  We describe basic types of free fields (1) neutral scalar bosons; (2) massive
vector bosons; (3) massless vector bosons; (4) charged scalar bosons; (5) Dirac
fermions; (6) Majorana fermions. We also consider free field perturbed by a
linear or quadratic perturbation involving a classical (c-number) function: (1)
neutral scalar bosons interacting with a linear source; (2) neutral scalar
bosons interacting with a mass-like perturbation; (3) massive photons
interacting with an classical current; (4) massless photons interacting with an
classical current. (5) charged scalar bosons interacting with an
electromagnetic potential; (6) Dirac fermions interacting with an
electromagnetic potential.
