Nicolas Lanchier
Assistant Professor in Mathematics
Ph.D., University of Rouen, France, 2005
Research interest --
Most mathematical models introduced in the life and social sciences literature that describe inherently spatial phenomena
of interacting populations consist of systems of ordinary differential equations.
These models, however, leave out any spatial structure or stochastic component, two factors that have been identified as key factors
in how communities are shaped.
The aim of my research is to understand the role of space and stochasticity in a wide variety of applied sciences such as ecology,
epidemiology, population genetics, opinion and cultural dynamics, through the mathematical analysis of stochastic processes known
as interacting particle systems.
In these models, members of the population (particles) such as atoms, cells, plants or agents, are located on the set of vertices
of a connected graph.
The latter has to be thought of as a network of interactions that dictates the dynamics of the system as particles interact only
locally with their neighbors, thus modeling the presence of a spatial structure.
The main objective of research in this area is to understand the macroscopic behavior and spatial patterns that emerge from the
microscopic interactions that describe the local dynamics of large systems.
Space in this context must be understood in a broad sense: an edge between two vertices of the underlying graph may be
synonymous of geographic proximity, but also friendship relation, adherence to the same political party, etc.
Impact in mathematics and applied sciences --
The field of interacting particle systems is simultaneously one of the most challenging topics of probability theory and a popular
modeling tool in applied sciences.
From the point of view of mathematics, the tradition is to establish analytical results about simple existing models, while from
the point of view of applied sciences, the emphasis is on the development of realistic models and predictions based on numerical
simulations, whereas it is known from past research that spatial simulations are difficult to interpret and might lead to
erroneous conclusions.
My research is a combination of these two aspects.
The challenge is to introduce models that are both mathematically tractable and able to capture the essence of biological and
sociological systems through fundamentally new mathematical features,
as opposed to straightforward generalizations of existing models including a large number of parameters that are artificially complex.
Interestingly, while this approach is highly motivated by applied sciences, it also gives rise to problems which, regardless of
their applications, are important mathematically, and to challenging proofs that usually consists of subtle combinations of measure
theory, probability theory (percolation, random walks, large deviations), graph theory and combinatorics.
Towards dynamic graphs and hypergraphs --
The traditional framework of interacting particle systems focuses on stochastic dynamics on static graphs.
My recent research extends this framework in two directions, again motivated by phenomena that are ubiquitous in nature, by considering
evolutions either on dynamic graphs or on hypergraphs.
Dynamic graph structures arise naturally when looking at systems of particles that have the ability to shape their spatial environment,
which results in systems involving dynamics on the graph but also dynamics of the graph, with a feedback between dynamics on and of the graph.
Hypergraph structures are employed to model systems including an intermediate mesoscopic scale, which results in systems in which all the
vertices of the same hyperedge are simultaneously updated.
See also
NSF Grant DMS-10-05282