2,413 research outputs found

### A Reverse Monte Carlo study of H+D Lyman alpha absorption from QSO spectra

A new method based on a Reverse Monte Carlo [RMC] technique and aimed at the
inverse problem in the analysis of interstellar (intergalactic) absorption
lines is presented. The line formation process in chaotic media with a finite
correlation length $(l > 0)$ of the stochastic velocity field (mesoturbulence)
is considered. This generalizes the standard assumption of completely
uncorrelated bulk motions $(l \equiv 0)$ in the microturbulent approximation
which is used for the data analysis up-to-now. It is shown that the RMC method
allows to estimate from an observed spectrum the proper physical parameters of
the absorbing gas and simultaneously an appropriate structure of the velocity
field parallel to the line-of-sight. The application to the analysis of the H+D
Ly$\alpha$ profile is demonstrated using Burles & Tytler [B&T] data for QSO
1009+2956 where the DI Ly$\alpha$ line is seen at $z_a = 2.504$. The results
obtained favor a low D/H ratio in this absorption system, although our upper
limit for the hydrogen isotopic ratio of about $4.5\times10^{-5}$ is slightly
higher than that of B&T (D/H = $3.0^{+0.6}_{-0.5} \times 10^{-5}$). We also
show that the D/H and N(HI) values are, in general, correlated, i.e. the
derived D-abundance may be badly dependent on the assumed hydrogen column
density. The corresponding confidence regions for an arbitrary and a fixed
stochastic velocity field distribution are calculated.Comment: 6 pages, LaTeX, 2 Postscript figures, to appear in "The Primordial
Nuclei and Their Galactic Evolution", eds. N. Prantzos, M. Tosi, R. von
Steiger (Kluwer: Dordrecht

### A Generalization of Metropolis and Heat-Bath Sampling for Monte Carlo Simulations

For a wide class of applications of the Monte Carlo method, we describe a
general sampling methodology that is guaranteed to converge to a specified
equilibrium distribution function. The method is distinct from that of
Metropolis in that it is sometimes possible to arrange for unconditional
acceptance of trial moves. It involves sampling states in a local region of
phase space with probability equal to, in the first approximation, the square
root of the desired global probability density function. The validity of this
choice is derived from the Chapman-Kolmogorov equation, and the utility of the
method is illustrated by a prototypical numerical experiment.Comment: RevTeX, 7 pages, 2 table

### Permanents of cyclic (0,1) matrices

AbstractAn efficient method is presented for evaluating the permanents Pnk of cyclic (0,1) matrices of dimension n and common row and column sum k. A general method is developed for finding recurrence rules for Pnk (k fixed); the recurrence rules are given in semiexplicit form for the range 4â‰¤kâ‰¤9. A table of Pnk is included for the range 4â‰¤kâ‰¤9, kâ‰¤nâ‰¤80. The Pnk are calculated in the formPnk=2+âˆ‘Ï„âˆ’1[kâˆ’12]TÏ„k(n)where the Ttk(n) satisfy recurrence rules given symbolically by the characteristic equations of certain (0, 1) matrices Î rk; the latter turn out to be identical with the r-th permanental compounds of certain simpler matrices Î 1k. Finally, formal expressions for Pnk are given which allow one to write down the solution to the generalized MÃ©nage Problem in terms of sums over scalar products of the iterates of a set of unit vectors

### On finite limit sets for transformations on the unit interval

AbstractAn infinite sequence of finite or denumerable limit sets is found for a class of many-to-one transformations of the unit interval into itself. Examples of four different types are studied in some detail; tables of numerical results are included. The limit sets are characterized by certain patterns; an algorithm for their generation is described and established. The structure and order of occurrence of these patterns is universal for the class

### On the Unicity of Smartphone Applications

Prior works have shown that the list of apps installed by a user reveal a lot
about user interests and behavior. These works rely on the semantics of the
installed apps and show that various user traits could be learnt automatically
using off-the-shelf machine-learning techniques. In this work, we focus on the
re-identifiability issue and thoroughly study the unicity of smartphone apps on
a dataset containing 54,893 Android users collected over a period of 7 months.
Our study finds that any 4 apps installed by a user are enough (more than 95%
times) for the re-identification of the user in our dataset. As the complete
list of installed apps is unique for 99% of the users in our dataset, it can be
easily used to track/profile the users by a service such as Twitter that has
access to the whole list of installed apps of users. As our analyzed dataset is
small as compared to the total population of Android users, we also study how
unicity would vary with larger datasets. This work emphasizes the need of
better privacy guards against collection, use and release of the list of
installed apps.Comment: 10 pages, 9 Figures, Appeared at ACM CCS Workshop on Privacy in
Electronic Society (WPES) 201

### Equations of state of elements based on the generalized Fermi-Thomas theory

The Fermi-Thomas model has been used to derive the equation of state of matter at high pressures and at various temperatures. Calculations have been carried out both without and with the exchange terms. Discussion of similarity transformations lead to the virial theorem and to correlation of solutions for different Z values

### Binary continuous random networks

Many properties of disordered materials can be understood by looking at
idealized structural models, in which the strain is as small as is possible in
the absence of long-range order. For covalent amorphous semiconductors and
glasses, such an idealized structural model, the continuous-random network, was
introduced 70 years ago by Zachariasen. In this model, each atom is placed in a
crystal-like local environment, with perfect coordination and chemical
ordering, yet longer-range order is nonexistent. Defects, such as missing or
added bonds, or chemical mismatches, however, are not accounted for. In this
paper we explore under which conditions the idealized CRN model without defects
captures the properties of the material, and under which conditions defects are
an inherent part of the idealized model. We find that the density of defects in
tetrahedral networks does not vary smoothly with variations in the interaction
strengths, but jumps from close-to-zero to a finite density. Consequently, in
certain materials, defects do not play a role except for being thermodynamical
excitations, whereas in others they are a fundamental ingredient of the ideal
structure.Comment: Article in honor of Mike Thorpe's 60th birthday (to appear in J.
Phys: Cond Matt.

### One Dimensional Nonequilibrium Kinetic Ising Models with Branching Annihilating Random Walk

Nonequilibrium kinetic Ising models evolving under the competing effect of
spin flips at zero temperature and nearest neighbour spin exchanges at
$T=\infty$ are investigated numerically from the point of view of a phase
transition. Branching annihilating random walk of the ferromagnetic domain
boundaries determines the steady state of the system for a range of parameters
of the model. Critical exponents obtained by simulation are found to agree,
within error, with those in Grassberger's cellular automata.Comment: 10 pages, Latex, figures upon request, SZFKI 05/9

### Monte Carlo approach of the islanding of polycrystalline thin films

We computed by a Monte Carlo method derived from the Solid on Solid model,
the evolution of a polycrystalline thin film deposited on a substrate during
thermal treatment. Two types of substrates have been studied: a single
crystalline substrate with no defects and a single crystalline substrate with
defects. We obtain islands which are either flat (i.e. with a height which does
not overcome a given value) or grow in height like narrow towers. A good
agreement was found regarding the morphology of numerical nanoislands at
equilibrium, deduced from our model, and experimental nanoislands resulting
from the fragmentation of YSZ thin films after thermal treatment.Comment: 20 pages, 7 figure

### Wang-Landau sampling for quantum systems: algorithms to overcome tunneling problems and calculate the free energy

We present a generalization of the classical Wang-Landau algorithm [Phys.
Rev. Lett. 86, 2050 (2001)] to quantum systems. The algorithm proceeds by
stochastically evaluating the coefficients of a high temperature series
expansion or a finite temperature perturbation expansion to arbitrary order.
Similar to their classical counterpart, the algorithms are efficient at thermal
and quantum phase transitions, greatly reducing the tunneling problem at first
order phase transitions, and allow the direct calculation of the free energy
and entropy.Comment: Added a plot showing the efficiency at first order phase transition

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