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Search Results: 1 - 10 of 325392 matches for " S. Stramaglia "
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Causal Information Approach to Partial Conditioning in Multivariate Data Sets
D. Marinazzo,M. Pellicoro,S. Stramaglia
Computational and Mathematical Methods in Medicine , 2012, DOI: 10.1155/2012/303601
Abstract: When evaluating causal influence from one time series to another in a multivariate data set it is necessary to take into account the conditioning effect of the other variables. In the presence of many variables and possibly of a reduced number of samples, full conditioning can lead to computational and numerical problems. In this paper, we address the problem of partial conditioning to a limited subset of variables, in the framework of information theory. The proposed approach is tested on simulated data sets and on an example of intracranial EEG recording from an epileptic subject. We show that, in many instances, conditioning on a small number of variables, chosen as the most informative ones for the driver node, leads to results very close to those obtained with a fully multivariate analysis and even better in the presence of a small number of samples. This is particularly relevant when the pattern of causalities is sparse.
Statistical mechanics approach to the phase unwrapping problem
S. Stramaglia,A. Refice,L. Guerriero
Physics , 2000, DOI: 10.1016/S0378-4371(99)00462-8
Abstract: The use of Mean-Field theory to unwrap principal phase patterns has been recently proposed. In this paper we generalize the Mean-Field approach to process phase patterns with arbitrary degree of undersampling. The phase unwrapping problem is formulated as that of finding the ground state of a locally constrained, finite size, spin-L Ising model under a non-uniform magnetic field. The optimization problem is solved by the Mean-Field Annealing technique. Synthetic experiments show the effectiveness of the proposed algorithm.
Cottingham formula and the pion electromagnetic mass difference at finite temperature
M. Ladisa,G. Nardulli,S. Stramaglia
Physics , 1999, DOI: 10.1016/S0370-2693(99)01055-2
Abstract: We generalize the Cottingham formula at finite (T\neq 0) temperature by using the imaginary time formalism. The Cottingham formula gives the theoretical framework to compute the electromagnetic mass differences of the hadrons using a dispersion relation approach. It can be also used in other contexts, such as non leptonic weak decays, and its generalization to finite temperature might be useful in evaluating thermal effects in these processes. As an application we compute the \pi^+-\pi^0 mass difference at T\neq 0; at small T we reproduce the behaviour found by other authors: \delta m^2(T)= \delta m^2(0)+\mathcal{O}(\alpha T^2), while for moderate T, near the deconfinement temperature, we observe deviations from this behaviour.
Monte Carlo study of the growth of striped domains
E. N. M. Cirillo,G. Gonnella,S. Stramaglia
Physics , 1997,
Abstract: We analyze the dynamical scaling behavior in a two-dimensional spin model with competing interactions after a quench to a striped phase. We measure the growth exponents studying the scaling of the interfaces and the scaling of the shrinking time of a ball of one phase plunged into the sea of another phase. Our results confirm the predictions found in previous papers. The correlation functions measured in the direction parallel and transversal to the stripes are different as suggested by the existence of different interface energies between the ground states of the model. Our simulations show anisotropic features for the correlations both in the case of single-spin-flip and spin-exchange dynamics.
A Matrix Model for Random Surfaces with Dynamical Holes
G. M. Cicuta,L. Molinari,E. Montaldi,S. Stramaglia
Physics , 1994, DOI: 10.1088/0305-4470/29/14/006
Abstract: A matrix model to describe dynamical loops on random planar graphs is analyzed. It has similarities with a model studied by Kazakov, few years ago, and the O(n) model by Kostov and collaborators. The main difference is that all loops are coherently oriented and empty. The free energy is analytically evaluated and the two critical phases are analyzed, where the free energy exhibits the same critical behaviour of Kazakov's model, thus confirming the universality of the description in the continuum limit (surface with small holes, and the tearing phase). A third phase occurs on the boundary separating the above phase regions, and is characterized by a different singular behaviour, presumably due to the orientation of loops.
Phase diagram of a generalized Winfree model
F. Giannuzzi,D. Marinazzo,G. Nardulli,M. Pellicoro,S. Stramaglia
Physics , 2008, DOI: 10.1103/PhysRevE.75.051104
Abstract: We study the phase diagram of a generalized Winfree model. The modification is such that the coupling depends on the fraction of synchronized oscillators, a situation which has been noted in some experiments on coupled Josephson junctions and mechanical systems. We let the global coupling k be a function of the Kuramoto order parameter r through an exponent z such that z=1 corresponds to the standard Winfree model, z<1 strengthens the coupling at low r (low amount of synchronization) and, at z>1, the coupling is weakened for low r. Using both analytical and numerical approaches, we find that z controls the size of the incoherent phase region, and one may make the incoherent behavior less typical by choosing z<1. We also find that the original Winfree model is a rather special case, indeed the partial locked behavior disappears for z>1. At fixed k and varying gamma, the stability boundary of the locked phase corresponds to a transition that is continuous for z<1 and first-order for z>1. This change in the nature of the transition is in accordance with a previous study on a similarly modified Kuramoto model.
Quartic Anharmonic Oscillator and Random Matrix Theory
G. M. Cicuta,S. Stramaglia,A. G. Ushveridze
Physics , 1995, DOI: 10.1142/S021773239600014X
Abstract: In this paper the relationship between the problem of constructing the ground state energy for the quantum quartic oscillator and the problem of computing mean eigenvalue of large positively definite random hermitean matrices is established. This relationship enables one to present several more or less closed expressions for the oscillator energy. One of such expressions is given in the form of simple recurrence relations derived by means of the method of orthogonal polynomials which is one of the basic tools in the theory of random matrices.
Cost functions for pairwise data clustering
L. Angelini,L. Nitti,M. Pellicoro,S. Stramaglia
Physics , 2001, DOI: 10.1016/S0375-9601(01)00373-5
Abstract: Cost functions for non-hierarchical pairwise clustering are introduced, in the probabilistic autoencoder framework, by the request of maximal average similarity between the input and the output of the autoencoder. The partition provided by these cost functions identifies clusters with dense connected regions in data space; differences and similarities with respect to a well known cost function for pairwise clustering are outlined.
Anisotropic dynamical scaling in a spin model with competing interactions
E. N. M. Cirillo,G. Gonnella,S. Stramaglia
Physics , 1997, DOI: 10.1103/PhysRevE.56.5065
Abstract: Results are presented for the kinetics of domain growth of a two-dimensional Ising spin model with competing interactions quenched from a disordered to a striped phase. The domain growth exponent are $\beta=1/2$ and $\beta=1/3$ for single-spin-flip and spin-exchange dynamics, as found in previous simulations. However the correlation functions measured in the direction parallel and transversal to the stripes are different as suggested by the existence of different interface energies between the ground states of the model. In the case of single-spin-flip dynamics an anisotropic version of the Ohta-Jasnow-Kawasaki theory for the pair scaling function can be used to fit our data.
Chaos in neural networks with a nonmonotonic transfer function
D. Caroppo,M. Mannarelli,G. Nardulli,S. Stramaglia
Physics , 1999, DOI: 10.1103/PhysRevE.60.2186
Abstract: Time evolution of diluted neural networks with a nonmonotonic transfer function is analitically described by flow equations for macroscopic variables. The macroscopic dynamics shows a rich variety of behaviours: fixed-point, periodicity and chaos. We examine in detail the structure of the strange attractor and in particular we study the main features of the stable and unstable manifolds, the hyperbolicity of the attractor and the existence of homoclinic intersections. We also discuss the problem of the robustness of the chaos and we prove that in the present model chaotic behaviour is fragile (chaotic regions are densely intercalated with periodicity windows), according to a recently discussed conjecture. Finally we perform an analysis of the microscopic behaviour and in particular we examine the occurrence of damage spreading by studying the time evolution of two almost identical initial configurations. We show that for any choice of the parameters the two initial states remain microscopically distinct.
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