Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: How to obtain the covariant form of Maxwell's equations from the
  continuity equation   The covariant Maxwell equations are derived from the continuity equation for
the electric charge. This result provides an axiomatic approach to Maxwell's
equations in which charge conservation is emphasized as the fundamental axiom
underlying these equations.

21
The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
29
The categories of this paper are: funct-an
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
14
The categories of this paper are: math-ph
37
The categories of this paper are: supr-con
20
The categories of this paper are: acc-phys
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin