Schwinger's Principle and Gauge Fixing in the Free Electromagnetic Field
  A manifestly covariant treatment of the free quantum eletromagnetic field, in a linear covariant gauge, is implemented employing the Schwinger's Variational Principle and the B-field formalism. It is also discussed the abelian Proca's model as an example of a system without constraints. 
C. A. M. de Melo, B. M. Pimentel and P. J. Pompeia