Central extensions and reciprocity laws
Charles Ehresmann's concepts in differential geometry
We outline some of the tools C. Ehresmann introduced in Differential Geometry (fiber bundles, connections, jets, groupoids, pseudogroups). We emphasize two aspects of C. Ehresmann's works: use of Cartan notations for the theory of connections and semi-holonomic jets.
Classification of (1,1) tensor fields and bihamiltonian structures
Consider a (1,1) tensor field J, defined on a real or complex m-dimensional manifold M, whose Nijenhuis torsion vanishes. Suppose that for each point p ∈ M there exist functions , defined around p, such that and , j = 1,...,m. Then there exists a dense open set such that we can find coordinates, around each of its points, on which J is written with affine coefficients. This result is obtained by associating to J a bihamiltonian structure on T*M.
Connections, local subgroupoids, and a holonomy Lie groupoid of a line bundle gerbe.
Connections on lie Algebroids and the Weil-Kostant Theorem
Continuous family groupoids.