488 research outputs found
The noisy voter model on complex networks
We propose a new analytical method to study stochastic, binary-state models
on complex networks. Moving beyond the usual mean-field theories, this
alternative approach is based on the introduction of an annealed approximation
for uncorrelated networks, allowing to deal with the network structure as
parametric heterogeneity. As an illustration, we study the noisy voter model, a
modification of the original voter model including random changes of state. The
proposed method is able to unfold the dependence of the model not only on the
mean degree (the mean-field prediction) but also on more complex averages over
the degree distribution. In particular, we find that the degree heterogeneity
---variance of the underlying degree distribution--- has a strong influence on
the location of the critical point of a noise-induced, finite-size transition
occurring in the model, on the local ordering of the system, and on the
functional form of its temporal correlations. Finally, we show how this latter
point opens the possibility of inferring the degree heterogeneity of the
underlying network by observing only the aggregate behavior of the system as a
whole, an issue of interest for systems where only macroscopic, population
level variables can be measured.Comment: 28 pages, 9 figure
Coupled dynamics of node and link states in complex networks: A model for language competition
Inspired by language competition processes, we present a model of coupled
evolution of node and link states. In particular, we focus on the interplay
between the use of a language and the preference or attitude of the speakers
towards it, which we model, respectively, as a property of the interactions
between speakers (a link state) and as a property of the speakers themselves (a
node state). Furthermore, we restrict our attention to the case of two socially
equivalent languages and to socially inspired network topologies based on a
mechanism of triadic closure. As opposed to most of the previous literature,
where language extinction is an inevitable outcome of the dynamics, we find a
broad range of possible asymptotic configurations, which we classify as: frozen
extinction states, frozen coexistence states, and dynamically trapped
coexistence states. Moreover, metastable coexistence states with very long
survival times and displaying a non-trivial dynamics are found to be abundant.
Interestingly, a system size scaling analysis shows, on the one hand, that the
probability of language extinction vanishes exponentially for increasing system
sizes and, on the other hand, that the time scale of survival of the
non-trivial dynamical metastable states increases linearly with the size of the
system. Thus, non-trivial dynamical coexistence is the only possible outcome
for large enough systems. Finally, we show how this coexistence is
characterized by one of the languages becoming clearly predominant while the
other one becomes increasingly confined to "ghetto-like" structures: small
groups of bilingual speakers arranged in triangles, with a strong preference
for the minority language, and using it for their intra-group interactions
while they switch to the predominant language for communications with the rest
of the population.Comment: 21 pages, 15 figure
Markets, herding and response to external information
We focus on the influence of external sources of information upon financial
markets. In particular, we develop a stochastic agent-based market model
characterized by a certain herding behavior as well as allowing traders to be
influenced by an external dynamic signal of information. This signal can be
interpreted as a time-varying advertising, public perception or rumor, in favor
or against one of two possible trading behaviors, thus breaking the symmetry of
the system and acting as a continuously varying exogenous shock. As an
illustration, we use a well-known German Indicator of Economic Sentiment as
information input and compare our results with Germany's leading stock market
index, the DAX, in order to calibrate some of the model parameters. We study
the conditions for the ensemble of agents to more accurately follow the
information input signal. The response of the system to the external
information is maximal for an intermediate range of values of a market
parameter, suggesting the existence of three different market regimes:
amplification, precise assimilation and undervaluation of incoming information.Comment: 30 pages, 8 figures. Thoroughly revised and updated version of
arXiv:1302.647
Resistance to learning and the evolution of cooperation
In many evolutionary algorithms, crossover is the main operator used in generating new
individuals from old ones. However, the usual mechanism for generating offsprings in spatially
structured evolutionary games has to date been clonation. Here we study the effect of
incorporating crossover on these models. Our framework is the spatial Continuous Prisoner's
Dilemma. For this evolutionary game, it has been reported that occasional errors (mutations) in
the clonal process can explain the emergence of cooperation from a non-cooperative initial
state. First, we show that this only occurs for particular regimes of low costs of cooperation.
Then, we display how crossover gets greater the range of scenarios where cooperative mutants
can invade selfish populations. In a social context, where crossover involves a general rule of
gradual learning, our results show that the less that is learnt in a single step, the larger the
degree of global cooperation finally attained. In general, the effect of step-by-step learning can
be more efficient for the evolution of cooperation than a full blast one
Fragmentation transition in a coevolving network with link-state dynamics
We study a network model that couples the dynamics of link states with the
evolution of the network topology. The state of each link, either A or B, is
updated according to the majority rule or zero-temperature Glauber dynamics, in
which links adopt the state of the majority of their neighboring links in the
network. Additionally, a link that is in a local minority is rewired to a
randomly chosen node. While large systems evolving under the majority rule
alone always fall into disordered topological traps composed by frustrated
links, any amount of rewiring is able to drive the network to complete order,
by relinking frustrated links and so releasing the system from traps. However,
depending on the relative rate of the majority rule and the rewiring processes,
the system evolves towards different ordered absorbing configurations: either a
one-component network with all links in the same state or a network fragmented
in two components with opposite states. For low rewiring rates and finite size
networks there is a domain of bistability between fragmented and non-fragmented
final states. Finite size scaling indicates that fragmentation is the only
possible scenario for large systems and any nonzero rate of rewiring.Comment: 10 pages, 13 figure
Binary and Multivariate Stochastic Models of Consensus Formation
A current paradigm in computer simulation studies of social sciences problems
by physicists is the emergence of consensus. The question is to establish when
the dynamics of a set of interacting agents that can choose among several
options (political vote, opinion, cultural features, etc.) leads to a consensus
in one of these options, or when a state with several coexisting social options
prevail. We consider here stochastic dynamic models naturally studied by
computer simulations. We will first review some basic results for the voter
model. This is a binary option stochastic model, and probably the simplest
model of collective behavior. Next we consider a model proposed by Axelrod for
the dissemination of culture. This model can be considered as a multivariable
elaboration of the voter model dynamics.Comment: (16 pages, 8 figures; for simililar work visit
http://www.imedea.uib.es/physdept
Recent Advances in Homogeneous Metal-Catalyzed Aerobic C–H Oxidation of Benzylic Compounds
Csp(3)-H oxidation of benzylic methylene compounds is an established strategy for the synthesis of aromatic ketones, esters, and amides. The need for more sustainable oxidizers has encouraged researchers to explore the use of molecular oxygen. In particular, homogeneous metal-catalyzed aerobic oxidation of benzylic methylenes has attracted much attention. This account summarizes the development of this oxidative strategy in the last two decades, examining key factors such as reaction yields, substrate: catalyst ratio, substrate scope, selectivity over other oxidation byproducts, and reaction conditions including solvents and temperature. Finally, several mechanistic proposals to explain the observed results will be discussed.(IT-774-13 (Basque Government) and CTQ2017-86630-P (Spanish Ministry of Economy and Competitiveness) projects
History and memory in digital culture. Methodological innovations and networks of stories [Editorial]
El dossier al que pertenece esta editorial se enmarca en el proyecto de investigación HISMEDI, Historia y Memoria Histórica on line. Retos y oportunidades para el conocimiento del pasado en Internet con referencia HAR-2015-63582-P (MINECO/FEDER
Egitura-zehaztapenerako ariketa ebatziak
104 p.Egitura-determinazioan jarduten duten ikasleen zein profesionalen trebakuntzan laguntzea da ariketa ebatziz osoturiko liburu honen helburu nagusia. Azken finean, lehen ariketa errazetatik hasita azken adibide konplexuenera, ikaslea gidatzea da liburu honen zeregina.
UV-Vis, IR, MS, eta NMR espektro sinpleak lantzen dira lehenik, ondoren gaur egungo esperimentu aurreratuagoetara helduz, seinale/banda/tontorren esanahaia eta beren ondorioa igarri nahi den konposatu ezezagunaren egituran adibide praktikoen bitartez azaltzen direlarik. Halaber, eskuratutako informazioa nola bildu, ustezko egitura kimikoa (ezaugarri estereokimikoak ere barne) proposatzeko eta esleipenerako irispideak ematen dira
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