content_image/326313.jpg content_image/326314.jpg content_image/326315.jpg content_image/326316.jpg content_image/326317.jpg
Using 

and 

<img src='content_image/326313.jpg'>

<img src='content_image/326314.jpg'>

where n k is the occupation number, as well as the commutation relation 

we obtain the 2-point correlator as 

<img src='content_image/326315.jpg'>

<img src='content_image/326316.jpg'>

Working through the integral, we obtain 

<img src='content_image/326317.jpg'>

