### Core team:

# Pierre-André Noël

### University of California

### Postdoctoral Researcher

#### Most recent papers:

Strategic tradeoffs in competitor dynamics on adaptive networks.

Laurent Hébert-Dufresne, Antoine Allard, Pierre-André Noël, Jean-Gabriel Young, Eric Libby. Scientific Reports, , 7, 2017.[pdf] [journal page] [arXiv]

**Abstract:**

Recent empirical work highlights the heterogeneity of social competitions such as political campaigns: proponents of some ideologies seek debate and conversation, others create echo chambers. While symmetric and static network structure is typically used as a substrate to study such competitor dynamics, network structure can instead be interpreted as a signature of the competitor strategies, yielding competition dynamics on adaptive networks. Here we demonstrate that tradeoffs between aggressiveness and defensiveness (i.e., targeting adversaries vs. targeting like-minded individuals) creates paradoxical behaviour such as non-transitive dynamics. And while there is an optimal strategy in a two competitor system, three competitor systems have no such solution; the introduction of extreme strategies can easily affect the outcome of a competition, even if the extreme strategies have no chance of winning. Not only are these results reminiscent of classic paradoxical results from evolutionary game theory, but the structure of social networks created by our model can be mapped to particular forms of payoff matrices. Consequently, social structure can act as a measurable metric for social games which in turn allows us to provide a game theoretical perspective on online political debates.

Spreading dynamics on complex networks: a general stochastic approach.

Pierre-André Noël, Antoine Allard, Laurent Hébert-Dufresne, Vincent Marceau, Louis J. Dubé. Preprint, 2014.[pdf] [journal page]

**Abstract:**

Dynamics on networks is considered from the perspective of Markov stochastic processes. We partially describe the state of the system through network motifs and infer any missing data using the available information. This versatile approach is especially well adapted for modelling spreading processes and/or population dynamics. In particular, the generality of our framework and the fact that its assumptions are explicitly stated suggests that it could be used as a common ground for comparing existing epidemics models too complex for direct comparison, such as agent-based computer simulations. We provide many examples for the special cases of susceptible-infectious-susceptible and susceptible-infectious-removed dynamics (eg, epidemics propagation) and we observe multiple situations where accurate results may be obtained at low computational cost. Our perspective reveals a subtle balance between the complex requirements of a realistic model and its basic assumptions.

The Social Zombie: Modelling undead outbreaks on social networks.

Laurent Hébert-Dufresne, Vincent Marceau, Pierre-André Noël, Antoine Allard, Louis J. Dubé. Mathematical Modelling of Zombies, 149-170, , 2014.[pdf] [journal page]

**Abstract:**

According to Mulder’s theory, the zombies will eventually fall on each other and make love. However, be it for love or evil, the cold, hard reality remains that the actions of the undead, just like those of the living, are structured by simple constraints of a social or spatiotemporal nature. In this chapter, we consider the underlying social network of the living and the horde behaviour of the undead. This model is then further improved by considering the adaptive nature of social interactions: people usually tend to avoid contact with zombies. Doing so captures the co-evolution of the human social network and of the zombie outbreak, which encourages humans to naturally barricade themselves in groups of survivors to better fight the undead menace. And then? Better stack goods, arm yourself and be patient, for the undead hordes are there to stay—hopefully dancing and making love.

Bond percolation on a class of correlated and clustered random graphs.

Antoine Allard, Laurent Hébert-Dufresne, Pierre-André Noël, Vincent Marceau, Louis J. Dubé. Journal of Physics A: Mathematical and Theoretical, , , 2012.[pdf] [journal page] [arXiv]

**Abstract:**

We introduce a formalism for computing bond percolation properties of a class of correlated and clustered random graphs. This class of graphs is a generalization of the configuration model where nodes of different types are connected via different types of hyperedges, edges that can link more than two nodes. We argue that the multitype approach coupled with the use of clustered hyperedges can reproduce a wide spectrum of complex patterns, and thus enhances our capability to model real complex networks. As an illustration of this claim, we use our formalism to highlight unusual behaviours of the size and composition of the components (small and giant) in a synthetic, albeit realistic, social network.

Exact solution of bond percolation on small arbitrary graphs.

Antoine Allard, Laurent Hébert-Dufresne, Pierre-André Noël, Vincent Marceau, Louis J. Dubé. EPL (Europhysics Letters), , 98, 2012.[pdf] [journal page] [arXiv]

**Abstract:**

We introduce a set of iterative equations that exactly solves the size distribution of components on small arbitrary graphs after the random removal of edges. We also demonstrate how these equations can be used to predict the distribution of the node partitions (ie, the constrained distribution of the size of each component) in undirected graphs. Besides opening the way to the theoretical prediction of percolation on arbitrary graphs of large but finite size, we show how our results find application in graph theory, epidemiology, percolation and fragmentation theory.

Structural preferential attachment: Stochastic process for the growth of scale-free, modular and self-similar systems.

Laurent Hébert-Dufresne, Antoine Allard, Vincent Marceau, Pierre-André Noël, Louis J. Dubé. Physical Review E, , 85, 2012.[pdf] [journal page] [arXiv]

**Abstract:**

Many complex systems have been shown to share universal properties of organization, such as scale independence, modularity, and self-similarity. We borrow tools from statistical physics in order to study structural preferential attachment (SPA), a recently proposed growth principle for the emergence of the aforementioned properties. We study the corresponding stochastic process in terms of its time evolution, its asymptotic behavior, and the scaling properties of its statistical steady state. Moreover, approximations are introduced to facilitate the modeling of real systems, mainly complex networks, using SPA. Finally, we investigate a particular behavior observed in the stochastic process, the peloton dynamics, and show how it predicts some features of real growing systems using prose samples as an example.

Propagation on networks: an exact alternative perspective.

Pierre-André Noël, Antoine Allard, Laurent Hébert-Dufresne, Vincent Marceau, Louis J. Dubé. Physical Review E, , 85, 2012.[pdf] [journal page] [arXiv]

**Abstract:**

By generating the specifics of a network structure only when needed (on-the-fly), we derive a simple stochastic process that exactly models the time evolution of susceptible-infectious dynamics on finite-size networks. The small number of dynamical variables of this birth-death Markov process greatly simplifies analytical calculations. We show how a dual analytical description, treating large scale epidemics with a Gaussian approximation and small outbreaks with a branching process, provides an accurate approximation of the distribution even for rather small networks. The approach also offers important computational advantages and generalizes to a vast class of systems.

Structural preferential attachment: Network organization beyond the link.

Laurent Hébert-Dufresne, Antoine Allard, Vincent Marceau, Pierre-André Noël, Louis J. Dubé. Physical Review Letters, , 107, 2011.[pdf] [journal page] [arXiv]

**Abstract:**

We introduce a mechanism which models the emergence of the universal properties of complex networks, such as scale independence, modularity and self-similarity, and unifies them under a scale-free organization beyond the link. This brings a new perspective on network organization where communities, instead of links, are the fundamental building blocks of complex systems. We show how our simple model can reproduce social and information networks by predicting their community structure and more importantly, how their nodes or communities are interconnected, often in a self-similar manner.

Modeling the dynamical interaction between epidemics on overlay networks.

Vincent Marceau, Pierre-André Noël, Laurent Hébert-Dufresne, Antoine Allard, Louis J. Dubé. Physical Review E, , 84, 2011.[pdf] [journal page] [arXiv]

**Abstract:**

Epidemics seldom occur as isolated phenomena. Typically, two or more viral agents spread within the same host population and may interact dynamically with each other. We present a general model where two viral agents interact via an immunity mechanism as they propagate simultaneously on two networks connecting the same set of nodes. By exploiting a correspondence between the propagation dynamics and a dynamical process performing progressive network generation, we develop an analytical approach that accurately captures the dynamical interaction between epidemics on overlay networks. The formalism allows for overlay networks with arbitrary joint degree distribution and overlap. To illustrate the versatility of our approach, we consider a hypothetical delayed intervention scenario in which an immunizing agent is disseminated in a host population to hinder the propagation of an undesirable agent (e.g., the spread of preventive information in the context of an emerging infectious disease).

Propagation dynamics on networks featuring complex topologies.

Laurent Hébert-Dufresne, Pierre-André Noël, Vincent Marceau, Antoine Allard, Louis J. Dubé. Physical Review E, , 82, 2010.[pdf] [journal page] [arXiv]

**Abstract:**

Analytical description of propagation phenomena on random networks has flourished in recent years, yet more complex systems have mainly been studied through numerical means. In this paper, a mean-field description is used to coherently couple the dynamics of the network elements (such as nodes, vertices, individuals, etc.) on the one hand and their recurrent topological patterns (such as subgraphs, groups, etc.) on the other hand. In a susceptible-infectious-susceptible (SIS) model of epidemic spread on social networks with community structure, this approach yields a set of ordinary differential equations for the time evolution of the system, as well as analytical solutions for the epidemic threshold and equilibria. The results obtained are in good agreement with numerical simulations and reproduce the behavior of random networks in the appropriate limits which highlights the influence of topology on the processes. Finally, it is demonstrated that our model predicts higher epidemic thresholds for clustered structures than for equivalent random topologies in the case of networks with zero degree correlation.br>

Adaptive networks: Coevolution of disease and topology.

Vincent Marceau, Pierre-André Noël, Laurent Hébert-Dufresne, Antoine Allard, Louis J. Dubé. Physical Review E, , 82, 2010.[pdf] [journal page] [arXiv]

**Abstract:**

Adaptive networks have been recently introduced in the context of disease propagation on complex networks. They account for the mutual interaction between the network topology and the states of the nodes. Until now, existing models have been analyzed using low complexity analytical formalisms, revealing nevertheless some novel dynamical features. However, current methods have failed to reproduce with accuracy the simultaneous time evolution of the disease and the underlying network topology. In the framework of the adaptive susceptible-infectious-susceptible (SIS) model of Gross et al.[Phys. Rev. Lett. 96, 208701 (2006)], we introduce an improved compartmental formalism able to handle this coevolutionary task successfully. With this approach, we analyze the interplay and outcomes of both dynamical elements, process and structure, on adaptive networks featuring different degree distributions at the initial stage.

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