5,623 research outputs found
Relativistic Positioning Systems: The Emission Coordinates
This paper introduces some general properties of the gravitational metric and
the natural basis of vectors and covectors in 4-dimensional emission
coordinates. Emission coordinates are a class of space-time coordinates defined
and generated by 4 emitters (satellites) broadcasting their proper time by
means of electromagnetic signals. They are a constitutive ingredient of the
simplest conceivable relativistic positioning systems. Their study is aimed to
develop a theory of these positioning systems, based on the framework and
concepts of general relativity, as opposed to introducing `relativistic
effects' in a classical framework. In particular, we characterize the causal
character of the coordinate vectors, covectors and 2-planes, which are of an
unusual type. We obtain the inequality conditions for the contravariant metric
to be Lorentzian, and the non-trivial and unexpected identities satisfied by
the angles formed by each pair of natural vectors. We also prove that the
metric can be naturally split in such a way that there appear 2 parameters
(scalar functions) dependent exclusively on the trajectory of the emitters,
hence independent of the time broadcast, and 4 parameters, one for each
emitter, scaling linearly with the time broadcast by the corresponding
satellite, hence independent of the others.Comment: 13 pages, 3 figures. Only format changed for a new submission.
Submitted to Class. Quantum Gra
QCD
We discuss QCD studies that will be possible at LEP2. We examine both
experimental and theoretical aspects of jets, fragmentation functions,
multiplicities and particle spectra.Comment: 44 pages, Latex, epsfig, 18 figures, to appear on the Report of the
Workshop on Physics at LEP2, CERN 96-01, vol. 1, 199
Positivity and conservation of superenergy tensors
Two essential properties of energy-momentum tensors T_{\mu\nu} are their
positivity and conservation. This is mathematically formalized by,
respectively, an energy condition, as the dominant energy condition, and the
vanishing of their divergence \nabla^\mu T_{\mu\nu}=0. The classical Bel and
Bel-Robinson superenergy tensors, generated from the Riemann and Weyl tensors,
respectively, are rank-4 tensors. But they share these two properties with
energy momentum tensors: the Dominant Property (DP) and the divergence-free
property in the absence of sources (vacuum). Senovilla defined a universal
algebraic construction which generates a basic superenergy tensor T{A} from any
arbitrary tensor A. In this construction the seed tensor A is structured as an
r-fold multivector, which can always be done. The most important feature of the
basic superenergy tensors is that they satisfy automatically the DP,
independently of the generating tensor A. In a previous paper we presented a
more compact definition of T{A} using the r-fold Clifford algebra. This form
for the superenergy tensors allowed to obtain an easy proof of the DP valid for
any dimension. In this paper we include this proof. We explain which new
elements appear when we consider the tensor T{A} generated by a
non-degree-defined r-fold multivector A and how orthogonal Lorentz
transformations and bilinear observables of spinor fields are included as
particular cases of superenergy tensors. We find some sufficient conditions for
the seed tensor A, which guarantee that the generated tensor T{A} is
divergence-free. These sufficient conditions are satisfied by some physical
fields, which are presented as examples.Comment: 19 pages, no figures. Language and minor changes. Published versio
Approaches based on LAMDA control applied to regulate HVAC systems for buildings
The control of HVAC (Heating Ventilation and Air Conditioning) systems is usually complex because its modeling in certain cases is difficult, since these systems have a large number of components. Heat exchangers, chillers, valves, sensors, and actuators, increase the non-linear characteristics of the complete model, so it is necessary to propose new control strategies that can handle the uncertainty generated by all these elements working together. On the other hand, artificial intelligence is a powerful tool that allows improving the performance of control systems with inexact models and uncertainties. This paper presents new control alternatives for HVAC systems based on LAMDA (Learning Algorithm for Multivariable Data Analysis). This algorithm has been used in the field of machine learning, however, we have taken advantage of its learning characteristics to propose different types of intelligent controllers to improve the performance of the overall control system in tasks of regulation and reference change. In order to perform a comparative analysis in the context of HVAC systems, conventional methods such as PID and Fuzzy-PID are compared with LAMDA-PID, LAMDA-Sliding Mode Control based on Z-numbers (ZLSMC), and Adaptive LAMDA. Specifically, two HVAC systems are implemented by simulations to evaluate the proposals: an MIMO (Multiple-input Multiple-output) HVAC system and an HVAC system with dead time, which are used to compare the results qualitatively and quantitatively. The results show that ZLSMC is the most robust controller, which efficiently controls HVAC systems in cases of reference changes and the presence of disturbances.European CommissionAgencia Estatal de InvestigaciĂłnJunta de Comunidades de Castilla-La Manch
The Role of Bacteria and Fungi on Forage Degradation \u3ci\u3ein Vitro\u3c/i\u3e
The study was conducted to evaluate the interactive role of bacteria and fungi on forage degradation in vitro. Samples of Cynodon spp. were incubated in a 48-h in vitro gas assay with incubation medium containing or not antimicrobial substances. Treatments were: antibiotic (Ab), antifungal (Af), negative control (i.e. without antimicrobials) or positive control (i.e. with both Ab and Af). Three replicate assays were conducted and, in each assay the gas volume was measured at 3, 6, 9, 12, 24, 36 and 48 h of incubation. Data of cumulative gas production in each flask in each assay was fitted to a one-pool logistic model which generated three kinetic parameters: total gas production, rate of gas production and lag time. For statistical analysis, data of triplicates in each run were averaged and each run was considered a replicate. All variables were significantly affected by treatments (P \u3c 0.05). Compared to negative control treatment, Ab decreased total gas production and the rate of gas production by 26 and 13 %, respectively, and increased the lag time by 5.5 hours. The inclusion of Af also decreased total gas production and the rate of gas production by 5 and 29%, respectively, whereas decreased the lag time by 1 hour. When both Ab and Af were included in the incubation medium, gas production was almost completely inhibited and no convergent data of fermentation parameters was generated. In conclusion, bacteria had a major role on forage degradation what, however, was increased by fungi activity. The mechanisms by which fungi interact with bacteria for degrading forage into the rumen needs to be elucidated
739 observed NEAs and new 2-4m survey statistics within the EURONEAR network
We report follow-up observations of 477 program Near-Earth Asteroids (NEAs)
using nine telescopes of the EURONEAR network having apertures between 0.3 and
4.2 m. Adding these NEAs to our previous results we now count 739 program NEAs
followed-up by the EURONEAR network since 2006. The targets were selected using
EURONEAR planning tools focusing on high priority objects. Analyzing the
resulting orbital improvements suggests astrometric follow-up is most important
days to weeks after discovery, with recovery at a new opposition also valuable.
Additionally we observed 40 survey fields spanning three nights covering 11 sq.
degrees near opposition, using the Wide Field Camera on the 2.5m Isaac Newton
Telescope (INT), resulting in 104 discovered main belt asteroids (MBAs) and
another 626 unknown one-night objects. These fields, plus program NEA fields
from the INT and from the wide field MOSAIC II camera on the Blanco 4m
telescope, generated around 12,000 observations of 2,000 minor planets (mostly
MBAs) observed in 34 square degrees. We identify Near Earth Object (NEO)
candidates among the unknown (single night) objects using three selection
criteria. Testing these criteria on the (known) program NEAs shows the best
selection methods are our epsilon-miu model which checks solar elongation and
sky motion and the MPC's NEO rating tool. Our new data show that on average 0.5
NEO candidates per square degree should be observable in a 2m-class survey (in
agreement with past results), while an average of 2.7 NEO candidates per square
degree should be observable in a 4m-class survey (although our Blanco
statistics were affected by clouds). At opposition just over 100 MBAs (1.6
unknown to every 1 known) per square degree are detectable to R=22 in a 2m
survey based on the INT data, while our two best ecliptic Blanco fields away
from opposition lead to 135 MBAs (2 unknown to every 1 known) to R=23.Comment: Published in Planetary and Space Sciences (Sep 2013
Null cone preserving maps, causal tensors and algebraic Rainich theory
A rank-n tensor on a Lorentzian manifold V whose contraction with n arbitrary
causal future directed vectors is non-negative is said to have the dominant
property. These tensors, up to sign, are called causal tensors, and we
determine their general properties in dimension N. We prove that rank-2 tensors
which map the null cone on itself are causal. It is known that, to any tensor A
on V there is a corresponding ``superenergy'' (s-e) tensor T{A} which always
has the dominant property. We prove that, conversely, any symmetric rank-2
tensor with the dominant property can be written in a canonical way as a sum of
N s-e tensors of simple forms. We show that the square of any rank-2 s-e tensor
is proportional to the metric if N<5, and that this holds for the s-e tensor of
any simple form for arbitrary N. Conversely, we prove that any symmetric rank-2
tensor T whose square is proportional to the metric must be, up to sign, the
s-e of a simple p-form, and that the trace of T determines the rank p of the
form. This generalises, both with respect to N and the rank p, the classical
algebraic Rainich conditions, which are necessary and sufficient conditions for
a metric to originate in some physical field, and has a geometric
interpretation: the set of s-e tensors of simple forms is precisely the set of
tensors which preserve the null cone and its time orientation. It also means
that all involutory Lorentz transformations (LT) can be represented as s-e
tensors of simple forms, and that any rank-2 s-e tensor is the sum of at most N
conformally involutory LT. Non-symmetric null cone preserving maps are shown to
have a causal symmetric part and are classified according to the null
eigenvectors of the skew-symmetric part. We thus obtain a complete
classification of all conformal LT and singular null cone preserving maps on V.Comment: 36 pages, no figures, LaTeX fil
Symmetric hyperbolic systems for a large class of fields in arbitrary dimension
Symmetric hyperbolic systems of equations are explicitly constructed for a
general class of tensor fields by considering their structure as r-fold forms.
The hyperbolizations depend on 2r-1 arbitrary timelike vectors. The importance
of the so-called "superenergy" tensors, which provide the necessary symmetric
positive matrices, is emphasized and made explicit. Thereby, a unified
treatment of many physical systems is achieved, as well as of the sometimes
called "higher order" systems. The characteristics of these symmetric
hyperbolic systems are always physical, and directly related to the null
directions of the superenergy tensor, which are in particular principal null
directions of the tensor field solutions. Generic energy estimates and
inequalities are presented too.Comment: 24 pages, no figure
Global effect of COVID-19 pandemic on physical activity, sedentary behaviour and sleep among 3- to 5-year-old children: a longitudinal study of 14 countries
Background: The restrictions associated with the 2020 COVID-19 pandemic has resulted in changes to young
childrenâs daily routines and habits. The impact on their participation in movement behaviours (physical activity,
sedentary screen time and sleep) is unknown. This international longitudinal study compared young childrenâs
movement behaviours before and during the COVID-19 pandemic.
Methods: Parents of children aged 3â5 years, from 14 countries (8 low- and middle-income countries, LMICs)
completed surveys to assess changes in movement behaviours and how these changes were associated with the
COVID-19 pandemic. Surveys were completed in the 12 months up to March 2020 and again between May and
June 2020 (at the height of restrictions). Physical activity (PA), sedentary screen time (SST) and sleep were assessed
via parent survey. At Time 2, COVID-19 factors including level of restriction, environmental conditions, and parental
stress were measured. Compliance with the World Health Organizations (WHO) Global guidelines for PA (180 min/
day [â„60 min moderate- vigorous PA]), SST (â€1 h/day) and sleep (10-13 h/day) for children under 5 years of age,
was determined.
Results: Nine hundred- forty-eight parents completed the survey at both time points. Children from LMICs were
more likely to meet the PA (Adjusted Odds Ratio [AdjOR] = 2.0, 95%Confidence Interval [CI] 1.0,3.8) and SST
(AdjOR = 2.2, 95%CI 1.2,3.9) guidelines than their high-income country (HIC) counterparts. Children who could go
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outside during COVID-19 were more likely to meet all WHO Global guidelines (AdjOR = 3.3, 95%CI 1.1,9.8) than
those who were not. Children of parents with higher compared to lower stress were less likely to meet all three
guidelines (AdjOR = 0.5, 95%CI 0.3,0.9).
Conclusion: PA and SST levels of children from LMICs have been less impacted by COVID-19 than in HICs. Ensuring
children can access an outdoor space, and supporting parentsâ mental health are important prerequisites for
enabling pre-schoolers to practice healthy movement behaviours and meet the Global guidelines
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