Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An Evolution Equation Approach to the Klein-Gordon Operator on Curved
  Spacetime   We develop a theory of the Klein-Gordon equation on curved spacetimes. Our
main tool is the method of (non-autonomous) evolution equations on Hilbert
spaces. This approach allows us to treat low regularity of the metric, of the
electromagnetic potential and of the scalar potential. Our main goal is a
construction of various kinds of propagators needed in quantum field theory.

4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
6
The categories of this paper are: astro-ph
26
The categories of this paper are: bayes-an
31
The categories of this paper are: ao-sci
12
The categories of this paper are: nucl-th
7
The categories of this paper are: hep-th
8
The categories of this paper are: hep-ex
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
18
The categories of this paper are: econ
33
The categories of this paper are: plasm-ph
13
The categories of this paper are: quant-ph