  We apply generalized Bogoliubov transformations to the transfer matrix of
relativistic field theories regularized on a lattice. We derive the conditions
these transformations must satisfy to factorize the transfer matrix into two
terms which propagate fermions and antifermions separately, and we solve the
relative equations under some conditions. We relate these equations to the
saddle point approximation of a recent bosonization method and to the
Foldy-Wouthuysen transformations which separate positive from negative energy
states in the Dirac Hamiltonian.
