that it yields
accurate estimates. We demonstrate our new methodology by application to a
temperature dataset measured at 97 monitoring stations in the state of
Oklahoma, US. Our results indicate that our proposed model provides a very good
fit to the data, and that it captures both the bulk and the tail dependence
structures accurately.Comment: 36 pages, 7 figure
Magnetic structures of -O resulting from competition of interplane exchange interactions
Solid oxygen is a unique molecular crystal whose phase diagram is mostly
imposed by magnetic ordering, i.e., each crystal phase has a specific magnetic
structure. However, recent experiments showed that high-pressure -phase
is implemented in different magnetic structures. In the present paper we study
the role of interplane exchange interactions in formation of the magnetic
structures with different stacking sequences of the close-packed planes. We
show that temperature-induced variation of intermolecular distances can give
rise to compensation of the exchange coupling between the nearest close-packed
planes and result in the phase transition between different magnetic structures
within -O. Variation of the magnetic ordering is, in turn,
accompanied by the step-wise variation of interplane distance governed by space
and angular dependence of interplane exchange constants.Comment: 16 pages, 6 figure
On factor copula-based mixed regression models
In this article, a copula-based method for mixed regression models is
proposed, where the conditional distribution of the response variable, given
covariates, is modelled by a parametric family of continuous or discrete
distributions, and the effect of a common latent variable pertaining to a
cluster is modelled with a factor copula. We show how to estimate the
parameters of the copula and the parameters of the margins, and we find the
asymptotic behaviour of the estimation errors. Numerical experiments are
performed to assess the precision of the estimators for finite samples. An
example of an application is given using COVID-19 vaccination hesitancy from
several countries. Computations are based on R package CopulaGAMM
Saturn's Icy Moon Rhea: a Prediction for Bulk Chemical Composition and Physical Structure at the Time of the Cassini Spacecraft First Flyby
I report a model for the formation of Saturn's family of mid-sized icy moons
to coincide with the first flypast of Rhea by the Cassini Orbiter spacecraft on
26 November 2005. It is proposed that these moons had condensed from a
concentric family of orbiting gas rings that were cast off some 4600 Myr ago by
the contracting proto-Saturnian cloud. Numerical and structural models for Rhea
are constructed on the basis of a computed bulk chemical mix of hydrated rock
(mass fraction 0.385), H2O ice (0.395), and NH3 ice (0.220). The large
proportion of NH3 in the ice mass inhibits the formation of the dense
crystalline phase II of H2O ice at the satellite's centre. This may explain the
absence of compressional features on the surface. The favoured model of Rhea
has a chemically uniform interior and is very cold. The satellite is nearly
isodense and the predicted value of the axial moment-of-inertia factor is
C/MR^2 = 0.399 +/- 0.004. NH3 is unstable at Saturn's distance from the Sun,
except near the polar regions of the satellite. Perhaps the Cassini Orbiter
will discover indirect evidence for NH3 through the sublimative escape of this
ice from the outer layers, especially near the equatorial zones. Wasting of NH3
would weaken the residual soil, so making the edges of craters soft and prone
to landslides. It will be exciting to learn what Cassini discovers.Comment: This paper was submitted to the Publications of the Astronomical
Society of Australia (PASA) on 30 November 200
Magnetoelastic nature of solid oxygen epsilon-phase structure
For a long time a crystal structure of high-pressure epsilon-phase of solid
oxygen was a mistery. Basing on the results of recent experiments that have
solved this riddle we show that the magnetic and crystal structure of
epsilon-phase can be explained by strong exchange interactions of
antiferromagnetic nature. The singlet state implemented on quaters of O2
molecules has the minimal exchange energy if compared to other possible singlet
states (dimers, trimers). Magnetoelastic forces that arise from the spatial
dependence of the exchange integral give rise to transformation of 4(O2)
rhombuses into the almost regular quadrates. Antiferromagnetic character of the
exchange interactions stabilizes distortion of crystal lattice in epsilon-phase
and impedes such a distortion in long-range alpha- and delta-phases.Comment: 11 pages, 4 figures, Changes: corrected typos, reference to the
recent paper is adde
Effects of spin-elastic interactions in frustrated Heisenberg antiferromagnets
The Heisenberg antiferromagnet on a compressible triangular lattice in the
spin- wave approximation is considered. It is shown that the interaction
between quantum fluctuations and elastic degrees of freedom stabilizes the low
symmetric L-phase with a collinear Neel magnetic ordering. Multi-stability in
the dependence of the on-site magnetization on an unaxial pressure is found.Comment: Revtex, 4 pages, 2 eps figure
- …