4,180 research outputs found
The collision of two-kinks defects
We have investigated the head-on collision of a two-kink and a two-antikink
pair that arises as a generalization of the model. We have evolved
numerically the Klein-Gordon equation with a new spectral algorithm whose
accuracy and convergence were attested by the numerical tests. As a general
result, the two-kink pair is annihilated radiating away most of the scalar
field. It is possible the production of oscillons-like configurations after the
collision that bounce and coalesce to form a small amplitude oscillon at the
origin. The new feature is the formation of a sequence of quasi-stationary
structures that we have identified as lump-like solutions of non-topological
nature. The amount of time these structures survives depends on the fine-tuning
of the impact velocity.Comment: 14 pages, 9 figure
Modulational instability of spatially broadband nonlinear optical pulses in four-state atomic systems
The modulational instability of broadband optical pulses in a four-state
atomic system is investigated. In particular, starting from a recently derived
generalized nonlinear Schr\"odinger equation, a wave-kinetic equation is
derived. A comparison between coherent and random phase wave states is made. It
is found that the spatial spectral broadening can contribute to the nonlinear
stability of ultra-short optical pulses. In practical terms, this could be
achieved by using random phase plate techniques.Comment: 9 pages, 3 figures, to appear in Phys. Rev.
Quantum Trivelpiece-Gould waves in a magnetized dense plasma
The dispersion relation for the electrostatic waves below the electron plasma
frequency in a dense quantum plasma is derived by using the magnetohydrodynamic
model. It is shown that in the classical case the dispersion relation reduces
to the expression obtained for the well-known Trivelpiece-Gould (TG) modes.
Attention is also devoted to the case of solitary waves associated with the
nonlinear TG modes.Comment: 8 pages, 0 figure
Intelligence artificielle et signification. À propos des limites et des possibilitées des sciences cognitives
L'auteur distingue, dans les.travaux sur l'intelligence artifi- cielle, deux approches : l'approche technologique et l'approche cognitiviste. Il montre que les rapprochements faits, dans l'approche cognitiviste, entre l'intelligence humaine et l'intelligence artificielle, ne vont pas de soi, et que les thèses sur l'intelligence artificielle sont largement tributaires de certaines spéculations rationalistes et empi- ristes de la philosophie classique. Il expose la principale difficulté que rencontre alors une compréhension de l'intelligence humaine à partir de l'approche cognitiviste, à savoir la nécessité d'oblitérer la dimension sémantique et de professer un solipsisme radical, et il plaide en faveur d'une compréhension de l'intelligence humaine selon ce qu'elle est dans la culture plutôt que dans les machines.In the literature on artificial intelligence, the author distinguishes two approaches : the technological approach and the cognitivist approach. He shows that the parallels made between human intelligence and artificial intelligence within the cognitivist approach do not go without saying, and that the theses on artificial intelligence are largely dependent upon certain rationalist and empiricist speculations of classical philosophy. He exposes the principal problem that then encounters an understanding of human intelligence in the light of the cognitivist approach, namely the necessity to obliterate the semantic dimension and to profess a radical solipsism, and he pleads for an understanding of human intelligence commensurable with what it is in culture rather than in machines
Mass for Plasma Photons from Gauge Symmetry Breaking
We derive the effective masses for photons in unmagnetized plasma waves using
a quantum field theory with two vector fields (gauge fields). In order to
properly define the quantum field degrees of freedom we re-derive the classical
wave equations on light-front gauge. This is needed because the usual scalar
potential of electromagnetism is, in quantum field theory, not a physical
degree of freedom that renders negative energy eigenstates. We also consider a
background local fluid metric that allows for a covariant treatment of the
problem. The different masses for the longitudinal (plasmon) and transverse
photons are in our framework due to the local fluid metric. We apply the
mechanism of mass generation by gauge symmetry breaking recently proposed by
the authors by giving a non-trivial vacuum-expectation-value to the second
vector field (gauge field). The Debye length is interpreted as an
effective compactification length and we compute an explicit solution for the
large gauge transformations that correspond to the specific mass eigenvalues
derived here. Using an usual quantum field theory canonical quantization we
obtain the usual results in the literature. Although none of these ingredients
are new to physicist, as far as the authors are aware it is the first time that
such constructions are applied to Plasma Physics. Also we give a physical
interpretation (and realization) for the second vector field in terms of the
plasma background in terms of known physical phenomena.
Addendum: It is given a short proof that equation (10) is wrong, therefore
equations (12-17) are meaningless. The remaining results are correct being
generic derivations for nonmagnetized plasmas derived in a covariant QFT
framework.Comment: v1: 1+6 pages v2: Several discussions rewritten; Abstract rewritten;
References added; v3: includes Addendu
Pulmonary Embolism Associated to HIV Infection
A presença de anticorpos antifosfolĂpidos Ă© frequente em doentes com infecção VIH principalmente em fases avançadas da doença. Apesar da elevada prevalĂŞncia de anticorpos antifosfolĂpidos, a sua associação a fenĂłmenos trombĂłticos Ă© rara, estando apenas descritos alguns casos.
Os autores apresentam um caso clĂnico cuja manifestação inaugural de uma infecção VIH foi um tromboembolismo pulmonar associado á presença de anticoagulante lĂşpico
Management Of Fishing Of The Broadband Anchovy (anchoviella Lepidentostole) (fowler, 1911), In South SĂŁo Paulo State, Brazil.
The broadband anchovy (Anchoviella lepidentostole) is one of the most important fishery resources for economic and social development of Iguape City and for the whole of the south coast of SĂŁo Paulo state. Nowadays this activity involves over 2,500 fishermen. This paper aims to evaluate its fishing, and discuss how the activity is managed. In order to conduct this study, production data and catch per unit effort (CPUE) data from the 1998-2000 period have been collected, and the characterisation of the activity has also been done. A decline in broadband anchovy abundance had been observed in the region up until 2004, when a new regulation was implemented, in an attempt to recover the resource, with the establishment of closed season fishing (defeso) during the catch season, and adjustments in the employed fishing gear. The results have shown a recovery of the abundance of broadband anchovy throughout the 2005-2010 period, showing thus far that the measures taken for the maintenance of the resource have been successful. The activity has been managed through discussions involving people from the fishing sector in order to determine the controversial actions to be taken, that are hard to enforce. In order to achieve a more satisfactory resource management, it is necessary to keep the participatory process, with adaptive co-management, based on discussion among all the sectors involved (from government and society), and constant monitoring of the activity.73691-
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