12,764 research outputs found
On Sasaki-Einstein manifolds in dimension five
We prove the existence of Sasaki-Einstein metrics on certain simply connected
5-manifolds where until now existence was unknown. All of these manifolds have
non-trivial torsion classes. On several of these we show that there are a
countable infinity of deformation classes of Sasaki-Einstein structures.Comment: 18 pages, Exposition was expanded and a reference adde
Self-organization, scaling and collapse in a coupled automaton model of foragers and vegetation resources with seed dispersal
We introduce a model of traveling agents ({\it e.g.} frugivorous animals) who
feed on randomly located vegetation patches and disperse their seeds, thus
modifying the spatial distribution of resources in the long term. It is assumed
that the survival probability of a seed increases with the distance to the
parent patch and decreases with the size of the colonized patch. In turn, the
foraging agents use a deterministic strategy with memory, that makes them visit
the largest possible patches accessible within minimal travelling distances.
The combination of these interactions produce complex spatio-temporal patterns.
If the patches have a small initial size, the vegetation total mass (biomass)
increases with time and reaches a maximum corresponding to a self-organized
critical state with power-law distributed patch sizes and L\'evy-like movement
patterns for the foragers. However, this state collapses as the biomass sharply
decreases to reach a noisy stationary regime characterized by corrections to
scaling. In systems with low plant competition, the efficiency of the foraging
rules leads to the formation of heterogeneous vegetation patterns with
frequency spectra, and contributes, rather counter-intuitively,
to lower the biomass levels.Comment: 11 pages, 5 figure
Derivation of the Blackbody Radiation Spectrum from a Natural Maximum-Entropy Principle Involving Casimir Energies and Zero-Point Radiation
By numerical calculation, the Planck spectrum with zero-point radiation is
shown to satisfy a natural maximum-entropy principle whereas alternative
choices of spectra do not. Specifically, if we consider a set of
conducting-walled boxes, each with a partition placed at a different location
in the box, so that across the collection of boxes the partitions are uniformly
spaced across the volume, then the Planck spectrum correspond to that spectrum
of random radiation (having constant energy kT per normal mode at low
frequencies and zero-point energy (1/2)hw per normal mode at high frequencies)
which gives maximum uniformity across the collection of boxes for the radiation
energy per box. The analysis involves Casimir energies and zero-point radiation
which do not usually appear in thermodynamic analyses. For simplicity, the
analysis is presented for waves in one space dimension.Comment: 11 page
Strong low-frequency quantum correlations from a four-wave mixing amplifier
We show that a simple scheme based on nondegenerate four-wave mixing in a hot
atomic vapor behaves like a near-perfect phase-insensitive optical amplifier,
which can generate bright twin beams with a measured quantum noise reduction in
the intensity difference of more than 8 dB, close to the best optical
parametric amplifiers and oscillators. The absence of a cavity makes the system
immune to external perturbations, and the strong quantum noise reduction is
observed over a large frequency range.Comment: 4 pages, 4 figures. Major rewrite of the previous version. New
experimental results and further analysi
Some Heuristic Semiclassical Derivations of the Planck Length, the Hawking Effect and the Unruh Effect
The formulae for Planck length, Hawking temperature and Unruh-Davies
temperature are derived by using only laws of classical physics together with
the Heisenberg principle. Besides, it is shown how the Hawking relation can be
deduced from the Unruh relation by means of the principle of equivalence; the
deep link between Hawking effect and Unruh effect is in this way clarified.Comment: LaTex file, 6 pages, no figure
Randomizing world trade. II. A weighted network analysis
Based on the misleading expectation that weighted network properties always
offer a more complete description than purely topological ones, current
economic models of the International Trade Network (ITN) generally aim at
explaining local weighted properties, not local binary ones. Here we complement
our analysis of the binary projections of the ITN by considering its weighted
representations. We show that, unlike the binary case, all possible weighted
representations of the ITN (directed/undirected, aggregated/disaggregated)
cannot be traced back to local country-specific properties, which are therefore
of limited informativeness. Our two papers show that traditional macroeconomic
approaches systematically fail to capture the key properties of the ITN. In the
binary case, they do not focus on the degree sequence and hence cannot
characterize or replicate higher-order properties. In the weighted case, they
generally focus on the strength sequence, but the knowledge of the latter is
not enough in order to understand or reproduce indirect effects.Comment: See also the companion paper (Part I): arXiv:1103.1243
[physics.soc-ph], published as Phys. Rev. E 84, 046117 (2011
Weakly nonlinear theory of grain boundary motion in patterns with crystalline symmetry
We study the motion of a grain boundary separating two otherwise stationary
domains of hexagonal symmetry. Starting from an order parameter equation
appropriate for hexagonal patterns, a multiple scale analysis leads to an
analytical equation of motion for the boundary that shares many properties with
that of a crystalline solid. We find that defect motion is generically opposed
by a pinning force that arises from non-adiabatic corrections to the standard
amplitude equation. The magnitude of this force depends sharply on the
mis-orientation angle between adjacent domains so that the most easily pinned
grain boundaries are those with an angle between four and eight degrees.
Although pinning effects may be small, they do not vanish asymptotically near
the onset of this subcritical bifurcation, and can be orders of magnitude
larger than those present in smectic phases that bifurcate supercritically
Coarsening in potential and nonpotential models of oblique stripe patterns
We study the coarsening of two-dimensional oblique stripe patterns by
numerically solving potential and nonpotential anisotropic Swift-Hohenberg
equations. Close to onset, all models exhibit isotropic coarsening with a
single characteristic length scale growing in time as . Further from
onset, the characteristic lengths along the preferred directions and
grow with different exponents, close to 1/3 and 1/2, respectively. In
this regime, one-dimensional dynamical scaling relations hold. We draw an
analogy between this problem and Model A in a stationary, modulated external
field. For deep quenches, nonpotential effects produce a complicated
dislocation dynamics that can lead to either arrested or faster-than-power-law
growth, depending on the model considered. In the arrested case, small isolated
domains shrink down to a finite size and fail to disappear. A comparison with
available experimental results of electroconvection in nematics is presented.Comment: 13 pages, 13 figures. To appear in Phys. Rev.
Modeling the mobility of living organisms in heterogeneous landscapes: Does memory improve foraging success?
Thanks to recent technological advances, it is now possible to track with an
unprecedented precision and for long periods of time the movement patterns of
many living organisms in their habitat. The increasing amount of data available
on single trajectories offers the possibility of understanding how animals move
and of testing basic movement models. Random walks have long represented the
main description for micro-organisms and have also been useful to understand
the foraging behaviour of large animals. Nevertheless, most vertebrates, in
particular humans and other primates, rely on sophisticated cognitive tools
such as spatial maps, episodic memory and travel cost discounting. These
properties call for other modeling approaches of mobility patterns. We propose
a foraging framework where a learning mobile agent uses a combination of
memory-based and random steps. We investigate how advantageous it is to use
memory for exploiting resources in heterogeneous and changing environments. An
adequate balance of determinism and random exploration is found to maximize the
foraging efficiency and to generate trajectories with an intricate
spatio-temporal order. Based on this approach, we propose some tools for
analysing the non-random nature of mobility patterns in general.Comment: 14 pages, 4 figures, improved discussio
Hydrodynamic reductions of the heavenly equation
We demonstrate that Pleba\'nski's first heavenly equation decouples in
infinitely many ways into a triple of commuting (1+1)-dimensional systems of
hydrodynamic type which satisfy the Egorov property. Solving these systems by
the generalized hodograph method, one can construct exact solutions of the
heavenly equation parametrized by arbitrary functions of a single variable. We
discuss explicit examples of hydrodynamic reductions associated with the
equations of one-dimensional nonlinear elasticity, linearly degenerate systems
and the equations of associativity.Comment: 14 page
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