  The self-energy-functional approach (SFA) is discussed in the context of
different variational principles for strongly correlated electron systems.
Formal analogies between static and dynamical variational approaches, different
types of approximation strategies and the relations to density-functional and
dynamical mean-field theory are emphasized. The discussion elucidates the
strengths of the SFA in the construction of new non-perturbative approximations
but also the limitations of the approach and thereby opens up future
perspectives.
