  Based on a canonical approach and functional-integration techniques, a series
expansion of Green's function of a scalar field, in the presence of a medium,
is obtained. A series expansion for Lifshitz-energy, in finite-temperature, in
terms of the susceptibility of the medium is derived and the whole formalism is
generalized to the case of electromagnetic field in the presence of some
dielectrics. A covariant formulation of the problem is presented.
