“…This extension induces noticeable changes: there appears a neccessity of interpretation of the initial conditions [31] and a neccesity to take into account the history (or memory) of the system from the initial instant, since we have to deal, instead of the classical state, with a so-called "'pseudo state"', that is an expanded state [32,33]. This infinite-dimensional system, as we expose it in the following sections, has been studied, from the point of view of its structural properties, i.e., controllability, observability [34,35] and, to some extent, of its stability [36].…”