  The many-body theory of degenerate systems is described in detail. More
generally, this theory applies to systems with an initial state that cannot be
described by a single Slater determinant. The double-source (or
closed-time-path) formalism of nonequilibrium quantum field theory is used to
derive an expression for the average value of a product of interacting fields
when the initial state is not the vacuum or a Slater determinant. Quantum group
techniques are applied to derive the hierarchy of unconnected Green functions
and the hierarchy of connected ones.
