7,557 research outputs found
Topological regulation of activation barriers on fractal substrates
We study phase-ordering dynamics of a ferromagnetic system with a scalar
order-parameter on fractal graphs. We propose a scaling approach, inspired by
renormalization group ideas, where a crossover between distinct dynamical
behaviors is induced by the presence of a length associated to the
topological properties of the graph. The transition between the early and the
asymptotic stage is observed when the typical size of the growing
ordered domains reaches the crossover length . We consider two
classes of inhomogeneous substrates, with different activated processes, where
the effects of the free energy barriers can be analytically controlled during
the evolution. On finitely ramified graphs the free energy barriers encountered
by domains walls grow logarithmically with while they increase as a
power-law on all the other structures. This produces different asymptotic
growth laws (power-laws vs logarithmic) and different dependence of the
crossover length on the model parameters. Our theoretical picture
agrees very well with extensive numerical simulations.Comment: 13 pages, 4 figure
Evidence of radius inflation in stars approaching the slow-rotator sequence
Average stellar radii in open clusters can be estimated from rotation periods
and projected rotational velocities under the assumption of random orientation
of the spin axis. Such estimates are independent of distance, interstellar
absorption, and models, but their validity can be limited by missing data
(truncation) or data that only represent upper/lower limits (censoring). We
present a new statistical analysis method to estimate average stellar radii in
the presence of censoring and truncation. We use theoretical distribution
functions of the projected stellar radius to define a likelihood
function in the presence of censoring and truncation. Average stellar radii in
magnitude bins are then obtained by a maximum likelihood parametric estimation
procedure. This method is capable of recovering the average stellar radius
within a few percent with as few as 10 measurements. Here it is
applied for the first time to the dataset available for the Pleiades. We find
an agreement better than 10 percent between the observed vs
relationship and current standard stellar models for 1.2
0.85 with no evident bias. Evidence of a systematic deviation at
level are found for stars with 0.8 0.6 approaching the
slow-rotator sequence. Fast-rotators ( < 2 d) agree with standard models
within 15 percent with no systematic deviations in the whole 1.2 0.5 range. The evidence found of a possible radius inflation
just below the lower mass limit of the slow-rotator sequence indicates a
possible connection with the transition from the fast to the slow-rotator
sequence.Comment: Accepted by Astronomy and Astrophysics, 11 pages, 6 figure
Fluctuation-dissipation relations and field-free algorithms for the computation of response functions
We discuss the relation between the fluctuation-dissipation relation derived
by Chatelain and Ricci-Tersenghi [C.Chatelain, J.Phys. A {\bf 36}, 10739
(2003); F. Ricci-Tersenghi, Phys.Rev.E 68, 065104(R) (2003)] and that by
Lippiello-Corberi-Zannetti [E. Lippiello, F. Corberi and M. Zannetti Phys. Rev.
E {\bf 72}, 056103 (2005)]. In order to do that, we re-derive the
fluctuation-dissipation relation for systems of discrete variables evolving in
discrete time via a stochastic non-equilibrium Markov process. The calculation
is carried out in a general formalism comprising the Chatelain, Ricci-Tersenghi
result and that by Lippiello-Corberi-Zannetti as special cases. The
applicability, generality, and experimental feasibility of the two approaches
is thoroughly discussed. Extending the analytical calculation to the variance
of the response function we show the vantage of field-free numerical methods
with respect to the standard method where the perturbation is applied. We also
show that the signal to noise ratio is better (by a factor ) in the
algorithm of Lippiello-Corberi-Zannetti with respect to that of Chatelain-Ricci
Tersenghi.Comment: 17 pages, 5 figures. To appear in Phys. Rev.
A Mathematical Model of Flavescence Dor\'ee Epidemiology
Flavescence dor\'ee (FD) is a disease of grapevine transmitted by an insect
vector, Ball. At present, no prophylaxis exists, so
mandatory control procedures (e.g. removal of infected plants, and insecticidal
sprays to avoid transmission) are in place in Italy and other European
countries. We propose a model of the epidemiology of FD by taking into account
the different aspects involved into the transmission process (acquisition of
the disease, latency and expression of symptoms, recovery rate, removal and
replacement of infected plants, insecticidal treatments, and the effect of
hotbeds). The model was constructed as a system of first order nonlinear ODEs
in four compartment variables. We perform a bifurcation analysis of the
equilibria of the model using the severity of the hotbeds as the control
parameter. Depending on the non-dimensional grapevine density of the vineyard
we find either a single family of equilibria in which the health of the
vineyard gradually deteriorates for progressively more severe hotbeds, or
multiple equilibria that give rise to sudden transitions from a nearly healthy
vineyard to a severely deteriorated one when the severity of the hotbeds
crosses a critical value. These results suggest some lines of intervention for
limiting the spread of the disease
Continuous measurements of real-life bidirectional pedestrian flows on a wide walkway
Employing partially overlapping overhead \kinectTMS sensors and automatic
pedestrian tracking algorithms we recorded the crowd traffic in a rectilinear
section of the main walkway of Eindhoven train station on a 24/7 basis. Beside
giving access to the train platforms (it passes underneath the railways), the
walkway plays an important connection role in the city. Several crowding
scenarios occur during the day, including high- and low-density dynamics in
uni- and bi-directional regimes. In this paper we discuss our recording
technique and we illustrate preliminary data analyses. Via fundamental
diagrams-like representations we report pedestrian velocities and fluxes vs.
pedestrian density. Considering the density range - ped/m, we
find that at densities lower than ped/m pedestrians in
unidirectional flows walk faster than in bidirectional regimes. On the
opposite, velocities and fluxes for even bidirectional flows are higher above
ped/m.Comment: 9 pages, 7 figure
Adapting the Proust Index to examine the macroeconomic variations in the European Union
The viability of the European project is coming under serious question due to a combination of persistent economic stagnation coupled with the migration crisis, which together fuel the relentless rise of anti-establishment and anti-E.U. sentiments across the continent. Is the magnitude of these crises and their obdurate resistance to progress leading the European project towards failure? One of the potential future paths of the E.U., a formal ‘two-speed’ Europe, is in the works regarding the sectors of E.U. defense and border control. If such a framework were to be applied as a solution to their economic problems, E.U. countries could target policies based on their economic performance rather than a one-size-fits-all E.U. policy. Although a current, intrinsic, ‘two-speed’ framework exists in the E.U., it encounters several difficulties (namely political will and through a form of social pressure) that undermine its success, ultimately rendering it unconvincing. I develop an alternative approach based solely on economic performance by adapting the Proust Index, a macroeconomic index originally devised by The Economist which aggregates seven indicators. This analysis firstly reveals clearly divergent patterns of economic trends within the E.U. In the context of a ‘two-speed’ Europe, it then provides a set of hypothetical 'definitions' along which E.U. member-states could be divided, something largely missing or at best vague in the few instances it appears in official statements and in economic literature. Ultimately, because the significant gap in performance between the two groups is increasing, this thesis suggests that urgent action needs to be taken to address it whichever option the E.U. chooses for its future
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