4,112 research outputs found
Asynchronous Variational Integrators
We describe a new class of asynchronous variational integrators (AVI) for nonlinear
elastodynamics. The AVIs are distinguished by the following attributes: (i)
The algorithms permit the selection of independent time steps in each element, and
the local time steps need not bear an integral relation to each other; (ii) the algorithms
derive from a spacetime form of a discrete version of Hamiltonâs variational
principle. As a consequence of this variational structure, the algorithms conserve
local momenta and a local discrete multisymplectic structure exactly.
To guide the development of the discretizations, a spacetime multisymplectic
formulation of elastodynamics is presented. The variational principle used incorporates
both configuration and spacetime reference variations. This allows a unified
treatment of all the conservation properties of the system.A discrete version of reference
configuration is also considered, providing a natural definition of a discrete
energy. The possibilities for discrete energy conservation are evaluated.
Numerical tests reveal that, even when local energy balance is not enforced
exactly, the global and local energy behavior of the AVIs is quite remarkable, a
property which can probably be traced to the symplectic nature of the algorith
Non-Gaussianity in the HILC foreground-reduced three-year WMAP CMB map
A detection or nondetection of primordial non-Gaussianity in the CMB data is
essential not only to test alternative models of the physics of the early
universe but also to discriminate among classes of inflationary models. Given
this far reaching consequences of such a non-Gaussianity detection for our
understanding of the physics of the early universe, it is important to employ
alternative indicators in order to have further information about the
Gaussianity features of CMB that may be helpful for identifying their origins.
In this way, a considerable effort has recently gone into the design of
non-Gaussianity indicators, and in their application in the search for
deviation from Gaussianity in the CMB data. Recently we have proposed two new
large-angle non-Gaussianity indicators which provide measures of the departure
from Gaussianity on large angular scales. We have used these indicators to
carry out analyses of Gaussianity of the single frequency bands and of the
available foreground-reduced {\it five-year} maps with and without the KQ75
mask. Here we extend and complement these studies by performing a new analysis
of deviation from Gaussianity of the {\it three-year} harmonic ILC (HILC)
foreground-reduced full-sky and KQ75 masked maps obtained from WMAP data. We
show that this full-sky foreground-reduced maps presents a significant
deviation from Gaussianity, which is brought down to a level of consistency
with Gaussianity when the KQ75 mask is employed.Comment: 6 pages, 1 figure. To appear in IJMPD (2010). V2: Corrected ref.[8]
A Study of Gaussianity in CMB band maps
The detection of non-Gaussianity in the CMB data would rule out a number of
inflationary models. A null detection of non-Gaussianity, instead, would
exclude alternative models for the early universe. Thus, a detection or
non-detection of primordial non-Gaussianity in the CMB data is crucial to
discriminate among inflationary models, and to test alternative scenarios.
However, there are various non-cosmological sources of non-Gaussianity. This
makes important to employ different indicators in order to detect distinct
forms of non-Gaussianity in CMB data. Recently, we proposed two new indicators
to measure deviation from Gaussianity on large angular scales, and used them to
study the Gaussianity of the raw band WMAP maps with and without the KQ75 mask.
Here we extend this work by using these indicators to perform similar analyses
of deviation from Gaussianity of the foreground-reduced Q, V, and W band maps.
We show that there is a significant deviation from Gaussianity in the
considered full-sky maps, which is reduced to a level consistent with
Gaussianity when the KQ75 mask is employed.Comment: 5 pages, 2 PS figures, uses ws-ijmpd.cls ; to be published in the
International Journal of Modern Physics
The Midpoint Rule as a Variational--Symplectic Integrator. I. Hamiltonian Systems
Numerical algorithms based on variational and symplectic integrators exhibit
special features that make them promising candidates for application to general
relativity and other constrained Hamiltonian systems. This paper lays part of
the foundation for such applications. The midpoint rule for Hamilton's
equations is examined from the perspectives of variational and symplectic
integrators. It is shown that the midpoint rule preserves the symplectic form,
conserves Noether charges, and exhibits excellent long--term energy behavior.
The energy behavior is explained by the result, shown here, that the midpoint
rule exactly conserves a phase space function that is close to the Hamiltonian.
The presentation includes several examples.Comment: 11 pages, 8 figures, REVTe
Electronic structure of silicon-based nanostructures
We have developed an unifying tight-binding Hamiltonian that can account for
the electronic properties of recently proposed Si-based nanostructures, namely,
Si graphene-like sheets and Si nanotubes. We considered the and
models up to first- and second-nearest neighbors, respectively. Our
results show that the Si graphene-like sheets considered here are metals or
zero-gap semiconductors, and that the corresponding Si nanotubes follow the
so-called Hamada's rule [Phys. Rev. Lett. {\bf 68}, 1579 1992]. Comparison to a
recent {\it ab initio} calculation is made.Comment: 12 pages, 6 Figure
Variational Integrators for Almost-Integrable Systems
We construct several variational integrators--integrators based on a discrete
variational principle--for systems with Lagrangians of the form L = L_A +
epsilon L_B, with epsilon << 1, where L_A describes an integrable system. These
integrators exploit that epsilon << 1 to increase their accuracy by
constructing discrete Lagrangians based on the assumption that the integrator
trajectory is close to that of the integrable system. Several of the
integrators we present are equivalent to well-known symplectic integrators for
the equivalent perturbed Hamiltonian systems, but their construction and error
analysis is significantly simpler in the variational framework. One novel
method we present, involving a weighted time-averaging of the perturbing terms,
removes all errors from the integration at O(epsilon). This last method is
implicit, and involves evaluating a potentially expensive time-integral, but
for some systems and some error tolerances it can significantly outperform
traditional simulation methods.Comment: 14 pages, 4 figures. Version 2: added informative example; as
accepted by Celestial Mechanics and Dynamical Astronom
An interface to retrieve personal memories using an iconic visual language
Relevant past events can be remembered when visualizing related pictures. The main difficulty is how to find these photos in a large personal collection. Query definition and image annotation are key issues to overcome this problem. The former is relevant due to the diversity of the clues provided by our memory when recovering a past moment and the later because images need to be annotated with information regarding those clues to be retrieved. Consequently, tools to recover past memories should deal carefully with these two tasks. This paper describes a user interface designed to explore pictures from personal memories. Users can query the media collection in several ways and for this reason an iconic visual language to define queries is proposed. Automatic and semi-automatic annotation is also performed using the image content and the audio information obtained when users show their images to others. The paper also presents the user interface evaluation based on tests with 58 participants
Incarceration, identity and resilience : understanding the long-term psychological impacts of racial trauma on Japanese Americans who were imprisoned during World War II
The purpose of this exploratory study was to deepen the understanding around the impacts of racial trauma and civil rights violations on Japanese Americansâ enduring sense of belonging and legitimacy in the United States. The study used semi-structured interviews with 13 Japanese Americans who were incarcerated during WWII to gather qualitative data around their experiences, in order to explore the long-term psychological impact of imprisonment and additionally, how the psychological effects are related to the current social environment. The major findings of this study are that formerly incarcerated Japanese Americans experience long term psychological consequences as a result of their imprisonment experiences and that these psychological effects shape their perception of modern day political and social contexts.
The major findings from the study fell into five major categories: 1) decreased feelings of safety 2) solidarity across racial lines 3) repetition of history 4) increased activism and community empowerment 5) present-day demagoguery and xenophobia. These findings contribute to the existing literature by expanding on the understanding of how Japanese Americansâ incarceration trauma response interacts with modern day social and political contexts. This study articulates that Japanese Americansâ incarceration trauma response is triggered by witnessing prejudice experienced by other minority groups in the United States and also by a xenophobic political and social climate. However, this study formulates that in addition to these negative impacts, Japanese Americansâ also experience positive outcomes, including increased empathy between oppressed groups
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