Lagrangian quantum field theory in momentum picture. III. Free vector   fields
  Free vector fields, satisfying the Lorenz condition, are investigated in details in the momentum picture of motion in Lagrangian quantum field theory. The field equations are equivalently written in terms of creation and annihilation operators and on their base the commutation relations are derived. Some problems concerning the vacuum and state vectors of free vector field are discussed. Special attention is paid to peculiarities of the massless case; in particular, the electromagnetic field is explored. Several Lagrangians, describing free vector fields, are considered and the basic consequences of them are pointed and compared. 
Bozhidar Z. Iliev (Institute for Nuclear Research and Nuclear Energy,   Bulgarian Academy of Sciences, Sofia, Bulgaria)