  A Green's function approach to the inclusive quasielastic ($e,e'$) scattering
is presented. The components of the nuclear response are written in terms of
the single-particle optical model Green's function. The explicit calculation of
the Green's function can be avoided by its spectral representation, which is
based on a biorthogonal expansion in terms of the eigenfunctions of the
non-Hermitian optical potential and of its Hermitian conjugate. This allows one
to treat final state interactions consistently in the inclusive ($e,e'$) and in
the exclusive ($e,e'N$) reactions. Numerical results for the longitudinal and
transverse response functions obtained in a nonrelativistic and in a
relativistic framework are presented and discussed also in comparison with
data.
