802 research outputs found
Stellar Equilibrium in 2+1 Dimensions
The hydrostatic equilibrium of a dimensional perfect fluid star in
asymptotically anti-de Sitter space is discussed. The interior geometry matches
the exterior black-hole solution. An upper mass limit is found, analogous
to Buchdahl's theorem in 3+1, and the possibility of collapse is discussed. The
case of a uniform matter density is solved exactly and a new interior solution
is presented.Comment: 11 pages, revtex, no figure
Cables and fire hazards
Besides describing the experiments conducted to develop a nonflammable cable, this article discusses several considerations regarding other hazards which might result from cable fires, particularly the toxicity and opacity of the fumes emitted by the burning cable. In addition, this article examines the effects of using the Oxygen Index as a gauge of quality control during manufacture
Couplings between Chern-Simons gravities and 2p-branes
The interaction between Chern-Simons (CS) theories and localized external
sources (2p-branes) is analyzed. This interaction generalizes the minimal
coupling between a point charge (0-brane) and a gauge connection. The external
currents that define the 2p-branes are covariantly constant (D-2p-1)-forms
coupled to (2p-1) CS forms. The general expression for the sources --charged
with respect to the corresponding gauge algebra-- is presented, focusing on two
special cases: 0-branes and (D-3)-branes.
In any dimension, 0-branes are constructed as topological defects produced by
a surface deficit of (D-2)-sphere in AdS space, and they are not constant
curvature spaces for D>3. They correspond to dimensionally continued black
holes with negative mass.
On the other hand, in the case of CS (super) gravities, the (D-3)-branes are
naked conical singularities (topological defects) obtained by identification of
points with a Killing vector. In 2+1 dimensions, extremal spinning branes of
this type are BPS states. Stable (D-3)-branes are shown to exist also in higher
dimensions, as well.
Classical field equations are also discussed and in the presence of sources
there is a large number of inequivalent and disconnected sectors in solution
space.Comment: 29 pages, no figures; version accepted in PRD; extended introduction
and several references added; some sections have been reorganized and several
minor corrections mad
Bootstrap inference in the presence of bias
We consider bootstrap inference for estimators which are (asymptotically)
biased. We show that, even when the bias term cannot be consistently estimated,
valid inference can be obtained by proper implementations of the bootstrap.
Specifically, we show that the prepivoting approach of Beran (1987, 1988),
originally proposed to deliver higher-order refinements, restores bootstrap
validity by transforming the original bootstrap p-value into an asymptotically
uniform random variable. We propose two different implementations of
prepivoting (plug-in and double bootstrap), and provide general high-level
conditions that imply validity of bootstrap inference. To illustrate the
practical relevance and implementation of our results, we discuss five
applications: (i) a simple location model for i.i.d. data, possibly with
infinite variance; (ii) regression models with omitted controls; (iii)
inference on a target parameter based on model averaging; (iv) ridge-type
regularized estimators; and (v) dynamic panel data models
Analysis of Deterioration in a Plasma Focus Device
Indexación: Scopus.The Plasma Focus (PF) is a kind of dense transient plasmas in with high-pulsed voltage. To produce devices for eld application it is necessary to obtain PF equipment able to operate for a long period of time. Thus, a reliability analysis is indispensable. In this work a reliability analysis program for plasma focus devices is presented. The program considers a criticality analysis using Failure Modes and Effects Criticality Analysis (FMECA) to identify the most important failure modes of the system. Said failure modes are studied operating the Plasma Focus for many cycles, obtaining from them the characteristic curves of V(t) and İ(t). Feature Extraction (FE) techniques are applied to obtain a list of parameters that correlate to the degrading process. Furthermore, Machine Learning tools are used to learn from the obtained data, linking the changes in these parameters during its life cycle to the decay of the system in hope for future implementation of a predictive maintenance system and a reference for data analysis and prediction in PFs. The study was applied to a portable plasma focus device operated at 2 joules of stored energy. © Published under licence by IOP Publishing Ltd.The work is supported by IAEA CRP contract 20370 and by grant ACT-1115, CONICYT, Chile.https://iopscience.iop.org/article/10.1088/1742-6596/1043/1/01204
The Black Hole in Three Dimensional Space Time
The standard Einstein-Maxwell equations in 2+1 spacetime dimensions, with a
negative cosmological constant, admit a black hole solution. The 2+1 black hole
-characterized by mass, angular momentum and charge, defined by flux integrals
at infinity- is quite similar to its 3+1 counterpart. Anti-de Sitter space
appears as a negative energy state separated by a mass gap from the continuous
black hole spectrum. Evaluation of the partition function yields that the
entropy is equal to twice the perimeter length of the horizon.Comment: This version is the one that appeared in PRL (1992), and has
important improvements with respect to the one previously submitted to the
archive. 13 pages, latex, no figure
Designing and prototyping of a novel textile-based 3D panel
The paper describes the designing and prototyping process of a novel textile-based building
component, named Textile Wall, conceived to quickly erect walls and roofs with a protective purpose
(humanitarian sheltering, flood protection, ballistic defense etc.), or simply to divide work and home
spaces. Textile Wall is an innovative foldable cell panel, lightweight and freestanding, made of semirigid
PVC foils and finishing textile layers. Stabilizing or insulating materials can fill its vertical cells.
The height and the dimension of cells, together with the membrane material (fabrics, mosquito netting, papers, composite fibers), can differ in relation to its final use and the expected durability. Man portable with a small logistical footprint, the use of digital technologies allows tailing the production of the Textile Wall in various, flexible and modular configurations. It can be deployed rapidly, without any engineering support; the easy set-up will encourage local users to repair or rebuild homes in case of disaster, taking into account own constructive and social background. The Textile Wall has been one of the results of the collaborative research project S(P)EEDKITS, founded from the European Union’s 7th Framework Programme - Security Theme (2012-2016), in which research institutes, universities, companies operating in the emergency sector and non-profit organizations have rethought shelters, medical care resources and other facilities provided in case of natural disaster and conflict
A fixed-time second order sliding mode observer for a class of nonlinear systems
This paper presents a second order fixed time sliding mode observer based on an extension of the super-twisting algorithm. This observer can be applied to a class of nonlinear system with a block-wise representation. The block structure provides a straightforward form to the application of the proposed second order sliding mode algorithm, yielding to finite-time convergence with a settling time independent to the system initial conditions. Finally, as numerical simulation example, the case of a linear induction motor is studied, exposing the efficiency and feasibility of the proposal
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