The ergodic theory of hyperbolic groups

These notes are a self-contained introduction to the use of dynamical and probabilistic methods in the study of hyperbolic groups. Most of this material is standard; however some of the proofs given are new, and some results are proved in greater generality than have appeared in the literature. These notes originated in a minicourse given at a workshop in Melbourne, July 11-15 2011.Comment: 37 pages, 5 figures; incorporates referee's comment

Consensus theories: an oriented survey

This article surveys seven directions of consensus theories: Arrowian results, federation consensus rules, metric consensus rules, tournament solutions, restricted domains, abstract consensus theories, algorithmic and complexity issues. This survey is oriented in the sense that it is mainly – but not exclusively – concentrated on the most significant results obtained, sometimes with other searchers, by a team of French searchers who are or were full or associate members of the Centre d'Analyse et de Mathématique Sociale (CAMS).Consensus theories ; Arrowian results ; aggregation rules ; metric consensus rules ; median ; tournament solutions ; restricted domains ; lower valuations ; median semilattice ; complexity

Aggregation of Weak Fuzzy Norms

[EN] Aggregation is a mathematical process consisting in the fusion of a set of values into a unique one and representing them in some sense. Aggregation functions have demonstrated to be very important in many problems related to the fusion of information. This has resulted in the extended use of these functions not only to combine a family of numbers but also a family of certain mathematical structures such as metrics or norms, in the classical context, or indistinguishability operators or fuzzy metrics in the fuzzy context. In this paper, we study and characterize the functions through which we can obtain a single weak fuzzy (quasi-)norm from an arbitrary family of weak fuzzy (quasi-)norms in two different senses: when each weak fuzzy (quasi-)norm is defined on a possibly different vector space or when all of them are defined on the same vector space. We will show that, contrary to the crisp case, weak fuzzy (quasi-)norm aggregation functions are equivalent to fuzzy (quasi-)metric aggregation functions.J.R.-L. acknowledges financial support from the research project PGC2018-095709-B-C21 funded by MCIN/AEI/10.13039/501100011033 and FEDER Una manera de hacer Europa.Pedraza Aguilera, T.; Ramos-Canós, J.; Rodríguez López, J. (2021). Aggregation of Weak Fuzzy Norms. Symmetry (Basel). 13(10):1-16. https://doi.org/10.3390/sym13101908116131

New results on the aggregation of norms

[EN] It is a natural question if a Cartesian product of objects produces an object of the same type. For example, it is well known that a countable Cartesian product of metrizable topological spaces is metrizable. Related to this question, Borsik and Dobos characterized those functions that allow obtaining a metric in the Cartesian product of metric spaces by means of the aggregation of the metrics of each factor space. This question was also studied for norms by Herburt and Moszynska. This aggregation procedure can be modified in order to construct a metric or a norm on a certain set by means of a family of metrics or norms, respectively. In this paper, we characterize the functions that allow merging an arbitrary collection of (asymmetric) norms defined over a vector space into a single norm (aggregation on sets). We see that these functions are different from those that allow the construction of a norm in a Cartesian product (aggregation on products). Moreover, we study a related topological problem that was considered in the context of metric spaces by Borsik and Dobos. Concretely, we analyze under which conditions the aggregated norm is compatible with the product topology or the supremum topology in each case.J. Rodríguez-López acknowledges financial support from FEDER/Ministerio de Ciencia, Innovación y Universidades-Agencia Estatal de Investigación Proyecto PGC2018-095709-B-C21. We kindly acknowledge the comments of all the reviewers of this paper which have contributed to improve it.Pedraza Aguilera, T.; Rodríguez López, J. (2021). New results on the aggregation of norms. Mathematics. 9(18):1-19. https://doi.org/10.3390/math918229111991