Generalized unitary Bogoliubov transformation that breaks fermion number   parity
  The standard Bogoliubov transformation is generalized to enable fermion number parity breaking. The new transformation can diagonalize fermion Hamiltonians that are quadratic in fermion and number parity operators. This new variational freedom cannot lower the energy of Hamiltonians that conserve number parity. However, it has value as a minimal mechanism for continuously varying between states of different fermion number, regardless of number parity. 
Jonathan E. Moussa
cond-mat.str-el