1,318 research outputs found
Universality of Zipf's Law
We introduce a simple and generic model that reproduces Zipf's law. By
regarding the time evolution of the model as a random walk in the logarithmic
scale, we explain theoretically why this model reproduces Zipf's law. The
explanation shows that the behavior of the model is very robust and universal.Comment: 5 eps files included. To be published in J. Phys. Soc. Jp
TSA infrared measurements for stress distribution on car elements
Because of the continuous evolution of the market in terms of quality and
performance, the car production industry is being subjected to more and more
pressing technological challenges. In this framework the use of an advanced
measurement technique such as thermoelasticity allows the engineers to have a
fast and reliable tool for experimental investigation, optimization and
validation of the finite element method (FEM) of those critical parts, such
as parts of car-frame tables (Marsili and Garinei, 2013; Ju et al., 1997). In
this work it is shown how the thermoelastic measurement technique can be used
to optimize a Ferrari car frame, as a method of experimental investigation
and as a technique of validation of numerical models.The measurement technique developed for this purpose is described together
with the calibration method used in the test benches normally used for
fatigue testing and qualification of this car's components. The results
obtained show a very good agreement with FEM models and also the possibility
of experimentally identifying the concentration levels of stress in critical
parts with a very high spatial resolution and testing the effective geometry
and material structure
Risk bubbles and market instability
We discuss a simple model of correlated assets capturing the feedback effects induced by portfolio investment in the covariance dynamics. This model predicts an instability when the volume of investment exceeds a critical value. Close to the critical point the model exhibits dynamical correlations very similar to those observed in real markets. Maximum likelihood estimates of the modelâs parameter for empirical data indeed confirms this conclusion. We show that this picture is confirmed by the empirical analysis for different choices of the time horizon
Dynamic instability in a phenomenological model of correlated assets
We show that financial correlations exhibit a non-trivial dynamic behavior. We introduce a simple phenomenological
model of a multi-asset financial market, which takes into account the impact of portfolio investment on price dynamics. This captures the fact that correlations determine the optimal portfolio but are affected by investment based on it. We show that such a feedback on correlations gives rise to an instability when the volume of investment exceeds a critical value. Close to the critical point the model exhibits dynamical correlations
very similar to those observed in real markets. Maximum likelihood estimates of the modelâs parameter for empirical data indeed confirm this conclusion, thus suggesting that real markets operate close to a dynamically
unstable point
Non perturbative renormalization group approach to surface growth
We present a recently introduced real space renormalization group (RG)
approach to the study of surface growth.
The method permits us to obtain the properties of the KPZ strong coupling
fixed point, which is not accessible to standard perturbative field theory
approaches. Using this method, and with the aid of small Monte Carlo
calculations for systems of linear size 2 and 4, we calculate the roughness
exponent in dimensions up to d=8. The results agree with the known numerical
values with good accuracy. Furthermore, the method permits us to predict the
absence of an upper critical dimension for KPZ contrarily to recent claims. The
RG scheme is applied to other growth models in different universality classes
and reproduces very well all the observed phenomenology and numerical results.
Intended as a sort of finite size scaling method, the new scheme may simplify
in some cases from a computational point of view the calculation of scaling
exponents of growth processes.Comment: Invited talk presented at the CCP1998 (Granada
Generalized Dielectric Breakdown Model
We propose a generalized version of the Dielectric Breakdown Model (DBM) for
generic breakdown processes. It interpolates between the standard DBM and its
analog with quenched disorder, as a temperature like parameter is varied. The
physics of other well known fractal growth phenomena as Invasion Percolation
and the Eden model are also recovered for some particular parameter values. The
competition between different growing mechanisms leads to new non-trivial
effects and allows us to better describe real growth phenomena.
Detailed numerical and theoretical analysis are performed to study the
interplay between the elementary mechanisms. In particular, we observe a
continuously changing fractal dimension as temperature is varied, and report an
evidence of a novel phase transition at zero temperature in absence of an
external driving field; the temperature acts as a relevant parameter for the
``self-organized'' invasion percolation fixed point. This permits us to obtain
new insight into the connections between self-organization and standard phase
transitions.Comment: Submitted to PR
Interacting Individuals Leading to Zipf's Law
We present a general approach to explain the Zipf's law of city distribution.
If the simplest interaction (pairwise) is assumed, individuals tend to form
cities in agreement with the well-known statisticsComment: 4 pages 2 figure
Financial instability from local market measures
We study the emergence of instabilities in a stylized model of a financial
market, when different market actors calculate prices according to different
(local) market measures. We derive typical properties for ensembles of large
random markets using techniques borrowed from statistical mechanics of
disordered systems. We show that, depending on the number of financial
instruments available and on the heterogeneity of local measures, the market
moves from an arbitrage-free phase to an unstable one, where the complexity of
the market - as measured by the diversity of financial instruments - increases,
and arbitrage opportunities arise. A sharp transition separates the two phases.
Focusing on two different classes of local measures inspired by real markets
strategies, we are able to analytically compute the critical lines,
corroborating our findings with numerical simulations.Comment: 17 pages, 4 figure
A New Stochastic Strategy for the Minority Game
We present a variant of the Minority Game in which players who where
successful in the previous timestep stay with their decision, while the losers
change their decision with a probability . Analytical results for different
regimes of and the number of players are given and connections to
existing models are discussed. It is shown that for the average
loss is of the order of 1 and does not increase with as for
other known strategies.Comment: 4 pages, 3 figure
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