Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Fundamental solution of the Volkov problem (characteristic
  representation)   The characteristic representation, or Goursat problem, for the
Klein-Fock-Gordon equation with Volkov interaction [1] is regarded. It is shown
that in this representation the explicit form of the Volkov propagator can be
obtained. Using the characteristic representation technique, the Schwinger
integral [2] in the Volkov problem can be calculated.

31
The categories of this paper are: ao-sci
7
The categories of this paper are: hep-th
29
The categories of this paper are: funct-an
19
The categories of this paper are: eess
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
11
The categories of this paper are: hep-lat
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
21
The categories of this paper are: adap-org
35
The categories of this paper are: mtrl-th
18
The categories of this paper are: econ