363 research outputs found
The Anti-FPU Problem
We present a detailed analysis of the modulational instability of the
zone-boundary mode for one and higher-dimensional Fermi-Pasta-Ulam (FPU)
lattices. Following this instability, a process of relaxation to equipartition
takes place, which we have called the Anti-FPU problem because the energy is
initially fed into the highest frequency part of the spectrum, at variance with
the original FPU problem (low frequency excitations of the lattice). This
process leads to the formation of chaotic breathers in both one and two
dimensions. Finally, the system relaxes to energy equipartition on time scales
which increase as the energy density is decreased. We show that breathers
formed when cooling the lattice at the edges, starting from a random initial
state, bear strong qualitative similarities with chaotic breathers
EnFilter: a Password Enforcement and Filter
EnFilter is a Proactive Password Checking System, designed to avoid password guessing attacks. It is made of a set of configurable filters, each one based on a specific pattern recognition measure that can be tuned by the system administrator depending on the adopted password policy. Filters use decision trees, lexical analysers, as well as Levenshtein distance based techniques. EnFilter is implemented for Windows 2000/2003/XP
Learning logic programs with negation as failure
Normal logic programs are usually shorter and easier to
write and understand than definite logic programs. As a consequence,
it is worth investigating their learnability, if Inductive Logic Program-
ming is to be proposed as an alternative tool for software development
and Software Engineering at large. In this paper we present an exten-
sion of the ILP system TRACY, called TRACY-not, able to learn normal
logic programs. The method is proved to be sound, in the sense that
it outputs a program which is complete and consistent w.r.t.the ex-
amples, and complete, in the sense that it does find a solution when it
exists. Compared to learning systems based on extensionality,TRACY
and TRACY not are less dependent on the kind and number of training
examples, which is due to the intensional evaluation of the hypothe-
ses and, for TRACY-not, to the possibility to have restricted hypothesis
spaces through the use of negation
Developing Real Estate Automated Valuation Models by Learning from Heterogeneous Data Sources
In this paper we propose a data acquisition methodology, and a Machine Learning solution for the partially automated evaluation of real estate properties. The novelty and importance of the approach lies in two aspects: (1) when compared to Automated Valuation Models (AVMs) as available to real estate operators, it is highly adaptive and non-parametric, and integrates diverse data sources; (2) when compared to Machine Learning literature that has addressed real estate applications, it is more directly linked to the actual business processes of appraisal companies: in this context prices that are advertised online are normally not the most relevant source of information, while an appraisal document must be proposed by an expert and approved by a validator, possibly with the help of technological tools. We describe a case study using a set of 7988 appraisal documents for residential properties in Turin, Italy. Open data were also used, including location, nearby points of interest, comparable property prices, and the Italian revenue service area code. The observed mean error as measured on an independent test set was around 21 K€, for an average property value of about 190 K€. The AVM described here can help the stakeholders in this process (experts, appraisal company) to provide a reference price to be used by the expert, to allow the appraisal company to validate their evaluations in a faster and cheaper way, to help the expert in listing a set of comparable properties, that need to be included in the appraisal document
Non-equilibrium steady states of long-range coupled harmonic chains
We perform a numerical study of transport properties of a one-dimensional
chain with couplings decaying as an inverse power of the
intersite distance and open boundary conditions, interacting with two heat
reservoirs. Despite its simplicity, the model displays highly nontrivial
features in the strong long-range regime, . At weak coupling with
the reservoirs, the energy flux departs from the predictions of perturbative
theory and displays anomalous superdiffusive scaling of the heat current with
the chain size. We trace back this behavior to the transmission spectrum of the
chain, which displays a self-similar structure with a characteristic
sigma-dependent fractal dimension.Comment: 9 pages, 8 figure
Classical and quantum harmonic mean-field models coupled intensively and extensively with external baths
We study the nonequilibrium steady-state of a fully-coupled network of N quantum harmonic oscillators, interacting with two thermal reservoirs. Given the long-range nature of the couplings, we consider two setups: one in which the number of particles coupled to the baths is fixed (intensive coupling) and one in which it is proportional to the size N (extensive coupling). In both cases, we compute analytically the heat fluxes and the kinetic temperature distributions using the nonequilibrium Green's function approach, both in the classical and quantum regimes. In the large N limit, we derive the asymptotic expressions of both quantities as a function of N and the temperature difference between the baths. We discuss a peculiar feature of the model, namely that the bulk temperature vanishes in the thermodynamic limit, due to a decoupling of the dynamics of the inner part of the system from the baths. At variance with the usual case, this implies that the steady-state depends on the initial state of the bulk particles. We also show that quantum effects are relevant only below a characteristic temperature that vanishes as 1/N. In the quantum low-temperature regime the energy flux is proportional to the universal quantum of thermal conductance
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