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In my textbook (see attached picture) there appears a functional derivative, but I honestly don't know how to evaluate a quantity like this. What should I do? I have tried to google but all I could find was how to take functional derivatives, where polynomials appeared under the integral, while here is an exponential.
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![Printed text with equations from a physics textbook: generating functional Z[ξ̄, ξ] as a path integral of e^(-S[ξ̄, c̄]) with action S, and the Green's function G(νν', τ - τ') defined as a second functional derivative of Z divided by Z[0, 0], with the fields being complex or Grassmann.](/data/attachments/66/66332-44ea9b88cc3ef5592490811f0ae501b0.jpg?hash=ROqbiMw-9V){kind=small}