Linear-quadratic control and quadratic differential forms

We consider the infinite time linear-quadratic control problem from a behavioral point of view. The performance functional is the integral of a quadratic differential form. A characterization of the stationary trajectories and of the local minima with respect to (left) compact support variations, as well as their relation to stability, are obtained. Finally, several theorems are derived that describe the optimal LQ trajectories with specified initial, and possibly terminal, conditions