2,356 research outputs found
Pair Partitioning in time reversal acoustics
Time reversal of acoustic waves can be achieved efficiently by the persistent
control of excitations in a finite region of the system. The procedure, called
Time Reversal Mirror, is stable against the inhomogeneities of the medium and
it has numerous applications in medical physics, oceanography and
communications. As a first step in the study of this robustness, we apply the
Perfect Inverse Filter procedure that accounts for the memory effects of the
system. In the numerical evaluation of such procedures we developed the Pair
Partitioning method for a system of coupled oscillators. The algorithm,
inspired in the Trotter strategy for quantum dynamics, obtains the dynamic for
a chain of coupled harmonic oscillators by the separation of the system in
pairs and applying a stroboscopic sequence that alternates the evolution of
each pair. We analyze here the formal basis of the method and discuss his
extension for including energy dissipation inside the medium.Comment: 6 pages, 4 figure
Descubriendo Patrones Craneofaciales Usando Datos Cefalométricos Multivariados para la Toma de Decisiones en Ortodoncia
Indexación: Web of Science; Scielo.The aim was to find craniofacial morphology patterns in a multivariate cephalometric database using a clustering technique. Cephalometric analysis was performed in a sample of 100 teleradiographs collected from Chilean orthodontic patients. Thirty cephalometric measurements were taken from commonly used analysis. The computed variables were used to perform a clustering analysis with the k-means algorithm to identify patterns of craniofacial morphology. The J48 decision tree was used to analyze each cluster, and the ANOVA test to determine the statistical differences between the clusters. Four clusters were found that had significant differences (P<0.001) in 24 of the 30 variables studied, suggesting that they represent different patterns of craniofacial form. Using the decision tree, 8 of the 30 variables appeared to be relevant for describing the clusters. The clustering analysis is effective in identifying different craniofacial patterns based on a multivariate database. The distinct clusters appear to be caused by differences in the compensation process of the facial structure responding to a genetically determined cranial and mandible form. The proposed method can be applied to several databases, creating specific classifications for each one of them.
KEY WORDS: Craniofacial patterns; Morphological patterns; Clustering technique; Orthodontics.RESUMEN: El objetivo fue encontrar patrones morfológicos craneofaciales, a partir de una base de datos cefalométricos multivariada, utilizando una técnica de clustering. Se realizó un análisis cefalométrico a una muestra de 100 telerradiografÃas pertenecientes a pacientes chilenos de ortodoncia. Treinta medidas cefalométricas obtenidas de los análisis más utilizados fueron registradas. Las variables computadas se utilizaron para realizar un análisis de clustering con el algoritmo k-medias, para identificar patrones de morfologÃa craneofacial. El árbol de decisión J48 se utilizó para analizar cada cluster, y test de ANOVA para determinar diferencias estadÃsticamente significativas entre los clusters. Se encontraron cuatro clusters con diferencia estadÃsticamente significativas (p<0,001) en 24 de las 30 variables estudiadas, lo que sugiere que efectivamente corresponden a diferentes patrones craneofaciales. Utilizando el árbol de decisión, se pudo determinar que 8 de las 30 variables resultaron ser relevantes en la definición de los clusters. El análisis de clustering es efectivo en identificar patrones morfológicos craneofaciales usando una base de datos multivariada. Los distintos cluster encontrados, aparentemente se formarÃan a partir de diferencias en el proceso de compensación de la estructura facial, en respuesta a la forma mandibular genéticamente determinada. El método propuesto puede ser aplicado a múltiples bases de datos, creando clasificaciones especÃficas para cada una de ellas.
PALABRAS CLAVE: Patrones craneofaciales; Patrones morfológicos; Técnica de clustering; Ortodoncia.http://ref.scielo.org/qdkkz
Semiclassical Theory of Time-Reversal Focusing
Time reversal mirrors have been successfully implemented for various kinds of
waves propagating in complex media. In particular, acoustic waves in chaotic
cavities exhibit a refocalization that is extremely robust against external
perturbations or the partial use of the available information. We develop a
semiclassical approach in order to quantitatively describe the refocusing
signal resulting from an initially localized wave-packet. The time-dependent
reconstructed signal grows linearly with the temporal window of injection, in
agreement with the acoustic experiments, and reaches the same spatial extension
of the original wave-packet. We explain the crucial role played by the chaotic
dynamics for the reconstruction of the signal and its stability against
external perturbations.Comment: 4 pages, 1 figur
Inflation Targeting: An international comparison of policy determinacy
This paper explores inflation targeting policy determinacy across sixteen economies explicitly following such rule. I find that these countries do not necessarily manage their monetary policy in a way that leads to determinacy, which is desirable from a welfare perspective. Data coming from developing economies tend to be less consistent with determinacy immediately after inflation targeting implementation, only increasing its "activeness" later in the period under scrutiny. Contrarily, developed economies tend to show a strong tendency towards determinacy in the first stage of inflation targeting implementation while showing an indeterminate solution after the target is achieved. I suggest a link between economic development and monetary policy effectiveness, which in turn is related with credibility
Tuning laser-induced bandgaps in graphene
Could a laser field lead to the much sought-after tunable bandgaps in
graphene? By using Floquet theory combined with Green's functions techniques,
we predict that a laser field in the mid-infrared range can produce observable
bandgaps in the electronic structure of graphene. Furthermore, we show how they
can be tuned by using the laser polarization. Our results could serve as a
guidance to design opto-electronic nano-devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
Statistical Signs of Social Influence on Suicides
Certain currents in sociology consider society as being composed of
autonomous individuals with independent psychologies. Others, however, deem our
actions as strongly influenced by the accepted standards of social behavior.
The later view was central to the positivist conception of society when in 1887
\'Emile Durkheim published his monograph Suicide (Durkheim, 1897). By treating
the suicide as a social fact, Durkheim envisaged that suicide rates should be
determined by the connections (or the lack of them) between people and society.
Under the same framework, Durkheim considered that crime is bound up with the
fundamental conditions of all social life and serves a social function. In this
sense, and regardless of its extremely deviant nature, crime events are somehow
capable to release certain social tensions and so have a purging effect in
society. The social effect on the occurrence of homicides has been previously
substantiated (Bettencourt et al., 2007; Alves et al., 2013), and confirmed
here, in terms of a superlinear scaling relation: by doubling the population of
a Brazilian city results in an average increment of 135 % in the number of
homicides, rather than the expected isometric increase of 100 %, as found, for
example, for the mortality due to car crashes. Here we present statistical
signs of the social influence on the suicide occurrence in cities. Differently
from homicides (superlinear) and fatal events in car crashes (isometric), we
find sublinear scaling behavior between the number of suicides and city
population, with allometric power-law exponents, and
, for all cities in Brazil and US, respectively. The fact that
the frequency of suicides is disproportionately small for larger cities reveals
a surprisingly beneficial aspect of living and interacting in larger and more
complex social networks.Comment: 7 pages, 4 figure
Reproducibility of a Titanium Plasma Vacuum Spark Discharge
The results of an extensive operation of a Vacuum Spark plasma using Titanium
electrodes in a 120 ns 150 kA discharge are presented. The hot spots are found
to form with a regular spacing in a zippering Z-pinch plasma, which forms close
to the cathode and extends to approximately two thirds of the anode separation
over a period of a few ns. The axis of the discharge is well defined by an
initial plasma from a Nd:YAG laser focussed onto the cathode electrode surface.
The statistics of the formation of the hot spots are given for the life of one
anode electrode. Between one and three hotspots form and the favored positions
are at 1.5 and 3.0 mm from the cathode and the strongest emission, as observed
in a filtered X-ray pinhole camera, comes from the hot spot closest to the
cathode. The emission spectra resolved between 50 and 350 \AA shows a wide
range of Ti ionization which allows the temperatures of the anode blow off
plasma, the Z-pinch and the hot spot plasma to be distinguished. These results
are compared with filtered PIN diode signals and filtered pinhole images.Comment: 5 pages, 4 figure
Highly stable microtubular cells for portable solid oxide fuel cell applications
In this work, extruded support tubes based on Nickel Oxide-YSZ (yttria stabilized-zirconia) were manufactured by Powder Extrusion Moulding (PEM). An YSZ layer is then deposited by dip coating as the electrolyte and subsequently, standard La0.8Sr0.2MnO3-δ (LSM)/YSZ composites were deposited by dip coating as oxygen electrodes. Microstructure of the anode support was optimized in order to achieve the maximum fuel utilization and as a consequence, a high performance of the cells. Experiments as a function of the fuel composition showed power densities above 500 mWcm−2 at 800 °C at 0.7 V, with high fuel utilization (∼75%). Long-term durability studies were also performed for a period above 1000 hours. The experiment was conducted at 800 °C using pure humidified hydrogen at a fixed voltage of 0.8 V. It was observed that the current density of the cell is significantly evolving during the initial period of about 100 hours, as a consequence of reconditioning of nickel particles at the anode support. Once the system is stabilized, no degradation was observed up to 1000 hours under operating conditions, obtaining current densities in the range of 400 mAcm−2 at 0.8 V and 800 °C.The project MAT2015-68078-R, financed by the Spanish Government (Ministerio de EconomÃa y Competitividad) and the Feder program of the European Union, is also acknowledged.Peer Reviewe
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