1,242 research outputs found
Context-Sensitive Dependency Pairs Framework
We show how to develop a dependency pair framework for proving termination of context-sensitive rewriting.Gutiérrez Gil, R. (2008). Context-Sensitive Dependency Pairs Framework. http://hdl.handle.net/10251/13625Archivo delegad
The lifetime problem of evaporating black holes: mutiny or resignation
It is logically possible that regularly evaporating black holes exist in
nature. In fact, the prevalent theoretical view is that these are indeed the
real objects behind the curtain in astrophysical scenarios. There are several
proposals for regularizing the classical singularity of black holes so that
their formation and evaporation do not lead to information-loss problems. One
characteristic is shared by most of these proposals: these regularly
evaporating black holes present long-lived trapping horizons, with absolutely
enormous evaporation lifetimes in whatever measure. Guided by the discomfort
with these enormous and thus inaccessible lifetimes, we elaborate here on an
alternative regularization of the classical singularity, previously proposed by
the authors in an emergent gravity framework, which leads to a completely
different scenario. In our scheme the collapse of a stellar object would result
in a genuine time-symmetric bounce, which in geometrical terms amounts to the
connection of a black-hole geometry with a white-hole geometry in a regular
manner. The two most differential characteristics of this proposal are: i) the
complete bouncing geometry is a solution of standard classical general
relativity everywhere except in a transient region that necessarily extends
beyond the gravitational radius associated with the total mass of the
collapsing object; and ii) the duration of the bounce as seen by external
observers is very brief (fractions of milliseconds for neutron-star-like
collapses). This scenario motivates the search for new forms of stellar
equilibrium different from black holes. In a brief epilogue we compare our
proposal with a similar geometrical setting recently proposed by Haggard and
Rovelli.Comment: 20 pages, 2 figures; v2: published version, references adde
Electromagnetism as an emergent phenomenon: a step-by-step guide
We give a detailed description of electrodynamics as an emergent theory from
condensed-matter-like structures, not only {\it per se} but also as a warm-up
for the study of the much more complex case of gravity. We will concentrate on
two scenarios that, although qualitatively different, share some important
features, with the idea of extracting the basic generic ingredients that give
rise to emergent electrodynamics and, more generally, to gauge theories. We
start with Maxwell's mechanical model for electrodynamics, where Maxwell's
equations appear as dynamical consistency conditions. We next take a superfluid
He-like system as representative of a broad class of fermionic quantum
systems whose low-energy physics reproduces classical electrodynamics (Dirac
and Maxwell equations as dynamical low-energy laws). An important lesson that
can be derived from both analyses is that the vector potential has a
microscopic physical reality and that it is only in the low-energy regime that
this physical reality is blurred in favour of gauge invariance, which in
addition turns out to be secondary to effective Lorentz invariance.Comment: 41 pages, 4 figures; v2: references added, version accepted for
publicatio
Evolutionary Algorithms for Community Detection in Continental-Scale High-Voltage Transmission Grids
Symmetry is a key concept in the study of power systems, not only because the admittance and Jacobian matrices used in power flow analysis are symmetrical, but because some previous studies have shown that in some real-world power grids there are complex symmetries. In order to investigate the topological characteristics of power grids, this paper proposes the use of evolutionary algorithms for community detection using modularity density measures on networks representing supergrids in order to discover densely connected structures. Two evolutionary approaches (generational genetic algorithm, GGA+, and modularity and improved genetic algorithm, MIGA) were applied. The results obtained in two large networks representing supergrids (European grid and North American grid) provide insights on both the structure of the supergrid and the topological differences between different regions. Numerical and graphical results show how these evolutionary approaches clearly outperform to the well-known Louvain modularity method. In particular, the average value of modularity obtained by GGA+ in the European grid was 0.815, while an average of 0.827 was reached in the North American grid. These results outperform those obtained by MIGA and Louvain methods (0.801 and 0.766 in the European grid and 0.813 and 0.798 in the North American grid, respectively)
Evolution of the spherical cavity radius generated around a subsurface emitter
The emitter discharge in subsurface drip irrigation can be affected by soil properties. A positive pressure develops at the emitter outlet where a spherical cavity is assumed to form. In steady-state conditions, the pressure in the soil relates to soil hydraulic 5 properties, the emitter discharge, and the cavity radius. This pressure in the soil is very sensitive to the cavity radius. In this paper, the development of the cavity around the emitter outlet was measured for various emitter discharges in laboratory tests carried out in containers with uniform loamy soils. A trend between soil pressure and emitter discharge was established that illustrates the performance of buried emitters in the 10 field. Its application to the prediction of water distribution in subsurface drip irrigation units and its effect on the estimation of irrigation performance is also show
Estado actual de los injertos óseos: Biología, función, conservación, riesgo de transmisión de enfermedades, inmunogenicidad e incorporación
Los dos factores determinantes de la magnitud y la velocidad de unión entre el injerto y el husped son la estabilidad mecánica del lecho y el contacto entre ambos. Si el injerto no presenta una adecuada estabilidad mecanica, el tejido de granulación que lo rodea dara lugar a una fibrosis que se interpondra entre el injerto y el lecho, dificultando su incorporación. De particular importancia resulta la vascularización del lecho de implantación, así como la riqueza del mismo en progenitores de celulas
endoteliales y fibroblastos, decisivas en la respuesta a un implante steoinductivo y/o osteoconductivo así como a estimulos angiogénicos (7,31,40,66,64)
Water distribution in laterals and units of subsurface drip irrigation. II: Field evaluation.
The performance of drip irrigation and subsurface drip irrigation (SDI) laterals has been compared. Two emitter models (one compensating and the other noncompensating) were assessed. Field tests were carried out with a pair of laterals working at the same inlet pressure. A procedure was developed that recorded head pressures at both lateral extremes and inlet flow during irrigation. Both models showed similar behavior and soil properties affected their discharge. On the other hand, the performance of a field SDI unit of compensating emitters was characterized by measuring pressures at different points and inlet flow. Finally, the distribution of water and soil pressure in the laterals and the unit were predicted and irrigation uniformity and soil pressure variability were also determined. Predictions agreed reasonably well with the experimental observations. Thus, the methodology proposed could be used to support the decision making for the design and management of SDI system
Charge, lattice and magnetism across the valence crossover in EuIrSi single crystals
We present a detailed study of the temperature evolution of the crystal
structure, specific heat, magnetic susceptibility and resistivity of single
crystals of the paradigmatic valence-fluctuating compound EuIrSi. A
comparison to stable-valent isostructural compounds EuCoSi (with
Eu), and EuRhSi, (with Eu) reveals an anomalously large
thermal expansion indicative of the lattice softening associated to valence
fluctuations. A marked broad peak at temperatures around 65-75 K is observed in
specific heat, susceptibility and the derivative of resistivity, as thermal
energy becomes large enough to excite Eu into a divalent state, which localizes
one f electron and increases scattering of conduction electrons. In addition,
the intermediate valence at low temperatures manifests in a moderately
renormalized electron mass, with enhanced values of the Sommerfeld coefficient
in the specific heat and a Fermi-liquid-like dependence of resistivity at low
temperatures. The high residual magnetic susceptibility is mainly ascribed to a
Van Vleck contribution. Although the intermediate/fluctuating valence duality
is to some extent represented in the interconfiguration fluctuation model
commonly used to analyze data on valence-fluctuating systems, we show that this
model cannot describe the different physical properties of EuIrSi with
a single set of parameters.Comment: 12 pages, 4 figures, 1 tabl
Wearable GPS patch ANTENNA on jeans fabric
In this work a wearable GPS adhesive copper sheet patch antenna on jeans fabric is designed,
simulated and tested including the human body impact and specific absorption rate (SAR). The proposed
textile antenna operates at the GPS L1 band (1575.42 MHz) and it has been designed by means of the
commercial full 3D electromagnetic CST Microwave Studio simulator. The original textile antenna presents a
-13.6 dB return loss at the GPS L1 frequency band with a fractional bandwidth of BW=3% (at S11=-10 dB),
with a gain G= 1.7 dBi and efficiency ¿ =23.1%. A homogeneous phantom has been considered in order to
simulate the SAR in the leg of an average human body. The impact of the human body on the antenna
performance implies a detuning frequency of 1.6% and G= 1.2 dBi; ¿ =19% at the GPS operation frequency.
In addition, several bending scenarios have been simulated to determine the degradation of the return loss,
realized gain and efficiency. The proposed antenna is compliant with specific absorption rate (SAR)
requirements from the IEEE C95.3 standard.Postprint (author's final draft
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