Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

29
The categories of this paper are: funct-an
17
The categories of this paper are: q-fin
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
15
The categories of this paper are: nucl-ex
12
The categories of this paper are: nucl-th
37
The categories of this paper are: supr-con
26
The categories of this paper are: bayes-an
1
The categories of this paper are: cs
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
5
The categories of this paper are: gr-qc