Taylor Expansion of Potential and Energy

In order to simplify this expression we first need to expand the factor y(r2) in 

Naturally, an expansion in terms of the reciprocal speed of light will finally result in a leading-order nonrelativistic description (with instantaneous transmission) of the interaction between two charges together with low-order relativistic corrections. 

Hidden in the above expression is the fact that all quantities that carry the index of the second charge 2 are to be taken at time tz although the interaction energy is sought at time fi. 

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