  We bosonize the long-wavelength excitations of interacting fermions in
arbitrary dimension by directly applying a suitable Hubbard-Stratonowich
transformation to the Grassmannian generating functional of the fermionic
correlation functions. With this technique we derive a surprisingly simple
expression for the single-particle Greens-function, which is valid for
arbitrary interaction strength and can describe Fermi- as well as Luttinger
liquids. Our approach sheds further light on the relation between bosonization
and the random-phase approximation, and enables us to study screening in a
non-perturbative way.
