  We continue our analysis of the consequences of the commutation relation
$[S,T]=\Id$, where $S$ and $T$ are two closable unbounded operators. The {\em
weak} sense of this commutator is given in terms of the inner product of the
Hilbert space $\H$ where the operators act. {We also consider what we call,
adopting a physical terminology}, a {\em nonlinear} extension of the above
commutation relations.
