[IEEE 2006 IEEE Conference on Computer Aided Control System Design, 2006 IEEE International Conference on Control Applications, 2006 IEEE International Symposium on Intelligent Control - Munich, Germany (2006.10.4-2006.10.6)] 2006 IEEE Conference on Computer Aided Control System Design, 2006 IEEE International Conference on Control Applications, 2006 IEEE International Symposium on Intelligent Control - Algebraic and geometrical conditions for the observability of the discrete state of a class of hybrid systems