“…We know that the operator P(A) is closable and that the following holds (cf. [46], [25], [20] and [18] for further information): Assuming that E is a function space on which translations are uniformly bounded and strongly continuous, the obvious choice for A j is −i∂/∂x j (notice also that E can be consisted of functions defined on some bounded domain [5], [25], [46], [47]). If P(x) = |η|≤N a η x η , x ∈ R n and E is such a space (for example, L p (R n ) with p ∈ [1, ∞), C 0 (R n ) or BUC(R n )), then it is not difficult to prove that P(A) is nothing else but the operator Let p ∈ [1, ∞].…”