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

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

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

Could Spain be less different? Exploring the effects of macroprudential policy on the house price cycle

Haciendo uso de un modelo basado en agentes del mercado inmobiliario español, exploramos los principales factores responsables de la mayor amplitud de los ciclos del precio de la vivienda en España —en comparación con la mayoría de los otros países europeos—, así como el potencial alcance de la política macroprudencial para reducir esta amplitud. En primer lugar, aprovechamos la disponibilidad de una calibración previa del modelo para el Reino Unido, país caracterizado por una menor amplitud del ciclo del precio de la vivienda, para mostrar el prominente papel desempeñado por las distribuciones de varias métricas de riesgo hipotecario: las ratios préstamo-valor, préstamo-ingresos y servicio de la deuda-ingresos. En segundo lugar, utilizamos el modelo para calibrar tanto un límite rígido de la ratio préstamo-valor como un límite suave de la ratio préstamo-ingresos con el objetivo de suavizar el ciclo del precio de la vivienda español de manera que su amplitud se iguale a la del equivalente británico. Finalmente, caracterizamos los efectos de estas políticas a lo largo de las diferentes fases del ciclo, encontrando que ambos instrumentos reducen el crecimiento del crédito y del precio durante la fase expansiva, mientras que hacen disminuir su declive durante la fase contractiva. Además, ambos instrumentos desencadenan un cambio en la composición de la actividad crediticia: en los dos casos se produce un aumento del crédito a inversores en vivienda de alquiler, pero mientras que el límite préstamo-valor conduce a una reducción del crédito a compradores de primera vivienda, con el límite préstamo-ingresos disminuye el crédito tanto a compradores de primera vivienda como a propietarios en busca de una nueva vivienda principal.Employing an agent-based model of the Spanish housing market, this paper explores the main drivers behind the large amplitude of the Spanish house price cycle —as compared to most other European countries—, as well as the scope for macroprudential policy to reduce this amplitude. First, we exploit the availability of a previous calibration to the UK, which has a less pronounced house price cycle, to show the prominent role played by the distributions of various mortgage risk metrics: loan-to-value, loan-to-income and debt-service-to-income ratios. Second, we use the model to calibrate both a hard loan-to-value limit and a soft loan-to-income limit to smooth the Spanish house price cycle and match the amplitude of the UK equivalent. Finally, we characterise the effects of these calibrated policies over the different phases of the cycle, finding that both instruments reduce credit and price growth during the expansionary phase and also reduce their decline during the contractionary phase. Moreover, both instruments lead to a compositional shift in lending: the loan-to-value policy from first-time buyers to buy-to-let investors and the loan-to-income policy from both first-time buyers and home movers to buy-to-let investors

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

Uncertainty, non-linear contagion and the credit quality channel: an application to the Spanish interbank market

Utilizando datos granulares de la Central de Información de Riesgos del Banco de España, estudiamos uno de los canales de contagio a través de los cuales se pueden transmitir las tensiones en los mercados financieros —el canal de la calidad crediticia—, enfocándonos en el mercado interbancario español. Proponemos un mecanismo de contagio no lineal, que depende de la estructura del balance bancario (específicamente, de la ratio de apalancamiento). Adicionalmente, modelizamos explícitamente la incertidumbre que rodea los análisis de los prestamistas acerca de la probabilidad de incumplimiento de sus prestatarios, incorporando así la ausencia de información completa en los mercados y las expectativas heterogéneas de los agentes en su evaluación de resultados futuros. Realizamos múltiples simulaciones que reflejan una amplia gama de posibles grados de tensión del sistema bancario y buscamos desentrañar los efectos de estos dos componentes clave del modelo al comparar nuestros resultados con los que se obtendrían empleando un modelo lineal y determinista. De esta forma, encontramos que, bajo el supuesto de no linealidad, las pérdidas son sustancialmente mayores para amplios grados intermedios de tensión del sistema, mientras que sus efectos se vuelven insignificantes para grados de tensión tanto muy bajos como muy altos. Asimismo, encontramos que incorporar los efectos de la incertidumbre genera un impacto menor relativo al supuesto de no linealidad, pero aun así relevante y más marcado en torno a los grados de tensión en los que diferentes partes del sistema se vuelven inestables. Curiosamente, las pérdidas pueden verse amplificadas o mitigadas con respecto al caso determinista, dependiendo del grado específico de tensión considerado. Finalmente, la interacción entre ambos componentes del modelo —contagio no lineal e incertidumbre— modifica el área para la que la incertidumbre tiene un impacto relevante.Using granular data from the Spanish Central Credit Register, we study the contagion of financial distress via the credit quality channel in the Spanish interbank market. We propose a non-linear contagion mechanism dependent on banks’ balance-sheet structure (specifically, their leverage ratios). Moreover, we explicitly model uncertainty in lenders’ assessments of the probability of default of their borrowers, thus incorporating agents’ lack of complete information and heterogeneous expectations in their assessment of future outcomes. We perform multiple simulations across a wide range of possible levels of stress in the system, and we focus on disentangling the effects of these two key model components by comparing the results of our model with those of a linear and deterministic counterpart. We find that non-linear contagion leads to substantially larger losses than its linear counterpart for a wide range of intermediate levels of stress in the system, while its effects become negligible for very low and very high stress levels. Regarding uncertainty, we find that its effects, while smaller than those of non-linear contagion, are nonetheless relevant and most important around levels of stress at which different parts of the system become unstable. Interestingly, losses can be amplified or mitigated with respect to the deterministic case depending on the specific level of stress considered. Finally, the interaction between these two model components - non-linear contagion and uncertainty - alters the area where uncertainty matters

Heterogeneous effects and spillovers of macroprudential policy in an agent-based model of the UK housing market

Desarrollamos un modelo basado en agentes para estudiar el impacto de experimentos de política macroprudencial sobre los principales indicadores del mercado inmobiliario del Reino Unido. La naturaleza heterogénea de este modelo nos permite evaluar el efecto de tales experimentos sobre los mercados de compra, alquiler y crédito hipotecario no solo de forma agregada, sino también a nivel de hogares individuales y subsegmentos tales como los compradores de primera vivienda, los propietarios, los inversores en vivienda para alquilar y los inquilinos. Este enfoque puede así ofrecer una visión general de los efectos desagregados de las políticas de estabilidad financiera. El modelo está calibrado con una amplia selección de fuentes de microdatos: incluye datos tanto de un destacado portal inmobiliario del Reino Unido como de regulación préstamo a préstamo. Mediante una serie de ejercicios de estática comparativa, investigamos el impacto de: i) un límite rígido de la ratio préstamo-valor, y ii) un límite suave de la ratio préstamo-ingresos, permitiendo que una determinada fracción de las nuevas hipotecas supere esta restricción. En primer lugar, encontramos que estos experimentos tienden a mitigar el ciclo de precios del mercado inmobiliario, lo que reduce la disponibilidad de crédito y, por tanto, el apalancamiento. En segundo lugar, un experimento que tenga por objetivo una particular medida del riesgo puede afectar también a otras métricas de riesgo; se hace así necesaria una cuidadosa calibración de la política para lograr una determinada reducción del riesgo. Por último, experimentos enfocados al mercado de compraventa de vivienda principal pueden tener efectos colaterales sobre el sector del alquiler, como resultado de un cambio composicional en la propiedad inmobiliaria desde ocupantes propietarios hacia inversores en vivienda para alquilar que afecta tanto a la oferta como a la demanda de viviendas en alquiler.We develop an agent-based model of the UK housing market to study the impact of macroprudential policy experiments on key housing market indicators. The heterogeneous nature of this model enables us to assess the effects of such experiments on the housing, rental and mortgage markets not only in the aggregate, but also at the level of individual households and sub-segments, such as first-time buyers, homeowners, buy-to-let investors, and renters. This approach can therefore offer a broad picture of the disaggregated effects of financial stability policies. The model is calibrated using a large selection of micro-data, including data from a leading UK real estate online search engine as well as loan-level regulatory data. With a series of comparative statics exercises, we investigate the impact of: i) a hard loan-to-value limit, and ii) a soft loan-to-income limit, allowing for a limited share of unconstrained new mortgages. We find that, first, these experiments tend to mitigate the house price cycle by reducing credit availability and therefore leverage. Second, an experiment targeting a specific risk measure may also affect other risk metrics, thus necessitating a careful calibration of the policy to achieve a given reduction in risk. Third, experiments targeting the owner-occupier housing market can spill over to the rental sector, as a compositional shift in home ownership from owner-occupiers to buy-to-let investors affects both the supply of and demand for rental properties

Wi-Fi Handshake: analysis of password patterns in Wi-Fi networks

This work is supported by the General Directorate of Culture, Education and University Management of Xunta de Galicia (Ref. ED431G/01, ED431D 2017/16), the Galician Network for Colorectal Cancer Research (Ref. ED431D 2017/23), Competitive Reference Groups (Ref. ED431C 2018/49) and the Spanish Ministry of Economy and Competitiveness via funding of the unique installation BIOCAI (UNLC08-1E-002, UNLC13-13-3503) and the European Regional Development Funds (FEDER). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.[Abstract]: This article seeks to provide a snapshot of the security of Wi-Fi access points in the metropolitan area of A Coruña. First, we discuss the options for obtaining a tool that allows the collection and storage of auditable information from Wi-Fi networks, from location to signal strength, security protocol or the list of connected clients. Subsequently, an analysis is carried out aimed at identifying password patterns in Wi-Fi networks with WEP, WPA and WPA2 security protocols. For this purpose, a password recovery tool called Hashcat was used to execute dictionary or brute force attacks, among others, with various word collections. The coverage of the access points in which passwords were decrypted is displayed on a heat map that represents various levels of signal quality depending on the signal strength. From the handshakes obtained, and by means of brute force, we will try to crack as many passwords as possible in order to create a targeted and contextualized dictionary both by geographical location and by the nature of the owner of the access point. Finally, we will propose a contextualized grammar that minimizes the size of the dictionary with respect to the most used ones and unifies the decryption capacity of the combination of all of them.Xunta de Galicia; ED431G/01Xunta de Galicia; ED431D 2017/16Xunta de Galicia; ED431D 2017/23Xunta de Galicia; ED431C 2018/4

The role of noise and initial conditions in the asymptotic solution of a bounded confidence, continuous-opinion model

We study a model for continuous-opinion dynamics under bounded confidence. In particular, we analyze the importance of the initial distribution of opinions in determining the asymptotic configuration. Thus, we sketch the structure of attractors of the dynamical system, by means of the numerical computation of the time evolution of the agents density. We show that, for a given bound of confidence, a consensus can be encouraged or prevented by certain initial conditions. Furthermore, a noisy perturbation is added to the system with the purpose of modeling the free will of the agents. As a consequence, the importance of the initial condition is partially replaced by that of the statistical distribution of the noise. Nevertheless, we still find evidence of the influence of the initial state upon the final configuration for a short range of the bound of confidence parameter

Unveiling the kinetic versatility of aryl-alcohol oxidases with different electron acceptors

Introduction: Aryl-alcohol oxidase (AAO) shows a pronounced duality as oxidase and dehydrogenase similar to that described for other glucose-methanol-choline (GMC) oxidase/dehydrogenase superfamily proteins involved in lignocellulose decomposition. In this work, we detail the overall mechanism of AAOs from Pleurotus eryngii and Bjerkandera adusta for catalyzing the oxidation of natural aryl-alcohol substrates using either oxygen or quinones as electron acceptors and describe the crystallographic structure of AAO from B. adusta in complex with a product analogue.Methods: Kinetic studies with 4-methoxybenzyl and 3-chloro-4- methoxybenzyl alcohols, including both transient-state and steady-state analyses, along with interaction studies, provide insight into the oxidase and dehydrogenase mechanisms of these enzymes. Moreover, the resolution of the crystal structure of AAO from B. adusta allowed us to compare their overall folding and the structure of the active sites of both AAOs in relation to their activities.Results and Discussion: Although both enzymes show similar mechanistic properties, notable differences are highlighted in this study. In B. adusta, the AAO oxidase activity is limited by the reoxidation of the flavin, while in P. eryngii the slower step takes place during the reductive half-reaction, which determines the overall reaction rate. By contrast, dehydrogenase activity in both enzymes, irrespective of the alcohol participating in the reaction, is limited by the hydroquinone release from the active site. Despite these differences, both AAOs are more efficient as dehydrogenases, supporting the physiological role of this activity in lignocellulosic decay. This dual activity would allow these enzymes to adapt to different environments based on the available electron acceptors