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{\bf  Problem Set 2}
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1) Consider the problem of electrons interacting with impurities with
concentration $n_i$, assume that the interaction potential is
weak and short range.
Assume the system is in equilibrium, at a temperature T.
a) Evaluate the following self energy  functions:

$\Sigma^t$, $ \Sigma^{>} $,   $ \Sigma^{<} $,
  $ \Sigma^{A} $ ,  $ \Sigma^{R} $  $ \Sigma^{K} $  

to second order in perturbation theory in the impurity potential.

%b) can you describe the physical meaning of these functions?



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\protect\caption{ Second Order and First Order Self Energies}
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