Mo
MA

Modelli   Matematici per
 le Applicazioni

Dipartimento di Matematica, Sapienza, Università di Roma


Seminari 2012

Torna ai seminari dell'anno corrente

23 Novembre

ore 12.00-14.00

Aula Consiglio

Irene Giardina

Birds, spins and entropy: a theoretical physics approach to collective animal behaviour

Flocking is a paradigmatic example of self-organized collective behaviour, where collective ordering emerges from the mutual interactions between  individuals. In this respect, it shares striking similarities with collective phenomena in inanimate systems that have long been studied by the physics community. Still, biological systems are more complicated than physical ones and it is not evident whether they can be described by the same kind of general laws so well understood in physics. Experimental findings often go beyond simple expectations, making this field even more fascinating. In this talk I will discuss our attempts to study collective animal behaviour using  a physicist's perspective. I will show how we used concepts and methods from statistical physics to make sense of experimental data of large flocks of birds. In particular, I will focus on the velocity correlation functions, that well capture the balance between consensus and independence among the large number of individuals in the group. We measured these correlations in real flocks of starlings and found that they exhibit a non-trivial scale-free behavior, indicating a surprisingly large degree of coordination and collective response. I will describe how we can use these correlations to systematically address the inverse problem, extract from the experimental measurements information on the underlying microscopic interactions, and build minimal (maximum entropy) models directly from the data.


26 Ottobre

ore 12.00-14.00

Aula Consiglio

Adriano Barra

Collective phenomena in immune systems
 
 
The immune system defends us from pathogens. But what is an antigen? Does each lymphocyte really know the complete ensemble of all the molecules of our body? Is there, instead, a systemic cooperation among these  cells resembling neurons in the brain? Is the immune system able to "think"? And, even more interesting, what is self? Is there a sharp separation between self and not-self?
From the pioneering papers by Jerne, Varela and Couthino, the idea of a network of B-clones interacting by exchanging antibodies was formulated as an hypothetical way for explaining tolerance toward self proteins by mature peripheral B-cells (beyond clonal deletion and receptor editing during the ontogenesis). However, despite anti-antibodies are commonly encountered,  evidence in favor of an extensive network is still lacking and even worse, always sharper experiments (e.g. the works by Goodnow) highlighted strong evidence of a phenomenon called "anergy": the latter shifts the responsibility of this lacking of attack by B-cells self-directed to a lacking of signalling by helpers cells (the second key needed for B-cell activation beyond the primary antigenic target, self or not-self): In a nutshell, B-cells need two activation signals, the former being the target (antigen) the latter being a "consensus" by the helpers. Helpers do not allow this consensus and the resulting B-cell with only one signal undergoes into a regime of "anergy". But how can helpers recognize those B-cells self-directed? The problem seems only shifted...  Coupled to a large introduction to theoretical immunology and its formalization through statistical mechanics, we try to revise this discussion within a thermodynamical framework by using the tools of disordered systems.

1 Giugno

ore 12.00-14.00

Aula Consiglio

Giovanni Alberti

Variational problems on the equilibrium (and quasistatic evolution) of drops

The classical (that is, geometric) model of capillarity accounts for many of the equilibrium shapes we commonly observe in liquid drops. In fact, even more complicated phenomena such as superhydrophobicity and contact angle hysteresis can be explained using simple variants of the basic model. In this lecture I will start from the classical theory to arrive, if time permits, to a recent work in collaboration with Antonio De Simone on the quasistatic evolution of drops.

4 Maggio

ore 12.00-14.00

Aula Consiglio

David Quéré

On the shapes of water


As we learnt from Young and Laplace, the cohesion of fluids makes them choose specific shapes, in particular spheres at a small scale. We discuss several ways to maintain this ideal shape on a solid, which leads to unique dynamical situations: water pearls do not stick, they run easily and they bounce - a little bit as if they were solid marbles. (But they are not. And the liquid nature of these pearls has interesting consequences on the dynamical shapes they adopt, for example.) The high mobility of liquid pearls implies that tiny forces are sufficient to move them, and we plan to present recent achievements where some asymmetric patterns at a solid surface permit the self-propulsion of the liquid. We would also like to discuss the effects of various fields to control these elusive drops.

30 Marzo

ore 12.00-14.00

Aula Consiglio

Lorenzo Giacomelli

Mathematical models of wetting phenomena

Wetting phenomena at "small" scales (a water drop on a glass, the precorneal tear-film) may be described by quite a few different mathematical models: diffuse interface ones, sharp ones such as the Navier-Stokes equations, and reduced ones such as the lubrication and the quasi-static approximations. Furthermore, they open up fundamental questions whose answer is yet debated, such as the description of the interface (if any) which separates "dry" from "wet" regions. Which model and which answer are most appropriate is likely to depend on the physics of the specific phenomena, and I will provide introductory information for most of them. However, all of these models are grounded on a basic and unifying physical mechanism: the balance of capillary and frictional (e.g. viscous) forces. Enlightening this principle will hopefully help understanding and enjoying the subsequent lectures within this series.

2 Marzo

ore 12.00-14.00

Aula Consiglio

Vittorio Loreto

Recent advances in language dynamics

Language dynamics is an emerging field that focuses on all processes related to the emergence, evolution and extinction of languages. Recently the study of the self-organization and evolution of language and meaning has recently led to the idea that a community of language users can be seen as a complex dynamical system that collectively solves the problem of developing a shared communication framework through the back-and-forth signaling between people.


In this talk I will review some of the progresses made in the last few years and highlight potential future directions for the research in this area. I will discuss in particular several examples corresponding to the early stages of the emergence of a language, namely the emergence of a common lexicon and the emergence of a shared set of linguistics categories. I will point out how synthetic modeling has nowadays reached sufficient maturity to contribute significantly to the ongoing debate in cognitive science. For instance it has been recently possible to reproduce in a numerical model the outcomes of an important experimental survey, the so-called World Color Survey (WCS).


In addition new experimental frameworks are becoming progressively available. Finally I will discuss the crucial issue in linguistics of whether structures of languages we adopt are the outcome of an individual-based process of optimization or rather the result of a complex socially-driven cultural negotiation. I will argue that a general scenario in language dynamics could be such that shared linguistic conventions would not emerge as attractors, but rather as metastable states.

3 Febbraio

ore 12.00-14.00

Aula Consiglio

Steven Strogatz

Social networks that balance themselves

Consider a fully-connected social network of people, companies, or countries, modeled as an undirected complete graph with real numbers on its edges. Positive edges link friends; negative edges link enemies. I'll discuss two simple models of how the edge weights of such networks might evolve over time, as they seek a balanced state in which "the enemy of my enemy is my friend." The mathematical techniques involve elementary ideas from linear algebra, random graphs, statistical physics, and differential equations. Some motivating examples from international relations and social psychology will also be discussed. This is joint work with Seth Marvel, Jon Kleinberg, and Bobby Kleinberg.

http://dueallamenouno.comunita.unita.it/2012/02/04/i-nemici-dei-miei-nemici/

13 gennaio

ore 12.00-14.00

Aula Consiglio

Maurizio Battaglia

Modeling volcano deformation made easy:constraining the source of the 2004-2011 volcano unrest at Mount St Helens (WA).

Precise measurements of ground deformation have become increasingly common as large networks of GPS receivers and borehole strainmeters have been established over the last decade. Complementing this continuous record are comparatively infrequent but spatially dense images of ground deformation from radar satellites, and a long historical record of leveling, Electronic Distance Meter, triangulation, and tilt data. Deformation can arise from tectonic and volcanic forces and from human activities such as aquifer withdrawal or geothermal exploitation. Mathematical models of how the crust deforms in response to different physical processes are required to characterize driving processes and constrain source location, size, orientation, and strength. This information is valuable for hazards forecasting and mitigation.