Measurements of Riemannian two-disks and two-spheres

We prove that any Riemannian two-sphere with area at most 1 can be continuously mapped onto a tree in a such a way that the topology of fibers is controlled and their length is less than 7.6. This result improves previous estimates and relies on a similar statement for Riemannian two-disks

Stable norms of non-orientable surfaces

We study the stable norm on the first homology of a closed, non-orientable surface equipped with a Riemannian metric. We prove that in every conformal class there exists a metric whose stable norm is polyhedral. Furthermore the stable norm is never strictly convex if the first Betti number of the surface is greater than two

Systolic geometry and simplicial complexity for groups

Twenty years ago Gromov asked about how large is the set of isomorphism classes of groups whose systolic area is bounded from above. This article introduces a new combinatorial invariant for finitely presentable groups called {\it simplicial complexity} that allows to obtain a quite satisfactory answer to his question. Using this new complexity, we also derive new results on systolic area for groups that specify its topological behaviour.Comment: 35 pages, 9 figure

Minimal length product over homology bases of manifolds

Minkowski's second theorem can be stated as an inequality for $n$-dimensional flat Finsler tori relating the volume and the minimal product of the lengths of closed geodesics which form a homology basis. In this paper we show how this fundamental result can be promoted to a principle holding for a larger class of Finsler manifolds. This includes manifolds for which first Betti number and dimension do no necessarily coincide, a prime example being the case of surfaces. This class of manifolds is described by a non-vanishing condition for the hyperdeterminant reduced modulo $2$ of the multilinear map induced by the fundamental class of the manifold on its first ${\mathbb Z}_2$-cohomology group using the cup product.Comment: 24 page

Marco, registro y concepción. Notas sobre las relaciones entre tres conceptos claves en didáctica

Las palabras «marco», «registro» y «medio» designan conceptos de amplia utilización por los investigadores en didáctica de las matemáticas, con el fin de modelizar situaciones o de analizar las actividades de los alumnos. Sin embargo, y a pesar de una literatura importante, su uso plantea problemas recurrentes para distinguirlas y relacionarlas (o vincularlas). Proponemos una solución a tales problemas, analizando las relaciones que mantienen entre ellos estos tres conceptos clave de la didáctica de las matemáticas. Mostraremos que la distinción entre «marco» (o «encuadre») y «concepción» debe ser buscada en el anclaje problemático de cada uno de estos conceptos; para los «marcos (o «encuadres»), se sitúa en primer plano el análisis matemático, para las «concepciones», tal lugar corresponde al análisis de las situaciones generadoras de obligaciones (o limitaciones) para el sujeto que aprende. En lo que a los «registros» se refiere, mostramos que son una herramienta indispensable para el funcionamiento de los «marcos» (o encuadres), como concepciones, y de sus relaciones; entre dos marcos, o dos concepciones, los registros tienen una función de mediación semiótica