3,671 research outputs found
The convolution theorem for nonlinear optics
We have expressed the nonlinear optical absorption of a semiconductor in
terms of its linear spectrum. We determined that the two-photon absorption
coefficient in a strong DC-electric field of a direct gap semiconductor can be
expressed as the product of a differential operator times the convolution
integral of the linear absorption without a DC-electric field and an Airy
function. We have applied this formalism to calculate the two-photon absorption
coefficient and nonlinear refraction for GaAs and ZnSe using their linear
absorption and have found excellent agreement with available experimental data.Comment: 8 pages, 2 figures (6 sub fugures
MaxEnt and dynamical information
The MaxEnt solutions are shown to display a variety of behaviors (beyond the
traditional and customary exponential one) if adequate dynamical information is
inserted into the concomitant entropic-variational principle. In particular, we
show both theoretically and numerically that power laws and power laws with
exponential cut-offs emerge as equilibrium densities in proportional and other
dynamics
A Newtonian approach to the cosmological dark fluids
We review the hydrodynamics of the dark sector components in Cosmology. For
this purpose we use the approach of Newtonian gravitational instability, and
thereafter we add corrections to arrive to a full relativistic description. In
Cosmology and Astrophysics, it is usual to decompose the dark sector into two
species, dark matter and dark energy. We will use instead a unified approach by
describing a single unified dark fluid with very simple assumptions, namely the
dark fluid is barotropic and its sound speed vanishes.Comment: 13 pages, To be published in 'Selected Topics of Computational and
Experimental Fluid Mechanics' Springer Book Series: Environmental Science and
Engineering: Environmental Scienc
On the exposure to mobile phone radiation in trains
This report presents theoretical estimates of the Power Density levels which
may be reached inside trains. Two possible sources of high levels of radiation
are discussed. The first one arises since the walls of the wagons are metallic
and therefore bounce back almost all radiation impinging on them. The second is
due to the simultaneous emission of a seemingly large number of nearby
telephones. The theoretical study presented here shows that Power Densities
stay at values below reference levels always.Comment: 9 pages, 1 figur
FLUKA Simulations of Energy Density Deposition from a ILC Bunch in different Spoiler Designs
FLUKA is used to simulate the energy deposition due to a direct bunch impact of the ILC beam in various candidate spoiler designs. The conclusions extracted will contribute to the overall optimisation process and identify areas where additional experimental data would be beneficial
Variational Principle underlying Scale Invariant Social Systems
MaxEnt's variational principle, in conjunction with Shannon's logarithmic
information measure, yields only exponential functional forms in
straightforward fashion. In this communication we show how to overcome this
limitation via the incorporation, into the variational process, of suitable
dynamical information. As a consequence, we are able to formulate a somewhat
generalized Shannonian Maximum Entropy approach which provides a unifying
"thermodynamic-like" explanation for the scale-invariant phenomena observed in
social contexts, as city-population distributions. We confirm the MaxEnt
predictions by means of numerical experiments with random walkers, and compare
them with some empirical data
On the symmetry of the Quadratic Assignment Problem through Elementary Landscape Decomposition
When designing meta-heuristic strategies to optimize the quadratic assignment problem (QAP), it is important to take into account the specific characteristics of the instance to be solved. One of the characteristics that has been pointed out as having the potential to affect the performance of optimization algorithms is the symmetry of the distance and flow matrices that form the QAP.
In this paper, we further investigate the impact of the symmetry of the QAP on the performance of meta-heuristic algorithms, focusing on local search based methods. The analysis is carried out using the elementary landscape decomposition (ELD) of the problem under the swap neighborhood. First, we study the number of local optima and the relative contribution of the elementary components on a benchmark composed of different types of instances. Secondly, we propose a specific local search algorithm based on the ELD in order to experimentally validate the effects of the symmetry. The analysis carried out shows that the symmetry of the QAP is a relevant feature that influences both the characteristics of the elementary components and the performance of local search based algorithms.IT1244-19,
PID2019-106453GA-I00/AEI/10.13039/501100011033,
H202
Anatomy of the attraction basins: Breaking with the intuition
olving combinatorial optimization problems efficiently requires the development of algorithms that consider the specific properties of the problems. In this sense, local search algorithms are designed over a neighborhood structure that partially accounts for these properties. Considering a neighborhood, the space is usually interpreted as a natural landscape, with valleys and mountains. Under this perception, it is commonly believed that, if maximizing, the solutions located in the slopes of the same mountain belong to the same attraction basin, with the peaks of the mountains being the local optima. Unfortunately, this is a widespread erroneous visualization of a combinatorial landscape. Thus, our aim is to clarify this aspect, providing a detailed analysis of, first, the existence of plateaus where the local optima are involved, and second, the properties that define the topology of the attraction basins, picturing a reliable visualization of the landscapes. Some of the features explored in this article have never been examined before. Hence, new findings about the structure of the attraction basins are shown. The study is focused on instances of permutation-based combinatorial optimization problems considering the 2-exchange and the insert neighborhoods. As a consequence of this work, we break away from the extended belief about the anatomy of attraction basins
Characterising the rankings produced by combinatorial optimisation problems and finding their intersections
The aim of this paper is to introduce the concept of intersection between combinatorial optimisation problems. We take into account that most algorithms, in their machinery, do not consider the exact objective function values of the solutions, but only a comparison between them. In this sense, if the solutions of an instance of a combinatorial optimisation problem are sorted into their objective function values, we can see the instances as (partial) rankings of the solutions of the search space. Working with specific problems, particularly, the linear ordering problem and the symmetric and asymmetric traveling salesman problem, we show that they can not generate the whole set of (partial) rankings of the solutions of the search space, but just a subset. First, we characterise the set of (partial) rankings each problem can generate. Secondly, we study the intersections between these problems: those rankings which can be generated by both the linear ordering problem and the symmetric/asymmetric traveling salesman problem, respectively. The fact of finding large intersections between problems can be useful in order to transfer heuristics from one problem to another, or to define heuristics that can be useful for more than one problem
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