**
Below is the ascii version of the abstract for 08-179.
The html version should be ready soon.**Rafael Tiedra de Aldecoa
Time delay for dispersive systems in quantum scattering theory
(455K, Postscript)
ABSTRACT. We consider time delay and symmetrised time delay (defined in terms of sojourn times) for quantum scattering pairs $\{H_0=h(P),H\}$, where $h(P)$ a dispersive operator of hypoelliptic-type. For instance $h(P)$ can be one of the usual elliptic operators such as the Schr\"odinger operator $h(P)=P^2$ or the square-root Klein-Gordon operator $h(P)=\sqrt{1+P^2}$. We show under general conditions that the symmetrised time delay exists for all smooth even localization functions. It is equal to the Eisenbud-Wigner time delay plus a contribution due to the non-radial component of the localization function. If the scattering operator $S$ commutes with some function of the velocity operator $\nabla h(P)$, then the time delay also exists and is equal to the symmetrised time delay. As an illustration of our results we consider the case of a one-dimensional Friedrichs Hamiltonian perturbed by a finite rank potential.
Our study put into evidence an integral formula relating the operator of differentiation with respect to the kinetic energy $h(P)$ to the time evolution of localization operators.