271 research outputs found
Intentions to Return of Irregular Migrants: Illegality as a Cause of Skill Waste
In this paper we show that highly skilled illegal migrants may be more likely to return home than migrants with low or no skills when illegality causes skill waste, i.e. reduced ability of making use of individual capabilities both in the labor and the financial markets. This result is in contrast with common wisdom on return migration, according to which low-skill individuals are more likely to go back home rather than high-skill migrants. The simple theoretical life-cycle framework that shows the former result is tested on a sample of illegal migrants crossing Italian borders in 2003. The estimation results confirm that highly skilled illegal migrants are more willing to return home.Illegal migration, labor skills, skill waste.
Percolation in high dimensions is not understood
The number of spanning clusters in four to nine dimensions does not fully
follow the expected size dependence for random percolation.Comment: 9-dimensional data and more points for large lattices added;
statistics improved, text expanded, table of exponents inserte
Reentrant phase diagram and pH effects in cross-linked gelatin gels
Experimental results have shown that the kinetics of bond formation in
chemical crosslinking of gelatin solutions is strongly affected not only by
gelatin and reactant concentrations but also by the solution pH. We present an
extended numerical investigation of the phase diagram and of the kinetics of
bond formation as a function of the pH, via Monte Carlo simulations of a
lattice model for gelatin chains and reactant agent in solution. We find a
reentrant phase diagram, namely gelation can be hindered either by loop
formation, at low reactant concentrations, or by saturation of active sites of
the chains via formation of single bonds with crosslinkers, at high reactant
concentrations. The ratio of the characteristic times for the formation of the
first and of the second bond between the crosslinker and an active site of a
chain is found to depend on the reactant reactivity, in good agreement with
experimental data.Comment: 8 pages, 8 figure
On the influence of time and space correlations on the next earthquake magnitude
A crucial point in the debate on feasibility of earthquake prediction is the
dependence of an earthquake magnitude from past seismicity. Indeed, whilst
clustering in time and space is widely accepted, much more questionable is the
existence of magnitude correlations. The standard approach generally assumes
that magnitudes are independent and therefore in principle unpredictable. Here
we show the existence of clustering in magnitude: earthquakes occur with higher
probability close in time, space and magnitude to previous events. More
precisely, the next earthquake tends to have a magnitude similar but smaller
than the previous one. A dynamical scaling relation between magnitude, time and
space distances reproduces the complex pattern of magnitude, spatial and
temporal correlations observed in experimental seismic catalogs.Comment: 4 Figure
Kinetics of bond formation in crosslinked gelatin gels
In chemical crosslinking of gelatin solutions, two different time scales
affect the kinetics of the gel formation in the experiments. We complement the
experimental study with Monte Carlo numerical simulations of a lattice model.
This approach shows that the two characteristic time scales are related to the
formation of single bonds crosslinker-chain and of bridges between chains. In
particular their ratio turns out to control the kinetics of the gel formation.
We discuss the effect of the concentration of chains. Finally our results
suggest that, by varying the probability of forming bridges as an independent
parameter, one can finely tune the kinetics of the gelation via the ratio of
the two characteristic times.Comment: 8 pages, 9 figures, revised versio
The balance between excitation and inhibition controls the temporal organization of neuronal avalanches
Neuronal avalanches, measured in vitro and in vivo, exhibit a robust critical
behaviour. Their temporal organization hides the presence of correlations. Here
we present experimental measurements of the waiting time distribution between
successive avalanches in the rat cortex in vitro. This exhibits a non-monotonic
behaviour, not usually found in other natural processes. Numerical simulations
provide evidence that this behaviour is a consequence of the alternation
between states of high and low activity, named up and down states, leading to a
balance between excitation and inhibition controlled by a single parameter.
During these periods both the single neuron state and the network excitability
level, keeping memory of past activity, are tuned by homeostatic mechanisms.Comment: 5 pages, 3 figures, to appear on Physical Review Letter
Multifractal Behaviour of n-Simplex Lattice
We study the asymptotic behaviour of resistance scaling and fluctuation of
resistance that give rise to flicker noise in an {\em n}-simplex lattice. We
propose a simple method to calculate the resistance scaling and give a
closed-form formula to calculate the exponent, , associated with
resistance scaling, for any n. Using current cumulant method we calculate the
exact noise exponent for n-simplex lattices.Comment: Latex, 9 pages including one figur
Current Distribution in the Three-Dimensional Random Resistor Network at the Percolation Threshold
We study the multifractal properties of the current distribution of the
three-dimensional random resistor network at the percolation threshold. For
lattices ranging in size from to we measure the second, fourth and
sixth moments of the current distribution, finding {\it e.g.\/} that
where is the conductivity exponent and is the
correlation length exponent.Comment: 10 pages, latex, 8 figures in separate uuencoded fil
Tolerance and Sensitivity in the Fuse Network
We show that depending on the disorder, a small noise added to the threshold
distribution of the fuse network may or may not completely change the
subsequent breakdown process. When the threshold distribution has a lower
cutoff at a finite value and a power law dependence towards large thresholds
with an exponent which is less than , the network is not sensitive
to the added noise, otherwise it is. The transition between sensitivity or not
appears to be second order, and is related to a localization-delocalization
transition earlier observed in such systems.Comment: 12 pages, 3 figures available upon request, plain Te
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