Perturbation expansion of the operators

A closed expression for X - which satisfies (149) - and hence Lpw is only possible for a free particle. However a formal expansion in powers of c is straightforward 

The somewhat unconventional counting of the orders in powers of c (actually we expand X = cX rather than X in powers of c ) is convenient for a later use (section 6). From the expansion of X one gets in an elementary, but rather tedious way  

---