Yang-Mills theory
The Landau gauge corresponds to the following
function 
,
where

is the space of functions on the four-dimensional spacetime:
In this case

is

and Eq. (
6) becomes:
The integration contour is

complex conjugate to

.
Don’t forget to insert

—
see Eq. (
5) !
(This is usually done by means of a Lagrange multiplier.)