502 research outputs found
Emergent multipolar spin correlations in a fluctuating spiral - The frustrated ferromagnetic S=1/2 Heisenberg chain in a magnetic field
We present the phase diagram of the frustrated ferromagnetic S=1/2 Heisenberg
J_1-J_2 chain in a magnetic field, obtained by large scale exact
diagonalizations and density matrix renormalization group simulations. A vector
chirally ordered state, metamagnetic behavior and a sequence of spin-multipolar
Luttinger liquid phases up to hexadecupolar kind are found. We provide
numerical evidence for a locking mechanism, which can drive spiral states
towards spin-multipolar phases, such as quadrupolar or octupolar phases. Our
results also shed light on previously discovered spin-multipolar phases in
two-dimensional quantum magnets in a magnetic field.Comment: 4+ pages, 4 figure
Exact EGB models for spherical static perfect fluids
We obtain a new exact solution to the field equations in the EGB modified
theory of gravity for a 5-dimensional spherically symmetric static
distribution. By using a transformation, the study is reduced to the analysis
of a single second order nonlinear differential equation. In general the
condition of pressure isotropy produces a first order differential equation
which is an Abel equation of the second kind. An exact solution is found. The
solution is examined for physical admissability. In particular a set of
constants is found which ensures that a pressure-free hypersurface exists which
defines the boundary of the distribution. Additionally the isotropic pressure
and the energy density are shown to be positive within the radius of the
sphere. The adiabatic sound speed criterion is also satisfied within the fluid
ensuring a subluminal sound speed. Furthermore, the weak, strong and dominant
conditions hold throughout the distribution. On setting the Gauss-Bonnet
coupling to zero, an exact solution for 5-dimensional perfect fluids in the
standard Einstein theory is obtained. Plots of the dynamical quantities for the
Gauss-Bonnet and the Einstein case reveal that the pressure is unaffected while
the the energy density increases under the influence of the Gauss-Bonnet term.Comment: 18 pages, Submitted for publicatio
The vanishing ideal of a finite set of points with multiplicity structures
Given a finite set of arbitrarily distributed points in affine space with
arbitrary multiplicity structures, we present an algorithm to compute the
reduced Groebner basis of the vanishing ideal under the lexicographic ordering.
Our method discloses the essential geometric connection between the relative
position of the points with multiplicity structures and the quotient basis of
the vanishing ideal, so we will explicitly know the set of leading terms of
elements of I. We split the problem into several smaller ones which can be
solved by induction over variables and then use our new algorithm for
intersection of ideals to compute the result of the original problem. The new
algorithm for intersection of ideals is mainly based on the Extended Euclidean
Algorithm.Comment: 12 pages,12 figures,ASCM 201
Derandomization of auctions
We study the role of randomization in seller optimal (i.e., profit maximization) auctions. Bayesian optimal auctions (e.g., Myerson, 1981) assume that the valuations of the agents are random draws from a distribution and prior-free optimal auctions either are randomized (e.g., Goldberg et al., 2006) or assume the valuations are randomized (e.g., Segal, 2003). Is randomization fundamental to profit maximization in auctions? Our main result is a general approach to derandomize single-item multi-unit unit-demand auctions while approximately preserving their performance (i.e., revenue). Our general technique is constructive but not computationally tractable. We complement the general result with the explicit and computationally-simple derandomization of a particular auction. Our results are obtained through analogy to hat puzzles that are interesting in their own right
A New View on Worst-Case to Average-Case Reductions for NP Problems
We study the result by Bogdanov and Trevisan (FOCS, 2003), who show that
under reasonable assumptions, there is no non-adaptive worst-case to
average-case reduction that bases the average-case hardness of an NP-problem on
the worst-case complexity of an NP-complete problem. We replace the hiding and
the heavy samples protocol in [BT03] by employing the histogram verification
protocol of Haitner, Mahmoody and Xiao (CCC, 2010), which proves to be very
useful in this context. Once the histogram is verified, our hiding protocol is
directly public-coin, whereas the intuition behind the original protocol
inherently relies on private coins
Utilizing field collected insects for next generation sequencing: effects of sampling, storage and DNA extraction methods
DNA sequencing technologies continue to advance the biological sciences, expanding
opportunities for genomic studies of nonâmodel organisms for basic and applied
questions. Despite these opportunities, many next generation sequencing protocols
have been developed assuming a substantial quantity of high molecular weight DNA
(>100 ng), which can be difficult to obtain for many study systems. In particular, the
ability to sequence fieldâcollected specimens that exhibit varying levels of DNA degradation
remains largely unexplored. In this study we investigate the influence of
five traditional insect capture and curation methods on DoubleâDigest Restriction
Enzyme Associated DNA (ddRAD) sequencing success for three wild bee species.
We sequenced a total of 105 specimens (between 7â13 specimens per species and
treatment). We additionally investigated how different DNA quality metrics (including
preâsequence concentration and contamination) predicted downstream sequencing
success, and also compared two DNA extraction methods. We report successful
library preparation for all specimens, with all treatments and extraction methods producing
enough highly reliable loci for population genetic analyses. Although results
varied between species, we found that specimens collected by net sampling directly
into 100% EtOH, or by passive trapping followed by 100% EtOH storage before pinning
tended to produce higher quality ddRAD assemblies, likely as a result of rapid
specimen desiccation. Surprisingly, we found that specimens preserved in propylene
glycol during field sampling exhibited lowerâquality assemblies. We provide recommendations
for each treatment, extraction method, and DNA quality assessment,
and further encourage researchers to consider utilizing a wider variety of specimens
for genomic analysesinfo:eu-repo/semantics/publishedVersio
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Organizational learningâCOVID-19 experience: innovative global practices for improving patient care in crisis response
Across human history, civilizations have responded to disasters and outbreaks of disease with increasingly complex, systematic approaches as a means of organizing chaos and protecting human life. The SARS-CoV-2 coronavirus (COVID-19) pandemic provides a unique opportunity to learn from the practice of disaster management and crisis-driven changes to patient care processes in hospital and emergent care environments worldwide. COVID-19 acts as an accelerant for process change and the need for redesign in systems where classical, linear evaluation methods most often inform carefully implemented service improvements. Strikingly, many innovative approaches and valuable lessons come from all over the globe where technology and access to resources have been most limited. This article answers the question, what can we learn about how to respond to future disasters from the evolution of disaster management as performed by helping professionals and policymakers during the past hundred-plus years and best practices seen today? Macro practitioners have co-created unique approaches within several global communities to help cope with COVID-19 and other disasters despite limited resources and seemingly unlimited needs. Referencing existing case studies of patient care responses during COVID-19 in Italy, Nigeria, South Africa, South Korea, and the United States, the authors document innovative practices and use of diverse technologies in local patient care systems. The article concludes by suggesting best practices for designing more robust, adaptive, and crisis ready responses to patient care, as well as the use of developmental evaluation as an agile approach to evaluating and improving patient services. It also suggests roles that helping professionals can play in the translation of big data systems of disaster management from organizations such as the Center for Disease Control, World Health Organization, non-governmental organizations (NGOs), and selected think tanks, among others
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The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and MaxwellâJĂŒttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.Original Publication: Antoine Bret, Laurent Gremillet and Mark Eric Dieckmann, Multidimensional electron beam-plasma instabilities in the relativistic regime, 2010, Physics of Plasmas, (17), 12, 120501-1-120501-36. http://dx.doi.org/10.1063/1.3514586 Copyright: American Institute of Physics http://www.aip.org/</p
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