  It is shown that a variational approach with fermionic squeezed states to
many-fermion systems such as pairing model is one of useful methods beyond the
usual Hartree-Fock-Bogoliubov approximation. A pairing-type quasi-spin squeezed
state is constructed and adopted as a trial state in the variational method. By
using this state, quantum fluctuations are taken into account. In pairing
model, dynamical and static approaches are discussed in the context of
comparing with the Hartree-Fock-Bogoliubov approximation about ground state
energy. As a possible extension to the O(4) model, prospective squeezed states
are investigated and the usefulness is discussed.
