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Search Results: 1 - 10 of 315 matches for " Laetitia Gauvin "
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Detecting the Community Structure and Activity Patterns of Temporal Networks: A Non-Negative Tensor Factorization Approach
Laetitia Gauvin, André Panisson, Ciro Cattuto
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0086028
Abstract: The increasing availability of temporal network data is calling for more research on extracting and characterizing mesoscopic structures in temporal networks and on relating such structure to specific functions or properties of the system. An outstanding challenge is the extension of the results achieved for static networks to time-varying networks, where the topological structure of the system and the temporal activity patterns of its components are intertwined. Here we investigate the use of a latent factor decomposition technique, non-negative tensor factorization, to extract the community-activity structure of temporal networks. The method is intrinsically temporal and allows to simultaneously identify communities and to track their activity over time. We represent the time-varying adjacency matrix of a temporal network as a three-way tensor and approximate this tensor as a sum of terms that can be interpreted as communities of nodes with an associated activity time series. We summarize known computational techniques for tensor decomposition and discuss some quality metrics that can be used to tune the complexity of the factorized representation. We subsequently apply tensor factorization to a temporal network for which a ground truth is available for both the community structure and the temporal activity patterns. The data we use describe the social interactions of students in a school, the associations between students and school classes, and the spatio-temporal trajectories of students over time. We show that non-negative tensor factorization is capable of recovering the class structure with high accuracy. In particular, the extracted tensor components can be validated either as known school classes, or in terms of correlated activity patterns, i.e., of spatial and temporal coincidences that are determined by the known school activity schedule.
User-based representation of time-resolved multimodal public transportation networks
Laura Alessandretti,Márton Karsai,Laetitia Gauvin
Computer Science , 2015,
Abstract: Multimodal transportation systems can be represented as time-resolved multilayer networks where different transportation modes connecting the same set of nodes are associated to distinct network layers. Their quantitative description became possible recently due to openly accessible datasets describing the geolocalised transportation dynamics of large urban areas. Advancements call for novel analytics, which combines earlier established methods and exploits the inherent complexity of the data. Here, our aim is to provide a novel user-based methodological framework to represent public transportation systems considering the total travel time, its variability across the schedule, and taking into account the number of transfers necessary. Using this framework we analyse public transportation systems in several French municipal areas. We incorporate travel routes and times over multiple transportation modes to identify efficient transportation connections and non-trivial connectivity patterns. The proposed method enables us to quantify the network's overall efficiency as compared to the specific demand and to the car alternative.
Phase diagram of a Schelling segregation model
Laetitia Gauvin,Jean Vannimenus,Jean-Pierre Nadal
Physics , 2009, DOI: 10.1140/epjb/e2009-00234-0
Abstract: The collective behavior in a variant of Schelling's segregation model is characterized with methods borrowed from statistical physics, in a context where their relevance was not conspicuous. A measure of segregation based on cluster geometry is defined and several quantities analogous to those used to describe physical lattice models at equilibrium are introduced. This physical approach allows to distinguish quantitatively several regimes and to characterize the transitions between them, leading to the building of a phase diagram. Some of the transitions evoke empirical sudden ethnic turnovers. We also establish links with 'spin-1' models in physics. Our approach provides generic tools to analyze the dynamics of other socio-economic systems.
Activity clocks: spreading dynamics on temporal networks of human contact
Laetitia Gauvin,André Panisson,Ciro Cattuto,Alain Barrat
Physics , 2013, DOI: 10.1038/srep03099
Abstract: Dynamical processes on time-varying complex networks are key to understanding and modeling a broad variety of processes in socio-technical systems. Here we focus on empirical temporal networks of human proximity and we aim at understanding the factors that, in simulation, shape the arrival time distribution of simple spreading processes. Abandoning the notion of wall-clock time in favour of node-specific clocks based on activity exposes robust statistical patterns in the arrival times across different social contexts. Using randomization strategies and generative models constrained by data, we show that these patterns can be understood in terms of heterogeneous inter-event time distributions coupled with heterogeneous numbers of events per edge. We also show, both empirically and by using a synthetic dataset, that significant deviations from the above behavior can be caused by the presence of edge classes with strong activity correlations.
Modeling urban housing market dynamics: can the socio-spatial segregation preserve some social diversity?
Laetitia Gauvin,Annick Vignes,Jean-Pierre Nadal
Computer Science , 2010,
Abstract: Addressing issues of social diversity, we introduce a model of housing transactions between agents who are heterogeneous in their willingness to pay. A key assumption is that agents' preferences for a location depend on both an intrinsic attractiveness and on the social characteristics of the neighborhood. The stationary space distribution of income is analytically and numerically characterized. The main results are that socio-spatial segregation occurs if -- and only if -- the social influence is strong enough, but even so, some social diversity is preserved at most locations. Comparison with data on the Paris housing market shows that the results reproduce general trends of price distribution and spatial income segregation.
Revealing latent factors of temporal networks for mesoscale intervention in epidemic spread
Laetitia Gauvin,André Panisson,Alain Barrat,Ciro Cattuto
Computer Science , 2015,
Abstract: The customary perspective to reason about epidemic mitigation in temporal networks hinges on the identification of nodes with specific features or network roles. The ensuing individual-based control strategies, however, are difficult to carry out in practice and ignore important correlations between topological and temporal patterns. Here we adopt a mesoscopic perspective and present a principled framework to identify collective features at multiple scales and rank their importance for epidemic spread. We use tensor decomposition techniques to build an additive representation of a temporal network in terms of mesostructures, such as cohesive clusters and temporally-localized mixing patterns. This representation allows to determine the impact of individual mesostructures on epidemic spread and to assess the effect of targeted interventions that remove chosen structures. We illustrate this approach using high-resolution social network data on face-to-face interactions in a school and show that our method affords the design of effective mesoscale interventions.
Bank Characteristics and Procyclicality: A Theoretical Approach  [PDF]
Marie-Sophie Gauvin
Journal of Financial Risk Management (JFRM) , 2014, DOI: 10.4236/jfrm.2014.33007
Abstract:

The 2007-2008 crisis highlighted liquidity management troubles. We witness a real estate asset price boom during the pre-crisis period and a difficulty for banks to raise funding afterwards. Consequently, bank choices in response to the conduct of the monetary policy along the cycle can be studied. Despite usual financial accelerator, the excessive (lack of) confidence of banks in the upward (down) phase explains procyclical balance sheet movements. Moreover, the monetary policy effects on bank behaviors vary according to their initial specifications. From a theoretical point of view, this paper examines the response of the banking sector to monetary authorities impulses, in function of their initial characteristics. So, the paper highlights a theoretical model, based on accounting identities, in which banks are distinguished in different categories according to their level of capitalization and liquidity. The principal result is that the less capitalized and liquid banks have more procyclical behaviors.

Amitié, amour et fragilité : essai d’un cynisme nouveau
Mathieu Gauvin
Phares : Revue Philosophique étudiante de l’Université Laval , 2006,
Abstract:
Harmonic Generation and Wave Mixing in Confinement Structures: An Account of Pump Depletion in Second-Order Nonlinear Multilayer Microcavities
Serge Gauvin,Maxime Boudreau
Journal of Nanomaterials , 2012, DOI: 10.1155/2012/943204
Abstract: In presence of weakly nonlinear media, it is tempting to neglect pump wave depletion when calculating the intensity of the various generated nonlinear components. However, even in the case of very weak pump wave depletion conditions, an optical structure that allows multipass optical paths, such as high finesse multilayer microcavities, could lead to significant cumulative pump wave depletion. In such conditions, neglecting pump depletion might lead to large computational errors. A matrix formalism devoted to such pump depletion in planar layered nonlinear structures without resort to the “bound” and “free” waves concept is described. A general approach that makes use of “intrinsic” and “extrinsic” waves concept is given, through a slight modification of the canonical propagation matrix. The theoretical results show that even in the case of very weak pump depletion conditions, the cumulative effect due to confinement actually leads to very significant effects. It turns out that taking into account the pump depletion is mandatory for numerous experimental conditions. This matrix formalism applies to absorbing media, and is extensible to include the case of anisotropic layers and cascading effects.
Banking Sector and Monetary Policy Transmission: Bank Capital, Credit and Risk-Taking Channels  [PDF]
Philippe Gilles, Marie-Sophie Gauvin, Nicolas Huchet
Modern Economy (ME) , 2013, DOI: 10.4236/me.2013.41010
Abstract:

In the literature, the question of central banks’ responsibility for triggering crises is raised when sustainable low interest rates lead to excessive banks’ risk exposures. However, such portfolio choices mainly depend on the various returns of assets and on the official interest rate, taking into account that the bank lending channel is affected by the bank capital channel. On the basis of a simple theoretical model including a solvency ratio, we show that during recessions a credit rationing is observed together with a flight to quality; during expansions monetary policy can induce both a fall in credit activity and an increase in financial instability. Then, regulatory capital arbitrages appear and still weaken productive loans. Conclusions can be drawn in terms of prudential policy, as the central bank may be powerless face to banking strategies if the regulatory framework is procyclical.

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