Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Scalar invariants in gravitational and electromagnetic fields   The paper discusses some scalar invariants in the gravitational field and
electromagnetic field by means of the characteristics of the quaternions. When
we emphasize some definitions of quaternion physical quantities, the speed of
light, mass density, energy density, power density, charge density, and spin
magnetic moment density etc. will remain the same respectively in the
gravitational and electromagnetic fields under the coordinate transformation.
The results explain why there are some relationships among different invariants
in the gravitational and electromagnetic fields.

36
The categories of this paper are: cmp-lg
0
The categories of this paper are: hep-ph
14
The categories of this paper are: math-ph
15
The categories of this paper are: nucl-ex
23
The categories of this paper are: solv-int
24
The categories of this paper are: dg-ga
34
The categories of this paper are: chem-ph
25
The categories of this paper are: patt-sol
6
The categories of this paper are: astro-ph
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
16
The categories of this paper are: stat
21
The categories of this paper are: adap-org
37
The categories of this paper are: supr-con