Free group automorphisms with parabolic boundary orbits

For $N\geq 4$, we show that there exist automorphisms of the free group $F_N$ which have a parabolic orbit in $\partial F_N$. In fact, we exhibit a technology for producing infinitely many such examples

Homomorphisms between diffeomorphism groups

For r at least 3, p at least 2, we classify all actions of the groups Diff^r_c(R) and Diff^r_+(S1) by C^p -diffeomorphisms on the line and on the circle. This is the same as describing all nontrivial group homomorphisms between groups of compactly supported diffeomorphisms on 1- manifolds. We show that all such actions have an elementary form, which we call topologically diagonal. As an application, we answer a question of Ghys in the 1-manifold case: if M is any closed manifold, and Diff(M)_0 injects into the diffeomorphism group of a 1-manifold, must M be 1 dimensional? We show that the answer is yes, even under more general conditions. Several lemmas on subgroups of diffeomorphism groups are of independent interest, including results on commuting subgroups and flows.Comment: Contains corrections and additional references. A revised version will appear in Ergodic Theory and Dynamical System

High temperature annealing of micrometric Zn2SiO4:Mn phosphor powders in fluidized bed

Micrometric Zn1.8Mn0.2SiO4 phosphor powders prepared by spray pyrolysis have been annealed between 900 and 1200°C under ambient air atmosphere to exalt their luminescence properties. Two original gas-solid fluidization processes have been tested in order to limit sintering phenomena, and the post-treated products have been compared with those annealed using a conventional process in crucible. The crystallinity, the size distribution, the outer morphology and the luminescence properties of powders before and after treatment have been analysed. Massive sintering phenomena occur in crucible from 1000°C, whereas the original granulometry and spherical morphology are preserved till 1100°C in fluidized bed. The luminescence efficiencies are comparable for the three processes and maximal after annealing at 1200°C. It has been established that residual ZnO and manganese ions at oxidation state higher than 2, still present after treatment at 1100°C, are detrimental to good luminescence efficiency. Both disappear from samples post-treated at 1200°C

Commutator length of annulus diffeomorphisms

We study the group of C^{r}-diffeomorphisms of the closed annulus that are isotopic to the identity. We show that, for r different from 3, the linear space of homogeneous quasi-morphisms on this group is one dimensional. Therefore, the commutator length on this group is (stably) unbounded. In particular, this provides an example of a manifold whose diffeomorphisms group is unbounded in the sense of Burago, Ivanov and Polterovich

Bonne année anomalistique !

International audienceQu'est-ce qu'une année ? Trois cent soixante-cinq jours sauf pour les années bissextiles qui comptent un jour de plus ? En général, on explique cela en disant que la Terre tourne autour du Soleil en trois cent soixante-cinq jours un quart. Si on consulte un dictionnaire, on trouve beaucoup de mots savants : connaissez-vous l'année anomalistique, l'année sidérale, ou l'année tropique ? Quelques explications ne sont peut-être pas inutiles

Géométriser l'espace : de Gauss à Perelman

International audienceIl y a exactement cent ans, Henri Poincaré publiait la preuve du théorème d'uniformisation. Il concluait ainsi une longue série de travaux, s'étalant sur tout le dix-neuvième siècle, et remontant au moins à Gauss. Il y a quatre ans, Grigori Perelman publiait sur internet trois articles majeurs qui mènent à la preuve de la conjecture de géométrisation de Thurston, elle-même directement issue du théorème d'uniformisation. Cela lui a valu la médaille Fields en 2006. Je voudrais esquisser l'histoire de cette aventure en tentant d'éviter tout aspect technique. Evidemment, je serai obligé de simplifier à l'extrême et de passer sous silence un grand nombre d'aspects importants

L'effet papillon

International audienceUn coup de feu éclate au Maroc... Incident sans importance, mais qui va changer la vie d'un couple d'américains, d'une nourrice mexicaine, d'une adolescente japonaise. Tout un jeu de petits effets et de grandes conséquences. Une réflexion sur la destinée humaine, soumise au hasard ou à la nécessité ? Beaucoup d'émotion dans Babel, ce film de Alejandro González Iñárritu sorti en 2006. Les critiques de cinéma n'ont pas manqué d'y voir une illustration du fameux effet papillon qui est probablement le phénomène mathématique le plus connu du grand public

Abuso espectacular y sistemático de derechos humanos en México

This paper serves as an introduction to the special issue on Human Rights. It starts with a general introduction to the concept and logic of human rights, as well as a framework for understanding the different types of law and international norms that are often referred to in discussions on Human Rights. The central part of the text discusses systemic or structural human rights abuses, which are often less visible than ‘spectacular’ events. The article goes on to explore why certain rights (for example economic rights) are harder to politicize as explicit human rights issues, giving consideration to both the institutional landscape, the social position of the affected social groups and the rights themselves. It concludes by stressing the importance of framing all human rights abuse as human rights abuse, which requires sustained research attention for systemic problems of vulnerable groups.Este artículo sirve como una introducción a la edición especial en Derechos Humanos. Empieza con una introducción general al concepto y lógica de los derechos humanos, así como también un marco referencial para entender los diferentes tipos de leyes y normas internacionales a los que se refieren usualmente en Derechos Humanos. La parte central del texto discute los abusos sistémicos y estructurales a los derechos humanos, los cuales son menos visibles que los eventos "espectaculares". El artículo continúa explorando por qué ciertos derechos (por ejemplo los económicos) son más difíciles de politizar como problemas explícitos de los derechos humanos, considerando tanto el escenario institucional, la posición social de los grupos afectados, y los derechos como tal. Concluye enfatizando la importancia del encuadre de todos los abusos de derechos humanos como abusos, lo cual requiere de atención por parte de los investigadores para los problemas sistemáticos de los grupos vulnerables

A finitely generated, locally indicable group with no faithful action by C^1 diffeomorphisms of the interval

According to Thurston's stability theorem, every group of C^1 diffeomorphisms of the closed interval is locally indicable (.e., every finitely generated subgroup factors through Z). We show that, even for finitely generated groups, the converse of this statement is not true. More precisely, we show that the semi-direct product between F_2 an Z^2, although locally indicable, does not embed into Diff_+^1 (]0,1[). (Here F_2 is any free subgroup of SL(2,Z), and its action on Z^2 is the projective one.) Moreover, we show that for every non-solvable subgroup G of SL(2,Z), the semi-direct product between G and Z^2 does not embed into Diff^1_+(S^1).Comment: 8 pages, no figur