1,720,983 research outputs found
Ground state of the Bethe lattice spin glass and running time of an exact optimization algorithm
Get PDFWe study the Ising spin glass on random graphs with fixed connectivity z and with a Gaussian distribution of the couplings, with mean mu and unit variance. We compute exact ground states by branch-and-cut with z=4,6 and system sizes up to 1280 spins, for different values of mu . We locate the spin-glass/ferromagnet phase transition Near the phase transition, we observe a sharp change of the median running time of our implementation of the algorithm, consistent with a change from a polynomial dependence on the system size, deep in the ferromagnetic phase, to slower than polynomial in the spin-glass phase
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Inverse inference of a quantum spin glass
Get PDFTreballs Finals de Màster en Física dels Sistemes Complexos i Biofísica, Facultat de Física, Universitat de Barcelona. Curs: 2022-2023. Tutor: Matteo PalassiniIn statistical physics, inverse problems arise when we need to design a manybody system with particular desired properties. Rather than calculating observables based on known model parameters, inverse problems involve inferring the parameters of a model based on observations. In my final degree project, we studied the inverse problem for the Viana-Bray spin glass model with a discrete distribution of the couplings, and with simulated annealing, we tried to infer the couplings of the system with the maximum pseudolikelihood method. In this project, we studied the inverse problem for the quantum Viana-Bray spin glass model with the same coupling distribution. The goal was to extend the pseudolikelihood maximization approach to a quantum spin glass with a transverse field since most research efforts have focused on the classical version and hardly anything is known about its quantum counterpart. To achieve this, we generated equilibrium configurations from the quantum partition function using quantum Monte Carlo techniques, and then we employed simulated annealing to maximize the pseudolikelihood function and infer the couplings of the system. We derived a modified version of the pseudolikelihood function from the initial proposal after closely following the approach used in the classical case. We found that when introducing the transverse field in the system, the algorithm was still able to infer the couplings; however, because of how the quantum system is treated, certain modifications had to be made to the pseudolikelihood method. Moreover, as in the classical case, we found that the algorithm performed the best around the phase transition boundaries
Dynamic message-passing approach to epidemic spreading
Get PDFTreballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2021, Tutor: Matteo PalassiniIn this work, we delve into the analysis of the propagation of infectious diseases described by the SIR epidemiological model on networks whose connections change over time. We use a fast dynamic message-passing method to estimate the marginal probabilities that each node in one of these networks is in a given state at a certain time. After testing the validity of the predictions given by this approach, we apply them to the problem of inferring the origin of an epidemic on a dynamic network
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Spectral density of Wishart matrices and the replica method
Get PDFTreballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2025, Tutor: Matteo PalassiniRandom matrix theory provides a powerful framework for understanding universal features of complex systems, especially in the large-size limit. In this work, we study the spectral density of Wishart-Laguerre random matrices using the Edwards-Jones formula. We reinterpret the averaged quantity in the Edwards-Jones formula as the partition function of a disordered system and apply tools from statistical physics. Our results illustrate how techniques from statistical physics of disordered systems naturally extend to random matrix theory, offering physical insight and analytical methods for exploring spectral properties in complex system
Quantum annealing via real-time Schrödinger dynamics
Get PDFTreballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2020, Tutor: Matteo PalassiniQuantum annealing provides a way to potentially solve optimisation problems faster than its classical counterpart, simulated annealing, but the suitability of this method is still open to debate. Here we present an example of simulated quantum annealing of a 2D Ising spin glass model by the real-time evolution governed by the Schrödinger equation. A bias towards certain ground states is observed for larger annealing times. The final states are compatible with the results obtained in a previous study using path-integral Monte Carlo for simulated quantum annealin
Ground-state energy fluctuations in the Sherrington-Kirkpatrick model
No full text4 pages, 4 eps figures. Some changes in the text, references corrected, plot of Tracy-Widom distribution addedThe probability distribution function (PDF) of the ground-state energy in the Sherrington-Kirkpatrick spin-glass model is numerically determined by collecting a large statistical sample of ground states, computed using a genetic algorithm. It is shown that the standard deviation of the ground-state energy per spin scales with the number of spins, N, as N^{-\rho} with \rho \simeq 0.765, but the value \rho=3/4 is also compatible with the data, while the previously proposed value \rho=5/6 is ruled out. The PDF satisfies finite-size scaling with a non-Gaussian asymptotic PDF, which can be fitted remarkably well by the Gumbel distribution for the m-th smallest element in a set of random variables, with m \simeq 6
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