Séminaires LEMMA

"Linking Mechanisms with Few Messages"

Maël LE TREUST (CNRS), en collaboration avec Tristan Tomala

Tuesday, 18 November 2025, 11h-12h

Lemma - 4 rue Blaise Desgoffe, 75006 Paris. Salle Maurice Desplas

Abstract : We investigate the mechanism design problem formulated by Jackson and Sonnenschein in 2007, where we consider that agents do not have enough messages to reveal their type. This problem is deeply related to the mismatched distortion-rate problem formulated by Lapidoth in 1997, in the Information Theory literature. The characterization of the set of single-letter incentive compatible distributions is an open problem. We provide inner and an outer bounds and we show these bounds match in several special cases.

"A theory of social ranking (with applications) "

Stefano MORETTI (Université Paris-Dauphine et CNRS)

Tuesday, 4 November 2025, 11h-12h

Lemma - 4 rue Blaise Desgoffe, 75006 Paris. Salle Maurice Desplas

Abstract : Seemingly unrelated situations, such as team sports, evaluating researchers, argumentation theory, explainable AI, and others, are characterized by a common problem: finding adequate solutions for ranking individuals or elements based on their performance in different groups or coalitions. Recent studies have examined this problem from a theoretical perspective using an axiomatic approach, introducing alternative fundamental properties that a social ranking solution should satisfy to properly convert a coalitional ranking into an individual one. In this seminar, I will focus on a particular solution, called lexicographic excellence, which aims to reward elements that appear frequently in the best coalitions. I’ll present some of its recently studied properties and axiomatic characterizations, and I’ll briefly introduce one of its applications to feature selection in ML models.

References:

Aleandri, M., Fritz, F., Moretti, S. Desirability and social ranking. Soc Choice Welf (2025). https://doi.org/10.1007/s00355-025-01590-1