“…the corresponding task is to envisage conditions on the operator kernel K(t) warranting preservation of positivity and trace of the solutions of the integrodifferential equation. In both cases the most general solution to the problem is not known, even not heuristically, while partial results have been recently obtained [29,30,31,32,33,34,35,34,36]. In particular we will consider how to obtain well-defined quantum memory kernels K(t).…”