The action functional contains the integrand ##\bar{\phi}(t)\mathcal{D}^{-1}\phi(t)##. By quadratic in the fields, do you mean ##\bar{\phi},\phi## or do you mean ##\bar{\phi},\mathcal{D}^{-1}\phi##?
For the second half of your response, thank you very much! I didn't know if ##\langle ab + cd...
How should the calculations be made in the correlation brackets? Or might you have a reference I could look at as a guide? Also, is the correlation bracket the same as a Green's function?
Hello (I'm reposting this from stack exchange, and thought this site may be more appropriate, so if you see it that's why),
I'm working through this paper, and have encountered "a little algebra shows that...", yet I'm not familiar enough with the topic at hand to figure this out. Here is the...