“…We are not primarily concerned with studying new concepts in the theory of hypercyclicity and our main intention is, in fact, to analyze the basic properties of a new important class of abstract second order (ill-posed) PDEs (cf. [3], [23], [26], [32], [34]- [35], [39], [42]- [44], [49] and [56]- [57] for further information in this direction).…”