DPT of electric properties

We use a superscript for orders in A, to distinguish from the expansion in powers of PT in orders of A is then straightforward [58] 

If we want to study the nrl and the relativistic corrections of the first-order property E or the second-order property E we must apply double perturbation theory. To formulate this we first switch to the DE with modified metric 

We must distinguish between two cases. For an electric perturbation fi is diagonal in the upper and lower components 

It does, probably not create any confusion that the same symbol Q is used for an operator acting on 4-component spinors and one acting on 2-component spinors. The case of a magnetic perturbation with 12 nondiagonal will be discussed in the next subsection. We start from 

The first subscript always refers to orders in c and the second subscript to orders in A. We write Eq and ipQ instead of 00 and oo- Expansion of (310) yields 

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