Partial Differential Equations and Group Theory

Ordinary differential control thPory (the classical theory) studies input/output re­ lations defined by systems of ordinary differential equations (ODE). The various con­ cepts that can be introduced (controllability, observability, invertibility, etc. ) must be tested on formal objects (matrices, vector fields, etc. ) by means of formal operations (multiplication, bracket, rank, etc. ), but without appealing to the explicit integration (search for trajectories, etc. ) of the given ODE. Many par