(Français) Café Sillon

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(Français) L’innatendu

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Seminar by Dr. Gerardo Iñiguez: Modelling complex contagion with tie heterogeneities

Dr. Gerardo Iñiguez from Institute for Research in Applied Mathematics and Systems, National Autonomous University of Mexico. He is going to talk about:

Modelling complex contagion with tie heterogeneities

Social influence, the effect that the past behaviour of acquaintances has on
our daily decisions, is arguably the main driving mechanism of many complex collective
phenomena in society, including the spreading of innovations, ideas and fads,
or the growth of political and social movements. Many of these processes have
been studied empirically in the past, particularly with regards to the
existence of so-called adoption cascades, where large number of people adopt
the same behaviour in a relatively short time. Moreover, they have been
modelled either as simple contagion (where adoption is driven by independent
contagion stimuli, like the Bass model of innovation diffusion), or as complex
contagion (where a threshold on the number of adopting neighbours in a
network determines spreading, like the Watts model of adoption cascades).
However, it is still unclear whether real spreading phenomena is regulated by
simple or complex contagion (or a combination of both), which makes it difficult to identify
cognitive and social mechanisms that may control the collective action of
large groups of people. Furthermore, most spreading models disregard one relevant
property of real social networks, namely tie heterogeneities in terms of social influence.
Influence arriving on social ties may vary from neighbour to neighbour, as it largely depends
on the nature and frequency of interaction with a given friend.

Here we close this gap on the understanding of social contagion by extending
the conventional Watts cascade model to account for tie heterogeneities. We focus on the case of a
bimodal weight distribution, such that spreading is determined by the adoption
threshold of nodes and the standard deviation of tie weights. We find that the
presence of tie heterogeneities induce unexpected dynamical behaviour, as
they either speed up or slow down contagion with respect to the unweighted
case, depending on the adoption threshold and weight standard
deviation. We found this effect to be present in both synthetic and real social networks,
which we verify via numerical simulations and an analytical approach based on
approximate master equations. Moreover, we find that our model with tie heterogeneities
bridges the theoretical descriptions of simple and complex contagion, and
thus hints at the existence of a single mechanism driving social spreading, regardless of the initial
hypotheses used to describe contagion stimuli between individuals. These
results may be instrumental in developing more accurate spreading models that manage to
gauge the rise and extent of real behavioural cascades in society.

Date: 10th January 2017, at 15:00
Place: Amphi J (ENS Lyon, site Monod).

Yannick PhD défense: Deep dive into social network and economic data: a data driven approach for uncovering temporal ties, human mobility and socioeconomic correlations

Date: Friday December 16th at 2pm
Place: ENS de Lyon (Site Descartes) in Amphitheatre Descartes.

This thesis is interdisciplinary, I will present some results in Network Science, Dynamic Graphs but also Sociology, Economy and Geography. For this reason, it was important for me to organize it in Descartes and if you do not hesitate to share to social science mailng lists.

You are also invited to the post PhD drinks with Monacan specialities that will be held after the defense in Salle Festive (check the map) from 4:30pm and drinks will continue well into the night till 3am.

The members of the jury are:

– Eric Fleury – ENS de Lyon, INRIA, LIP

– Crespelle Christophe – UCBL Lyon 1, LIP
– Karsai Marton -ENS de Lyon, INRIA, LIP

– Magnien Clémence – CNRS, LIP6
– Jari Saramaki – Aalto University

– Cardon Dominique – Science Po Paris, Medialab
– Lambiotte Renaud – University of Namur

In this thesis, I have carried out data-driven studies based on rich, large-scale combined data sets including social links between users (calls and SMS), their demographic parameters (age and gender), their mobility and their economic information such as income and spendings. These seven studies bring insights in network science but also in sociology, economy and geography. The questions asked are very diversified. How can one quantify the loss of temporal information caused by the aggregation of link streams into series of graphs? How can one infer mobility of a user from his or her localisations of calls? Is it possible to transmit SMS in a dense region by using the density of phones, the mobility of users and the locality of the messages? How can one quantify and prove empirically the social stratification of the society at a large population scale? I present, for this last question, a first socio-economic study with a data-driven approach. It has been possible to study, at a very large scale, the stratification of the society, the existence of “rich-clubs”, the spatial segregation and purchase patterns for each social class.

Paper on Socioeconomic correlations and stratification in social-communication networks

Published in Journal of the Royal Society Interface, 13 125 (2016)


The uneven distribution of wealth and individual economic capacities are among the main forces, which shape modern societies and arguably bias the emerging social structures. However, the study of correlations between the social network and economic status of individuals is difficult due to the lack of large-scale multimodal data disclosing both the social ties and economic indicators of the same population. Here, we close this gap through the analysis of coupled datasets recording the mobile phone communications and bank transaction history of one million anonymized individuals living in a Latin American country. We show that wealth and debt are unevenly distributed among people in agreement with the Pareto principle; the observed social structure is strongly stratified, with people being better connected to others of their own socioeconomic class rather than to others of different classes; the social network appears to have assortative socioeconomic correlations and tightly connected ‘rich clubs’; and that individuals from the same class live closer to each other but commute further if they are wealthier. These results are based on a representative, society-large population, and empirically demonstrate some long-lasting hypotheses on socioeconomic correlations, which potentially lay behind social segregation, and induce differences in human mobility.

Seminar: The power of local search for clustering by Vincent Viallat Cohen Addad

Abstract :

What are the performance guarantees of the algorithms used in practice
for clustering problems?
In a paper with Phil Klein and Claire Mathieu, we give the
first polynomial-time approximation schemes for the following problems:
(1) uniform facility location in edge-weighted planar graphs;
(2) $k$-median and $k$-means in edge-weighted planar graphs;
(3) $k$-means in Euclidean space of bounded dimension.
The algorithm is a standard, widely-used local search heuristic where
the local neighborhood of a solution $S$ consists of all solutions
obtained from $S$ by removing and adding $1/\eps^{O(1)}$ centers.
In a second line of work with Chris Schwiegelshohn, we show that this
algorithm also performs near-optimaly for several types of instances
that aim at characterizing the structure of real-world instances
stemming from machine learning and data analysis.
Those results make a step toward understanding the success of local
search heuristics in practice.

When: Friday Nov, 25th 2016
Where: Amphi B – ENS de Lyon — Site Monod

(Français) Poste Mdc section 27 IUT/Lyon1 — LIP/ENS de lyon

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(Français) Lello

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Controversy Mapping a Travel Companion

HDR dissertation by Tommaso Venturini under the Supervision of Pablo Jensen Ecole Normale Supérieure de Lyon

Jury: Richard Rogers, Dominique Pestre, Dominique Vinck, didier bigo, Pablo Jensen, Eric Fleury

Monday September 26th, 10:30AM. ENS de Lyon.

(Français) Le Danton

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