Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Physical Meaning of Gauge Transformations in Classical
  Electrodynamics   The structure of electrodynamics based on the variational principle together
with causality and space-time homogeneity is analyzed. It is proved that in
this case the 4-potential is defined uniquely. Therefore, the approach where
Maxwell equations and the Lorentz law of force are regarded as cornerstones of
the theory is not equivalent to the one described above.

30
The categories of this paper are: q-alg
7
The categories of this paper are: hep-th
23
The categories of this paper are: solv-int
32
The categories of this paper are: atom-ph
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
6
The categories of this paper are: astro-ph
9
The categories of this paper are: nlin
14
The categories of this paper are: math-ph
37
The categories of this paper are: supr-con
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
22
The categories of this paper are: chao-dyn
25
The categories of this paper are: patt-sol
4
The categories of this paper are: cond-mat
19
The categories of this paper are: eess