8,062 research outputs found
Resummed propagators in multi-component cosmic fluids with the eikonal approximation
We introduce the eikonal approximation to study the effect of the large-scale
motion of cosmic fluids on their small-scale evolution. This approach consists
in collecting the impact of the long-wavelength displacement field into a
single or finite number of random variables, whose statistical properties can
be computed from the initial conditions. For a single dark matter fluid, we
show that we can recover the nonlinear propagators of renormalized perturbation
theory. These are obtained with no need to assume that the displacement field
follows the linear theory. Then we extend the eikonal approximation to many
fluids. In particular, we study the case of two non-relativistic components and
we derive their resummed propagators in the presence of isodensity modes.
Unlike the adiabatic case, where only the phase of small-scale modes is
affected by the large-scale advection field, the isodensity modes change also
the amplitude on small scales. We explicitly solve the case of cold dark
matter-baryon mixing and find that the isodensity modes induce only very small
corrections to the resummed propagators.Comment: 18 pages, 9 figures, matches published version as PR
Opinion dynamics and decision of vote in bipolar political systems
A model of the opinion dynamics underlying the political decision is
proposed. The analysis is restricted to a bipolar scheme with a possible third
political area. The interaction among voters is local but the final decision
strongly depends on global effects such as, for example, the rating of the
governments. As in the realistic case, the individual decision making process
is determined by the most relevant personal interests and problems. The
phenomenological analysis of the national vote in Italy and Germany has been
carried out and a prediction of the next Italian vote as a function of the
government rating is presented.Comment: 8 pages, 1 figure. To be published in International Journal of Modern
Physics
Comparison of parametric, orthogonal, and spline functions to model individual lactation curves for milk yield in Canadian Holsteins
Test day records for milk yield of 57,390 first lactation Canadian Holsteins were analyzed with a linear model that included the fixed effects of herd-test date and days in milk (DIM) interval nested within age and calving season.
Residuals from this model were analyzed as a new variable and fitted with a five parameter model, fourth-order Legendre polynomials, with linear, quadratic and cubic spline models with three knots. The fit of the models was
rather poor, with about 30%-40% of the curves showing an adjusted R-square lower than 0.20 across all models. Results underline a great difficulty in modelling individual deviations around the mean curve for milk yield. However, the Ali and Schaeffer (5 parameter) model and
the fourth-order Legendre polynomials were able to detect two basic shapes of individual deviations among the mean curve. Quadratic and, especially, cubic spline functions had better fitting performances but a poor predictive
ability due to their great flexibility that results
in an abrupt change of the estimated curve when data are missing. Parametric and orthogonal polynomials seem to be robust and affordable under this standpoint
The hard X-ray tails in neutron star low mass X-ray binaries: BeppoSAX observations and possible theoretical explanation of the GX 17+2 case
We report results of a new spectral analysis of two BeppoSAX observations of
the Z source GX 17+2. In one of the two observations the source exhibits a
powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work
by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL
of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have
shown that bulk (dynamical) Comptonization of soft photons can be a possible
alternative mechanism for producing hard X-ray tails in such systems. We start
from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX
to better investigate the physical processes at work. We use GX 17+2 as a
representative case. Moreover, we suggest that weakening (or disappearance) of
the hard X-ray tail can be explained by increasing radiation pressure
originated at the surface of the neutron star (NS). As a result the high
radiation pressure stops the bulk inflow and consequently this radiation
feedback of the NS surface leads to quenching the bulk Comptonization.Comment: 6 pages, 3 figures, Accepted for publication in Ap
An allometry-based approach for understanding forest structure, predicting tree-size distribution and assessing the degree of disturbance
Tree-size distribution is one of the most investigated subjects in plant
population biology. The forestry literature reports that tree-size distribution
trajectories vary across different stands and/or species, while the metabolic
scaling theory suggests that the tree number scales universally as -2 power of
diameter. Here, we propose a simple functional scaling model in which these two
opposing results are reconciled. Basic principles related to crown shape,
energy optimization and the finite size scaling approach were used to define a
set of relationships based on a single parameter, which allows us to predict
the slope of the tree-size distributions in a steady state condition. We tested
the model predictions on four temperate mountain forests. Plots (4 ha each,
fully mapped) were selected with different degrees of human disturbance
(semi-natural stands vs. formerly managed). Results showed that the size
distribution range successfully fitted by the model is related to the degree of
forest disturbance: in semi-natural forests the range is wide, while in
formerly managed forests, the agreement with the model is confined to a very
restricted range. We argue that simple allometric relationships, at individual
level, shape the structure of the whole forest community.Comment: 22 pages, 4 figure
A new Comptonization model for low-magnetized accreting neutron stars in low mass X-ray binaries
We developed a new model for the X-ray spectral fitting \xspec package which
takes into account the effects of both thermal and dynamical (i.e. bulk)
Comptonization. The model consists of two components: one is the direct
blackbody-like emission due to seed photons which are not subjected to
effective Compton scattering, while the other one is a convolution of the
Green's function of the energy operator with a blackbody-like seed photon
spectrum. When combined thermal and bulk effects are considered, the analytic
form of the Green's function may be obtained as a solution of the diffusion
Comptonization equation. Using data from the BeppoSAX, INTEGRAL and RXTE
satellites, we test our model on the spectra of a sample of six persistently
low magnetic field bright neutron star Low Mass X-ray Binaries, covering three
different spectral states. Particular attention is given to the transient
powerlaw-like hard X-ray (> 30 keV) tails that we interpret in the framework of
the bulk motion Comptonization process. We show that the values of the best-fit
delta-parameter, which represents the importance of bulk with respect to
thermal Comptonization, can be physically meaningful and can at least
qualitatively describe the physical conditions of the environment in the
innermost part of the system. Moreover, we show that in fitting the thermal
Comptonization spectra to the X-ray spectra of these systems, the best-fit
parameters of our model are in excellent agreement with those of COMPTT, a
broadly used and well established XSPEC model.Comment: 15 pages, 8 figures, accepted for publication in Ap
Combinatorial approach to Modularity
Communities are clusters of nodes with a higher than average density of
internal connections. Their detection is of great relevance to better
understand the structure and hierarchies present in a network. Modularity has
become a standard tool in the area of community detection, providing at the
same time a way to evaluate partitions and, by maximizing it, a method to find
communities. In this work, we study the modularity from a combinatorial point
of view. Our analysis (as the modularity definition) relies on the use of the
configurational model, a technique that given a graph produces a series of
randomized copies keeping the degree sequence invariant. We develop an approach
that enumerates the null model partitions and can be used to calculate the
probability distribution function of the modularity. Our theory allows for a
deep inquiry of several interesting features characterizing modularity such as
its resolution limit and the statistics of the partitions that maximize it.
Additionally, the study of the probability of extremes of the modularity in the
random graph partitions opens the way for a definition of the statistical
significance of network partitions.Comment: 8 pages, 4 figure
Total thyroidectomy associated to chemotherapy in primary squamous cell carcinoma of the thyroid
Primary squamous cell carcinoma of the thyroid (PSCCT) is a rare malignant disease with rapid fatal prognosis. The onset is generally characterized by sudden bilateral latero-cervical lymphadenopathy. The Authors report patient of 58-year-old who referred for evaluation of rapidly aggravating bilateral latero-cervical lymphadenopathy. The US highlighted the presence of a hypoechoic nodular lesion characterized by peri and intra-nodular vascularization. Multilayer CT showed diffused involvement of mediastinal and bilateral latero-cervical lymph nodes, with no evidence of primary pulmonary neoplasia or elsewhere. The patient underwent total thyroidectomy. The peri-isthmic tissue was removed due to the presence of a small roundish formation, that was due to lymph node metastasis at histological examination. Histological diagnosis: PSCCT. The immunohistochemical panel of the thyroid lesion was indispensable for the differential diagnosis between PSCCT, medullary carcinoma, anaplastic carcinoma, and thyroid metastasis of neoplasia with unknown primitiveness. The patient underwent chemotherapeutic treatment with Carboplatin and Paclitaxel with modest improvement of dysphagia symptoms and reduction of 10-15% of the target lesions. The clinical course was characterized by loco-regional progression of the disease with exitus in 10 months after diagnosis. Survival and quality of life after surgical therapy and chemotherapy were like that of patients undergoing only chemotherapy. Due to the extreme rarity of the neoplasia, 60 cases described in Literature, no exclusive guidelines are reported for PSCCT. More extensive case studies are needed to evaluate the effects of total thyroidectomy with intent R0/R1 on improving survival and quality of life of patients with PSCCT
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