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Biomedical subjects

M Marsili

Publications and source records attributed to M Marsili.

At least 19 recordsLinked to original sources
PubMed2001

Continuum time limit and stationary states of the minority game.

We discuss in detail the derivation of stochastic differential equations for the continuum time limit of the minority game. We show that all properties of the minority game can be understood by a careful theoretical analysis of such equations. In particular, (i) we confirm that the stationary state properties are given by the ground state configurations of a disordered (soft) spin system, (ii) we derive the full stationary state distribution, (iii) we characterize the dependence on initial conditions in the symmetric phase, and (iv) we clarify the behavior of the system as a function of the learning rate. This leaves us with a complete and coherent picture of the collective behavior of the minority game. Strikingly we find that the temperaturelike parameter, which is introduced in the choice behavior of individual agents turns out to play the role, at the collective level, of the inverse of a thermodynamic temperature.

Journal Article↗
PubMed2001

Learning to coordinate in a complex and nonstationary world.

We study analytically and by computer simulations a complex system of adaptive agents with finite memory. Borrowing the framework of the minority game and using the replica formalism we show the existence of an equilibrium phase transition as a function of the ratio between the memory lambda and the learning rates Gamma of the agents. We show that, starting from a random configuration, a dynamic phase transition also exists, which prevents agents from reaching optimal coordination. Furthermore, in a nonstationary environment, we show by numerical simulations that the phase transition becomes discontinuous.

Game Theory↗
PubMed2001

Data clustering and noise undressing of correlation matrices.

We discuss an approach to data clustering. We find that maximum likelihood leads naturally to an Hamiltonian of Potts variables that depends on the correlation matrix and whose low temperature behavior describes the correlation structure of the data. For random, uncorrelated data sets no correlation structure emerges. On the other hand, for data sets with a built-in cluster structure, the method is able to detect and recover efficiently that structure. Finally we apply the method to financial time series, where the low-temperature behavior reveals a nontrivial clustering.

Journal Article↗
PubMed2001

Haemostatic molecular markers in patients undergoing radical retropubic prostatectomy for prostate cancer and submitted to prophylaxis with unfractioned or low molecular weight heparin.

BACKGROUND: Deep vein thrombosis and subsequently pulmonary embolism are the most common causes of increased post-operative morbidity and mortality in patients with pelvic or abdominal cancer. Aim of the study was to evaluate variations in coagulative parameters induced by two accepted primary prophylaxis patterns: standardized low doses of unfractioned heparin (UFH) or single doses of low molecular weight heparin (LMWH) in cancer patients submitted to radical retropubic prostatectomy. METHODS: Fifty patients (45-75 yr) were randomly assigned two groups. Group 1 received UFH (5000 units s.c. x 3 daily); group 2 received calcium nadroparin (single daily dose of 0.3 ml s.c.). In both groups prophylaxis began preoperatively and was maintained throughout the entire hospital-stay. Blood cell, platelet count, coagulative system exploring tests, thrombotic molecular markers, and physiological inhibitors of coagulation were determined at baseline conditions and on the first and seventh day after surgery. RESULTS: Preoperative values of fibrinogen, F1+2 fragment, TAT and D-dimer resulted over normal range in both groups. A significant increase of these markers was observed also during the post-operative period. PT, aPTT, ATIII, PC, total and free PS showed the most substantial changes on the 1st post operative day, though their values ranged within normal levels on the three sampling times. The levels of haemostatic markers demonstrated a baseline hypercoagulability, probably related to cancer and thrombin activation caused by prostatectomy. Despite this thrombophylic state, neither of the two groups presented symptomatic bleeding or thromboembolic complications. CONCLUSIONS: These results prove that a single daily dose of nadroparin has been safe and efficient as a thrice-daily dose of UFH, with a better risk/benefit relationship.

Aged↗
PubMed2000

Nonequilibrium phase transition in a model for social influence

We present extensive numerical simulations of the Axelrod's model for social influence, aimed at understanding the formation of cultural domains. This is a nonequilibrium model with short range interactions and a remarkably rich dynamical behavior. We study the phase diagram of the model and uncover a nonequilibrium phase transition separating an ordered (culturally polarized) phase from a disordered (culturally fragmented) one. The nature of the phase transition can be continuous or discontinuous depending on the model parameters. At the transition, the size of cultural regions is power-law distributed.

Journal Article↗
PubMed2000

Statistical mechanics of systems with heterogeneous agents: minority games

We study analytically a simple game theoretical model of heterogeneous interacting agents. We show that the stationary state of the system is described by the ground state of a disordered spin model which is exactly solvable within the simple replica symmetric ansatz. Such a stationary state differs from the Nash equilibrium where each agent maximizes her own utility. The latter turns out to be characterized by a replica symmetry broken structure. Numerical results fully agree with our analytical findings.

Journal Article↗
PubMed2000

Relevance of memory in minority games

By considering diffusion on De Bruijn graphs, we study in detail the dynamics of the histories in the minority game, a model of competition between adaptative agents. Such graphs describe the structure of the temporal evolution of M bit strings, each node standing for a given string, i.e., a history in the minority game. We show that the frequency of visit of each history is not given by 1/2(M) in the limit of large M when the transition probabilities are biased. Consequently, all quantities of the model do significantly depend on whether the histories are real or uniformly and randomly sampled. We expose a self-consistent theory of the case of real histories, which turns out to be in very good agreement with numerical simulations.

Journal Article↗
PubMed2000

First-order phase transition in a nonequilibrium growth process

We introduce a simple continuous model for nonequilibrium surface growth. The dynamics of the system is defined by the Kardar-Parisi-Zhang equation with a Morse-like potential representing a short range interaction between the surface and the substrate. The mean field solution displays a nontrivial phase diagram with a first-order transition between a growing and a bound surface, associated with a region of coexisting phases, and a tricritical point where the transition becomes second order. Numerical simulations in three dimensions show quantitative agreement with mean field results, and the features of the phase space are preserved even in two dimensions.

Journal Article↗
PubMed1999

Searching for a general anaesthesia protocol for rapid detoxification from opioids.

The technique for ultra rapid opioid detoxification is designed to shorten the detoxification period by precipitating withdrawal by the administration of opioid antagonists such as naloxone or naltrexone. This procedure is performed under deep sedation or general anaesthesia to ensure that the patient does not consciously experience the acute withdrawal phase. This strategy has aroused controversy regarding the risk of sedation or anaesthesia in this situation. In the present study, ultra rapid opioid detoxification was carried out in 12 opiate-addicted patients by infusion of naloxone 4 mg for a period of 5 h using controlled ventilation during general anaesthesia, induced and maintained with midazolam, propofol and atracurium. Invasive cardiovascular and respiratory monitoring was performed, and withdrawal signs were evaluated using a graduated scale. Anaesthesia was maintained for another hour after the completion of the naloxone infusion. The validity of this anaesthesia protocol was confirmed by the relative lack of change in the patients' haemodynamic values associated with mild signs of withdrawal.

Adult↗
PubMed1999

Scale invariant dynamics of surface growth.

We describe in detail and extend a recently introduced nonperturbative renormalization group (RG) method for surface growth. The scale invariant dynamics which is the key ingredient of the calculation is obtained as the fixed point of a RG transformation relating the representation of the microscopic process at two different coarse-grained scales. We review the RG calculation for systems in the Kardar-Parisi-Zhang (KPZ) universality class and compute the roughness exponent for the strong coupling phase in dimensions from 1 to 9. Discussions of the approximations involved and possible improvements are also presented. Moreover, very strong evidence of the absence of a finite upper critical dimension for KPZ growth is presented. Finally, we apply the method to the linear Edwards-Wilkinson dynamics where we reproduce the known exact results, proving the ability of the method to capture qualitatively different behaviors.

Journal Article↗
PubMed1999

Phase transition and symmetry breaking in the minority game.

We show that the minority game, a model of interacting heterogeneous agents, can be described as a spin system and displays a phase transition between a symmetric phase and a symmetry broken phase where the game's outcome is predictable. As a result a "spontaneous magnetization" arises in the spin formalism.

Journal Article↗