I'm a bit embarrassed to post this, as it might be trivial but what the hey? I'm stuck!

The problem is to reduce this:

into this:

There is going to be a factor of -2i in the process.

If it matters, there is an integration over allkin the unreduced form. These (minus a couple of numerical factors) are the intregrands.

More information upon request.

-Dan

Just for completeness, the Minkowski four-vector for k is . In case you need it.