35,759 research outputs found
Dressing a Naked Singularity: an Example
Considering the evolution of a perfect fluid with self-similarity of the
second kind, we have found that an initial naked singularity can be trapped by
an event horizon due to collapsing matter. The fluid moves along time-like
geodesics with a self-similar parameter . Since the metric
obtained is not asymptotically flat, we match the spacetime of the fluid with a
Schwarzschild spacetime. All the energy conditions are fulfilled until the
naked singularity.Comment: 14 pages, 1 figure. This version corrects an error in the calculus of
the pressure and in the conclusion
Influence of Refractory Periods in the Hopfield model
We study both analytically and numerically the effects of including
refractory periods in the Hopfield model for associative memory. These periods
are introduced in the dynamics of the network as thresholds that depend on the
state of the neuron at the previous time. Both the retrieval properties and the
dynamical behaviour are analyzed.Comment: Revtex, 7 pages, 7 figure
Decay of distance autocorrelation and Lyapunov exponents
This work presents numerical evidences that for discrete dynamical systems
with one positive Lyapunov exponent the decay of the distance autocorrelation
is always related to the Lyapunov exponent. Distinct decay laws for the
distance autocorrelation are observed for different systems, namely exponential
decays for the quadratic map, logarithmic for the H\'enon map and power-law for
the conservative standard map. In all these cases the decay exponent is close
to the positive Lyapunov exponent. For hyperbolic conservative systems, the
power-law decay of the distance autocorrelation tends to be guided by the
smallest Lyapunov exponent.Comment: 7 pages, 8 figure
Black string corrections in variable tension braneworld scenarios
Braneworld models with variable tension are investigated, and the corrections
on the black string horizon along the extra dimension are provided. Such
corrections are encrypted in additional terms involving the covariant
derivatives of the variable tension on the brane, providing profound
consequences concerning the black string horizon variation along the extra
dimension, near the brane. The black string horizon behavior is shown to be
drastically modified by the terms corrected by the brane variable tension. In
particular, a model motivated by the phenomenological interesting case
regarding Eotvos branes is investigated. It forthwith provides further physical
features regarding variable tension braneworld scenarios, heretofore concealed
in all previous analysis in the literature. All precedent analysis considered
uniquely the expansion of the metric up to the second order along the extra
dimension, what is able to evince solely the brane variable tension absolute
value. Notwithstanding, the expansion terms aftermath, further accomplished in
this paper from the third order on, elicits the successive covariant
derivatives of the brane variable tension, and their respective coupling with
the extrinsic curvature, the Weyl tensor, and the Riemann and Ricci tensors, as
well as the scalar curvature. Such additional terms are shown to provide sudden
modifications in the black string horizon in a variable tension braneworld
scenarioComment: 12 pages, 5 figures, accepted in PR
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