1,988 research outputs found
Traversable wormholes in a string cloud
We study spherically symmetric thin-shell wormholes in a string cloud
background in (3+1)-dimensional spacetime. The amount of exotic matter required
for the construction, the traversability and the stability under radial
perturbations, are analyzed as functions of the parameters of the model.
Besides, in the Appendices a non perturbative approach to the dynamics and a
possible extension of the analysis to a related model are briefly discussed.Comment: 21 pages, 10 figures; accepted for publication in Int. J. Mod. Phys.
Global action-angle coordinates for completely integrable systems with noncompact invariant submanifolds
The obstruction to the existence of global action-angle coordinates of
Abelian and noncommutative (non-Abelian) completely integrable systems with
compact invariant submanifolds has been studied. We extend this analysis to the
case of noncompact invariant submanifolds.Comment: 13 pages, to be published in J. Math. Phys. (2007
Mate choice promotes inbreeding avoidance in the two-spotted spider mite
Since inbreeding in Tetranychus urticae can reduce offspring fitness, sexual selection may favour disassortative mate choice with respect to relatedness of the mating partners. We tested whether T. urticae shows this preference for mating with unrelated partners. We chose an experimental set-up with high potential for female choosiness, since females only mate once and are therefore expected to be the choosier gender. An adult virgin female was placed together with two adult males from the same population. One male was unrelated and the other male was related—a brother with whom she had grown up. Significantly more copulations (64%) took place with the unrelated male. Time to mating did not depend on the female-to-male relatedness. The remaining (non-copulating) male tried to interfere with the ongoing mating in the majority of cases, but this interference did not depend on the female-to-male relatedness. These results imply that T. urticae (a) can recognize kin (via genetic and/or environmental similarity) and (b) has the potential to avoid inbreeding through mate choice
Elastic properties of grafted microtubules
We use single-particle tracking to study the elastic properties of single
microtubules grafted to a substrate. Thermal fluctuations of the free
microtubule's end are recorded, in order to measure position distribution
functions from which we calculate the persistence length of microtubules with
contour lengths between 2.6 and 48 micrometers. We find the persistence length
to vary by more than a factor of 20 over the total range of contour lengths.
Our results support the hypothesis that shearing between protofilaments
contributes significantly to the mechanics of microtubules.Comment: 9 pages, 3 figure
Thin-shell wormholes with a generalized Chaplygin gas
In this article, spherically symmetric thin-shell wormholes supported by a
generalized Chaplygin gas are constructed and their stability under
perturbations preserving the symmetry is studied. Wormholes with charge and
with a cosmological constant are analyzed and the results are compared with
those obtained for the original Chaplygin gas, which was considered in a
previous work. For some values of the parameters, one stable configuration is
also present and a new extra unstable solution is found.Comment: 14 pages, 6 figures; v2: typos corrected and minor rewordin
Discrete Model of Ideological Struggle Accounting for Migration
A discrete in time model of ideological competition is formulated taking into
account population migration. The model is based on interactions between global
populations of non-believers and followers of different ideologies. The complex
dynamics of the attracting manifolds is investigated.
Conversion from one ideology to another by means of (i) mass media influence
and (ii) interpersonal relations is considered. Moreover a different birth rate
is assumed for different ideologies, the rate being assumed to be positive for
the reference population, made of initially non-believers. Ideological
competition can happen in one or several regions in space. In the latter case,
migration of non-believers and adepts is allowed; this leads to an enrichment
of the ideological dynamics. Finally, the current ideological situation in the
Arab countries and China is commented upon from the point of view of the
presently developed mathematical model. The massive forced conversion by
Ottoman Turks in the Balkans is briefly discussed.Comment: 24 pages, with 5 figures and 52 refs.; prepared for a Special issue
of Advances in Complex System
Stability of Chaplygin gas thin-shell wormholes
In this paper we construct spherical thin-shell wormholes supported by a
Chaplygin gas. For a rather general class of geometries we introduce a new
approach for the stability analysis of static solutions under perturbations
preserving the symmetry. We apply this to wormholes constructed from
Schwarzschild, Schwarzschild-de Sitter, Schwarzschild-anti de Sitter and
Reissner-Nordstrom metrics. In the last two cases, we find that there are
values of the parameters for which stable static solutions exist.Comment: 14 pages, 5 figures; v2: minor changes and new references added.
Accepted for publication in Physical Review
Spectral functions in itinerant electron systems with geometrical frustration
The Hubbard model with geometrical frustration is investigated in a metallic
phase close to half-filling. We calculate the single particle spectral function
for the triangular lattice within dynamical cluster approximation, which is
further combined with non-crossing approximation and fluctuation exchange
approximation to treat the resulting cluster Anderson model. It is shown that
frustration due to non-local correlations suppresses short-range
antiferromagnetic fluctuations and thereby assists the formation of heavy
quasi-particles near half-filling.Comment: 4 pages, 5 eps figure
Transition Matrix Monte Carlo Reweighting and Dynamics
We study an induced dynamics in the space of energy of single-spin-flip Monte
Carlo algorithm. The method gives an efficient reweighting technique. This
dynamics is shown to have relaxation times proportional to the specific heat.
Thus, it is plausible for a logarithmic factor in the correlation time of the
standard 2D Ising local dynamics.Comment: RevTeX, 5 pages, 3 figure
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