content_image/360075.jpg content_image/360076.jpg content_image/360077.jpg content_image/360078.jpg content_image/360079.jpg content_image/360080.jpg content_image/360081.jpg content_image/360082.jpg
The corresponding expression for I$_{VP}$ ( ω ) in the Feynman gauge can be found, e.g., in Ref. [58]. The contribution of this diagram reads 

<img src='content_image/360075.jpg'>

The contribution of the diagram D is given by the expression 

<img src='content_image/360076.jpg'>

where the operator I ml VP can be derived within the two-time Green function method. In the present work we omit this term, assuming its value is relatively small. 

The reducible contributions G , H and I are given by 

<img src='content_image/360077.jpg'>

<img src='content_image/360078.jpg'>

<img src='content_image/360079.jpg'>

<img src='content_image/360080.jpg'>

<img src='content_image/360081.jpg'>

<img src='content_image/360082.jpg'>

