1,720,968 research outputs found
Fisica generale. Meccanica
No full textQuesto eserciziario è rivolto a tutti gli studenti dei corsi diFisica Generale I della laurea triennale. Copre gli argomenti relativialla Meccanica ed in particolare alla Meccanica del punto e allaMeccanica dei sistemi. Eì esclusa invece la parte di Termodinamica,trattata in molti corsi di laurea in maniera separata. L'uso dell'eserciziario non prevede specifiche propedeuticità, senon buone basi di matematica elementare. Lo studente troverà in ognicaso un'estesa sezione di richiami di analisi matematica e digeometria. Inoltre nello svolgimento degli esercizi si è cercato dirichiamare di volta in volta tutti gli strumenti necessari per laricerca della soluzione. Il volume è diviso in due parti che coprono la Dinamica del punto e la Dinamica dei sistemi, ulteriormente suddivisi in capitoli e paragrafi tematici che tendono a seguire il normale flusso e ordinamento dei corsi di laurea. All'interno di ogni capitolo sono proposti diversi esercizi che tentano di coprire il più possibile lo spettro di problemi di Meccanica proponibili a livello dei corsi di Fisica Generale. Abbiamo cercato di compiere uno sforzo di astrazione per sviscerare, a partire da problemi specifici, considerazioni generali per giungere alla definizione di esercizi prototipici. Ad ogni esercizio è associato un indicatore di difficoltà (in una scala da 1 a 3) che permette allo studente di programmare lo studio affrontando eserciziprogressivamente più complessi e di verificare le propriecompetenze. Un paragrafo di riepilogo chiude ogni sezionepresentando esercizi per la cui soluzione è necessario un buoncontrollo di tutti gli strumenti appresi nei paragrafi precedenti
Friendship, collaboration and semantics in Flickr: from social interaction to semantic similarity
No full textWe study the semantic assortativity in the social networks hosted by the Flickr folksonomy, based both on the contact data and on the group membership data provided by the users. The social network built this way are complex one. Besides, one observes a clear assortativity pattern, stronger than in a suitable null model adopted for a comparison. Nevertheless, such semantical similarity does not appear to develop during the community evolution, but is rather the result of a pre-existing shared background between users. © 2010 ACM
On the Emergence of Syntactic Structures: Quantifying and Modeling Duality of Patterning
No full textThe complex organization of syntax in hierarchical structures is one of the core design features of human language. Duality of patterning refers, for instance, to the organization of the meaningful elements in a language at two distinct levels: a combinatorial level, where meaningless forms are combined into meaningful forms; and a compositional level, where meaningful forms are composed into larger lexical units. The question remains wide open regarding how such structures could have emerged. The aim of this paper is to address these two aspects in a self-consistent way. First, we introduce suitable measures to quantify the level of combinatoriality and compositionality in a language, and we present a framework to estimate these observables in human natural languages. Second, we show that a recently introduced multi-agent modeling scheme, namely the Blending Game, provides a mathematical framework to address the problem of how a population of individuals can bootstrap combinatoriality and compositionality. Theoretical predictions based on this model turn out to be in good agreement with empirical data. It is remarkable that the two sides of duality of patterning emerge simultaneously as a consequence of a pure cultural dynamics in a simulated environment that contains meaningful relations, provided a simple constraint on message transmission fidelity is also considered
Experimental tribe: a general platform for web-gamig and social computation
No full textIn the last few years the Web has been progressively acquiring the status of an in- frastructure for “social computing” that allows to coordinate the cognitive abilities of human agents in online communities, and steer the collective user activity to- wards predefined goals. This general trend is also triggering the adoption of web- games as a very interesting laboratory to run experiments in the social-sciences and whenever the contribution of human beings is crucially required for research purposes. This paper introduces Experimental Tribe (ET), a novel general purpose web-based platform for social computation
The role of the disorder on the anomalous diffusion in 3d crowded media: simulations and nmr experiments
No full textThe dynamics of correlated novelties
No full textNovelties are a familiar part of daily life. They are also fundamental to the evolution of biological systems, human society, and technology. By opening new possibilities, one novelty can pave the way for others in a process that Kauffman has called expanding the adjacent possible . The dynamics of correlated novelties, however, have yet to be quantified empirically or modeled mathematically. Here we propose a simple mathematical model that mimics the process of exploring a physical, biological, or conceptual space that enlarges whenever a novelty occurs. The model, a generalization of Polya's urn, predicts statistical laws for the rate at which novelties happen (Heaps' law) and for the probability distribution on the space explored (Zipf's law), as well as signatures of the process by which one novelty sets the stage for another. We test these predictions on four data sets of human activity: the edit events of Wikipedia pages, the emergence of tags in annotation systems, the sequence of words in texts, and listening to new songs in online music catalogues. By quantifying the dynamics of correlated novelties, our results provide a starting point for a deeper understanding of the adjacent possible and its role in biological, cultural, and technological evolution
Molecular dynamics simulation of vascular network formation
No full textEndothelial cells are responsible for the formation of the capillary blood vessel network. We describe a system of endothelial cells by means of two-dimensional molecular dynamics simulations of point-like particles. Cell motion is governed by the gradient of the concentration of a chemical substance that they produce (chemotaxis). The typical time of degradation of the chemical substance introduces a characteristic length in the system. We show that point-like model cells form network resembling structures tuned by this characteristic length, before collapsing altogether. Successively, we improve the non-realistic point-like model cells by introducing an isotropic repulsive force between them and a velocity dependent force mimicking the observed peculiarity of endothelial cells of preserving the direction of their motion (persistence). This more realistic model does not show a clear network formation. We ascribe this partial fault in reproducing the experiments to the static geometry of our model cells that, in reality, change their shapes by elongating toward neighboring cells
Vertex intrinsic fitness: How to produce arbitrary scale-free networks
No full textWe study a recent model of random networks based on the presence of an intrinsic character of the vertices called fitness. The vertex fitnesses are drawn from a given probability distribution density. The edges between pairs of vertices are drawn according to a linking probability function depending on the fitnesses of the two vertices involved. We study here different choices for the probability distribution densities and the linking functions. We find that, irrespective of the particular choices, the generation of scale-free networks is straightforward. We then derive the general conditions under which scale-free behavior appears. This model could then represent a possible explanation for the ubiquity and robustness of such structures
Structural disorder and anomalous diffusion in random packing of spheres
Full text linkNowadays Nuclear Magnetic Resonance diffusion (dNMR) measurements of water molecules in heterogeneous systems have broad applications in material science, biophysics and medicine. Up to now, microstructural rearrangement in media has been experimentally investigated by studying the diffusion coefficient (D(t)) behavior in the tortuosity limit. However, this method is not able to describe structural disorder and transitions in complex systems. Here we show that, according to the continuous time random walk framework, the dNMR measurable parameter alpha, quantifying the anomalous regime of D(t), provides a quantitative characterization of structural disorder and structural transition in heterogeneous systems. To demonstrate this, we compare a measurements obtained in random packed monodisperse micro-spheres with Molecular Dynamics simulations of disordered porous media and 3D Monte Carlo simulation of particles diffusion in these kind of systems. Experimental results agree well with simulations that correlate the most used parameters and functions characterizing the disorder in porous media
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