2,216 research outputs found
Directed cycles and related structures in random graphs: II--Dynamic properties
We study directed random graphs (random graphs whose edges are directed) as
they evolve in discrete time by the addition of nodes and edges. For two
distinct evolution strategies, one that forces the graph to a condition of near
acyclicity at all times and another that allows the appearance of nontrivial
directed cycles, we provide analytic and simulation results related to the
distributions of degrees. Within the latter strategy, in particular, we
investigate the appearance and behavior of the strong components that were our
subject in the first part of this study.Comment: submitted to Physica
Early appraisal of the fixation probability in directed networks
In evolutionary dynamics, the probability that a mutation spreads through the
whole population, having arisen in a single individual, is known as the
fixation probability. In general, it is not possible to find the fixation
probability analytically given the mutant's fitness and the topological
constraints that govern the spread of the mutation, so one resorts to
simulations instead. Depending on the topology in use, a great number of
evolutionary steps may be needed in each of the simulation events, particularly
in those that end with the population containing mutants only. We introduce two
techniques to accelerate the determination of the fixation probability. The
first one skips all evolutionary steps in which the number of mutants does not
change and thereby reduces the number of steps per simulation event
considerably. This technique is computationally advantageous for some of the
so-called layered networks. The second technique, which is not restricted to
layered networks, consists of aborting any simulation event in which the number
of mutants has grown beyond a certain threshold value, and counting that event
as having led to a total spread of the mutation. For large populations, and
regardless of the network's topology, we demonstrate, both analytically and by
means of simulations, that using a threshold of about 100 mutants leads to an
estimate of the fixation probability that deviates in no significant way from
that obtained from the full-fledged simulations. We have observed speedups of
two orders of magnitude for layered networks with 10000 nodes
Comer fuera de casa en Brasil hoy
This article aims to explore the meanings and practices of eating out in Brazilian daily life. Since the mid-1980s, this practice has been spreading and diversifying in Brazil. Based on a series of three qualitative/quantitative surveys carried out between 2005 and 2015 in major Brazilian urban centres, we identified a clear distinction between eating out at weekdays and at weekends. At weekdays, by association with work, commitments and studies, this practice is perceived as both an obligation and an expense. At weekends, however, its association with leisure, family and friends makes it a pleasant investment and a cultural product. In this context, the meanings of eating at home versus eating out change, as well as the sense of the meals, selection of restaurants, sociability patterns and even the ethos of eating.Este artículo tiene como objetivo explorar los significados y las prácticas del ‘comer fuera de casa’ en la vida diaria brasileña. Desde mediados de la década de 1980, esta práctica se ha extendido y diversificado en Brasil. Sobre la base de una serie de tres encuestas cualitativas/cuantitativas, llevadas a cabo entre 2005 y 2015 en los principales centros urbanos de Brasil, identificamos una clara distinción entre comer afuera los días de semana y los fines de semana. En los días de la semana, por asociación con el trabajo, los compromisos y los estudios, esta práctica se percibe como una obligación y un gasto. Los fines de semana, sin embargo, su asociación con el ocio, la familia y los amigos la convierte en una inversión agradable y un producto cultural. En este contexto, los significados de comer en casa versus comer afuera cambian, así como el sentido de las comidas, la selección de restaurantes, los patrones de sociabilidad e incluso el ethos de la comida
Crack Cocaine Use and its Relationship with Violence and Hiv
OBJECTIVES: To evaluate crack cocaine use practices, risk behaviors associated with HIV infection among drug users, and their involvement with violence. INTRODUCTION: HIV infections are frequent among drug users due to risky sexual behavior. It is generally accepted that crack cocaine use is related to increased levels of violence. Several reports point to an increase in violence from those involved in drug trafficking. Although HIV infections and risky sexual behavior among drug users have been quite well studied, there are few studies that evaluate violence as it relates to drugs, particularly crack. METHODS: A total of 350 drug users attending drug abuse treatment clinics in São Paulo, Brazil were interviewed about their risky behaviors. Each patient had a serological HIV test done. RESULTS: HIV prevalence was 6.6% (4.0 to 10.2). Violence was reported by 97% (94.7 to 99.1) of the subjects (including cases without personal involvement). Acts of violence such as verbal arguments, physical fights, threats, death threats, theft, and drug trafficking were significantly higher among crack users. A decrease in frequency of sexual intercourse was observed among users of injected drugs, though prostitution was observed as a means of obtaining drugs. A high number of crack cocaine users had a history of previous imprisonment, many for drug-related infractions. DISCUSSION: The data presented are in accordance with other reports in the literature, and they show a correlation between drug use, imprisonment, violence, and drug trafficking. CONCLUSION: A high HIV prevalence and associated risky sexual behaviors were observed among crack cocaine users. The society and the authorities that deal with violence related to crack users and drug trafficking should be aware of these problems
The emergence and historical evolution of innovation networks:On the factors promoting and hampering patent collaboration in technological lagging economies
Collaboration and research networks are nowadays central to innovation because they favor knowledge interactions and complex approaches to challenging problems. This study explores the factors underlying the emergence and evolution of innovation networks in the past, using as example the case of Spain, a backward country regarding R&D performance. Combining, for the first time, historical patent data, social network analysis, and discrete choice regression techniques we test distinct institutional, geographical, and sectoral factors that triggered or hampered collaboration over the long term, i.e., the growth in the connections of individual co-patentees within innovation groups. The findings are relevant and demonstrate, inter alia, that in the Spanish case the length of intellectual monopolies did not foster collaboration, while geographical/technological diversification was key to enhance collaborative patterns in the past. The analysis also demonstrates that the likelihood of increasing collaboration over time depended on the initial level of connections (degree) the patentee had, confirming the existence of preferential attachment, even within the context of an emerging and disconnected network. However, belonging to larger innovation groups (size of the network components) did not promote per se greater interactions, suggesting that institutional weaknesses and backward innovation trends prevented the existence of positive payoffs from increased connectivity. The results have direct R&D policy implications for both nowadays developing countries and innovation leaders.</p
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