On Tue, 9 Mar 2010, Raoul wrote:
> Sure this general procedure is the better approach. But I don't know
> how this could work here. I even don't know a canonical form for the
> operator expressions.
For expressions with ladder operators a_i, a^+_i the normal form is
canonical (all a^+ at the beginning, all a at the end; inside each group,
order them, say, according to i, the number of the oscillator).
Andrey