My purpose here is to convey to you some idea of the scope and the depth of the work for which we are today honoring Pierre Deligne with the Fields Medal. Deligne's work centers around the remarkable relations, first envisioned by Weil, which exist between the cohomological structure of algebraic varieties over the complex numbers, and the diophantine structure of algebraic varieties over finit...

In this thesis work a complete study of the four quadrants coronagraph is presented This new coronagraph o ers performances of extremely large theorical rejection factor These performances open possibilities of exo planets detection around nearby stars The rst laboratory results gave an attenuation rate higher than on the central peak Thus I naturally studied all the possible uses of this coron...