12,114 research outputs found
Restricted Mobility Improves Delay-Throughput Trade-offs in Mobile Ad-Hoc Networks
In this paper we revisit two classes of mobility models which are widely used to repre-sent users ’ mobility in wireless networks: Random Waypoint (RWP) and Random Direction (RD). For both models we obtain systems of partial differential equations which describe the evolution of the users ’ distribution. For the RD model, we show how the equations can be solved analytically both in the stationary and transient regime adopting standard mathematical techniques. Our main contributions are i) simple expressions which relate the transient dura-tion to the model parameters; ii) the definition of a generalized random direction model whose stationary distribution of mobiles in the physical space corresponds to an assigned distribution
Adjusted empirical likelihood estimation of the youden index and associated threshold for the bigamma model
The Youden index is a widely used measure in the framework of medical diagnostic, where the effectiveness of a biomarker (screening marker or predictor) for classifying a disease status is studied. When the biomarker is continuous, it is important to determine the threshold or cut-off point to be used in practice for the discrimination between diseased and healthy populations. We introduce a new method based on adjusted empirical likelihood for quantiles aimed to estimate the Youden index and its associated threshold. We also include bootstrap based confidence intervals for both of them. In the simulation study, we compare this method with a recent approach based on the delta method under the bigamma scenario. Finally, a real example of prostatic cancer, well known in the literature, is analyzed to provide the reader with a better understanding of the new metho
First-principles study of structural, elastic, and bonding properties of pyrochlores
Density Functional Theory calculations have been performed to obtain lattice
parameters, elastic constants, and electronic properties of ideal pyrochlores
with the composition ABO (where A=La,Y and B=Ti,Sn,Hf, Zr). Some
thermal properties are also inferred from the elastic properties. A decrease of
the sound velocity (and thus, of the Debye temperature) with the atomic mass of
the B ion is observed. Static and dynamical atomic charges are obtained to
quantify the degree of covalency/ionicity. A large anomalous contribution to
the dynamical charge is observed for Hf, Zr, and specially for Ti. It is
attributed to the hybridization between occupied states of oxygen and
unoccupied d states of the B cation. The analysis based on Mulliken population
and deformation charge integrated in the Voronoi polyhedra indicates that the
ionicity of these pyrochlores increases in the order Sn--Ti--Hf--Zr. The charge
deformation contour plots support this assignment.Comment: Modified contact details, and acknowledgment
Convergence in Energy-Lowering (Disordered) Stochastic Spin Systems
We consider stochastic processes, S^t \equiv (S_x^t : x \in Z^d), with each
S_x^t taking values in some fixed finite set, in which spin flips (i.e.,
changes of S_x^t) do not raise the energy. We extend earlier results of
Nanda-Newman-Stein that each site x has almost surely only finitely many flips
that strictly lower the energy and thus that in models without zero-energy
flips there is convergence to an absorbing state. In particular, the assumption
of finite mean energy density can be eliminated by constructing a
percolation-theoretic Lyapunov function density as a substitute for the mean
energy density. Our results apply to random energy functions with a
translation-invariant distribution and to quite general (not necessarily
Markovian) dynamics.Comment: 11 page
Renormalization-group at criticality and complete analyticity of constrained models: a numerical study
We study the majority rule transformation applied to the Gibbs measure for
the 2--D Ising model at the critical point. The aim is to show that the
renormalized hamiltonian is well defined in the sense that the renormalized
measure is Gibbsian. We analyze the validity of Dobrushin-Shlosman Uniqueness
(DSU) finite-size condition for the "constrained models" corresponding to
different configurations of the "image" system. It is known that DSU implies,
in our 2--D case, complete analyticity from which, as it has been recently
shown by Haller and Kennedy, Gibbsianness follows. We introduce a Monte Carlo
algorithm to compute an upper bound to Vasserstein distance (appearing in DSU)
between finite volume Gibbs measures with different boundary conditions. We get
strong numerical evidence that indeed DSU condition is verified for a large
enough volume for all constrained models.Comment: 39 pages, teX file, 4 Postscript figures, 1 TeX figur
Stationary uphill currents in locally perturbed Zero Range Processes
Uphill currents are observed when mass diffuses in the direction of the
density gradient. We study this phenomenon in stationary conditions in the
framework of locally perturbed 1D Zero Range Processes (ZRP). We show that the
onset of currents flowing from the reservoir with smaller density to the one
with larger density can be caused by a local asymmetry in the hopping rates on
a single site at the center of the lattice. For fixed injection rates at the
boundaries, we prove that a suitable tuning of the asymmetry in the bulk may
induce uphill diffusion at arbitrarily large, finite volumes. We also deduce
heuristically the hydrodynamic behavior of the model and connect the local
asymmetry characterizing the ZRP dynamics to a matching condition relevant for
the macroscopic problem
Can cooperation slow down emergency evacuations?
We study the motion of pedestrians through obscure corridors where the lack
of visibility hides the precise position of the exits. Using a lattice model,
we explore the effects of cooperation on the overall exit flux (evacuation
rate). More precisely, we study the effect of the buddying threshold (of
no--exclusion per site) on the dynamics of the crowd. In some cases, we note
that if the evacuees tend to cooperate and act altruistically, then their
collective action tends to favor the occurrence of disasters.Comment: arXiv admin note: text overlap with arXiv:1203.485
Renormalization Group results for lattice surface models
We study the phase diagram of statistical systems of closed and open
interfaces built on a cubic lattice. Interacting closed interfaces can be
written as Ising models, while open surfaces as Z(2) gauge systems. When the
open surfaces reduce to closed interfaces with few defects, also the gauge
model can be written as an Ising spin model. We apply the lower bound
renormalization group (LBRG) transformation introduced by Kadanoff (Phys. Rev.
Lett. 34, 1005 (1975)) to study the Ising models describing closed and open
surfaces with few defects. In particular, we have studied the Ising-like
transition of self-avoiding surfaces between the random-isotropic phase and the
phase with broken global symmetry at varying values of the mean curvature. Our
results are compared with previous numerical work. The limits of the LBRG
transformation in describing regions of the phase diagram where not
ferromagnetic ground-states are relevant are also discussed.Comment: 24 pages, latex, 5 figures (available upon request to
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