Critical behavior and Griffiths effects in the disordered contact process

We study the nonequilibrium phase transition in the one-dimensional contact process with quenched spatial disorder by means of large-scale Monte-Carlo simulations for times up to $10^9$ and system sizes up to $10^7$ sites. In agreement with recent predictions of an infinite-randomness fixed point, our simulations demonstrate activated (exponential) dynamical scaling at the critical point. The critical behavior turns out to be universal, even for weak disorder. However, the approach to this asymptotic behavior is extremely slow, with crossover times of the order of $10^4$ or larger. In the Griffiths region between the clean and the dirty critical points, we find power-law dynamical behavior with continuously varying exponents. We discuss the generality of our findings and relate them to a broader theory of rare region effects at phase transitions with quenched disorder.Comment: 10 pages, 8 eps figures, final version as publishe

Dynamic networks and directed percolation

We introduce a model for dynamic networks, where the links or the strengths of the links change over time. We solve the model by mapping dynamic networks to the problem of directed percolation, where the direction corresponds to the evolution of the network in time. We show that the dynamic network undergoes a percolation phase transition at a critical concentration $p_c$, which decreases with the rate $r$ at which the network links are changed. The behavior near criticality is universal and independent of $r$. We find fundamental network laws are changed. (i) For Erd\H{o}s-R\'{e}nyi networks we find that the size of the giant component at criticality scales with the network size $N$ for all values of $r$, rather than as $N^{2/3}$. (ii) In the presence of a broad distribution of disorder, the optimal path length between two nodes in a dynamic network scales as $N^{1/2}$, compared to $N^{1/3}$ in a static network.Comment: 10 pages 5 figures; corrected metadata onl

Community Detection in Multiplex Networks

A multiplex network models different modes of interaction among same-type entities. In this article we provide a taxonomy of community detection algorithms in multiplex networks. We characterize the different algorithms based on various properties and we discuss the type of communities detected by each method. We then provide an extensive experimental evaluation of the reviewed methods to answer three main questions: to what extent the evaluated methods are able to detect ground-truth communities, to what extent different methods produce similar community structures and to what extent the evaluated methods are scalable. One goal of this survey is to help scholars and practitioners to choose the right methods for the data and the task at hand, while also emphasizing when such choice is problematic.Comment: 55 pages. Accepted for publication on ACM Computing Surveys in a shorter versio

Monte-Carlo simulations of disordered non-equilibrium phase transitions

This thesis focuses on the effects of both correlated and non-correlated disorder on non-equilibrium phase transitions, specifically those found in the d-dimensional contact process. These effects are studied by means of extensive Monte-Carlo simulations. The scaling behavior of various parameters is evaluated for both cases, and the results are compared with theory. For the correlated disorder case, the stationary density in the vicinity of the transition is also examined, and found to be smeared. The behavior in both cases can be understood as the results of rare regions where the system is locally free of disorder. For point-like defects, i.e., uncorrelated disorder, the rare regions are of finite size and cannot undergo a true phase transition. Instead, they fluctuate slowly which gives rise to Griffiths effects. In contrast, if the rare regions are infinite in at least one dimension, a stronger effect occurs: each rare region can independently undergo the phase transition and develop a nonzero steady state density. This leads to a smearing of the global transition --Abstract, page iv

Dynamic and interacting complex networks

Thesis (Ph.D.)--Boston UniversityPLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at [email protected]. Thank you.This thesis employs methods of statistical mechanics and numerical simulations to study some aspects of dynamic and interacting complex networks. The mapping of various social and physical phenomena to complex networks has been a rich field in the past few decades. Subjects as broad as petroleum engineering, scientific collaborations, and the structure of the internet have all been analyzed in a network physics context, with useful and universal results. In the first chapter we introduce basic concepts in networks, including the two types of network configurations that are studied and the statistical physics and epidemiological models that form the framework of the network research, as well as covering various previously-derived results in network theory that are used in the work in the following chapters. In the second chapter we introduce a model for dynamic networks, where the links or the strengths of the links change over time. We solve the model by mapping dynamic networks to the problem of directed percolation, where the direction corresponds to the time evolution of the network. We show that the dynamic network undergoes a percolation phase transition at a critical concentration Pc, that decreases with the rate r at which the network links are changed. The behavior near criticality is universal and independent of r. We find that for dynamic random networks fundamental laws are changed: i) The size of the giant component at criticality scales with the network size N for all values of r, rather than as N^(2/3) in static network, ii) In the presence of a broad distribution of disorder, the optimal path length between two nodes in a dynamic network scales as N^(1/2), compared to N^(1/3) in a static network. The third chapter consists of a study of the effect of quarantine on the propagation of epidemics on an adaptive network of social contacts. For this purpose, we analyze the susceptible-infected-recovered model in the presence of quarantine, where susceptible individuals protect themselves by disconnecting their links to infected neighbors with probability w and reconnecting them to other susceptible individuals chosen at random. Starting from a single infected individual, we show by an analytical approach and simulations that there is a phase transition at a critical rewiring (quarantine) threshold We separating a phase (w wc) where the disease does not spread out. We find that in our model the topology of the network strongly affects the size of the propagation and that wc increases with the mean degree and heterogeneity of the network. We also find that wc is reduced if we perform a preferential rewiring, in which the rewiring probability is proportional to the degree of infected nodes. In the fourth chapter, we study epidemic processes on interconnected network systems, and find two distinct regimes. In strongly-coupled network systems, epidemics occur simultaneously across the entire system at a critical value f3e· In contrast, in weakly-coupled network systems, a mixed phase exists below f3e, where an epidemic occurs in one network but does not spread to the coupled network. We derive an expression for the network and disease parameters that allow this mixed phase and verify it numerically. Public health implications of communities comprising these two classes of network systems are also mentioned.2031-01-0

Monte Carlo Simulations of the Smeared Phase Transition in a Contact Process with Extended Defects

We study the nonequilibrium phase transition in a contact process with extended quenched defects by means of Monte Carlo simulations. We find that the spatial disorder correlations dramatically increase the effects of the impurities. As a result, the sharp phase transition is completely destroyed by smearing. This is caused by effects similar to but stronger than the usual Griffiths phenomena, namely, rare strongly coupled spatial regions can undergo the phase transition independently from the bulk system. We determine both the stationary density in the vicinity of the smeared transition and its time evolution, and compare the simulation results to a recent theory based on extremal statistics

Le prêt numérique contrôlé des livres de bibliothèque au Canada

This paper explores legal considerations for how libraries in Canada can lend digital copies of books. It is an adaptation of A Whitepaper on Controlled Digital Lending of Library Books by David R. Hansen and Kyle K. Courtney, and draws heavily on this source in its content, with the permission of the authors. Our paper considers the legal and policy rationales for the process—“controlled digital lending”—in Canada, as well as a variety of risk factors and practical considerations that can guide libraries seeking to implement such lending, with the intention of helping Canadian libraries to explore controlled digital lending in our own Canadian legal and policy context. Our goal is to help libraries and their lawyers become better informed about controlled digital lending as an approach, offer the basis of the legal rationale for its use in Canada, and suggest situations in which this rationale might be strongest.Cet article tient compte des considérations juridiques concernant la façon dont les bibliothèques au Canada peuvent prêter des copies numériques de livres. Il s’agit d’une adaptation de A Whitepaper on Controlled Digital Lending of Library Books par David R. Hansen et Kyle K. Courtney et son contenu s’inspire largement de cette source, avec la permission des auteurs. Notre article examine les justifications juridiques et politiques du processus - « prêt numérique contrôlé » - au Canada ainsi qu’une variété de facteurs de risque et des considérations pratiques qui peuvent guider les bibliothèques cherchant à mettre en place un tel prêt dans le but d’aider les bibliothèques canadiennes à explorer le prêt numérique contrôlé dans notre propre contexte juridique et politique canadien. Notre objectif est d’aider les bibliothèques et leurs avocats à être mieux informés sur le prêt numérique contrôlé en tant qu’approche, d’offrir la base du raisonnement juridique pour son utilisation au Canada et de suggérer des situations dans lesquelles ce raisonnement pourrait être le plus fort