Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: On Einstein's equations for spacetimes admitting a non-null Killing
  field
16
The categories of this paper are: stat
2
The categories of this paper are: math
6
The categories of this paper are: astro-ph
34
The categories of this paper are: chem-ph
0
The categories of this paper are: hep-ph
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th
18
The categories of this paper are: econ
11
The categories of this paper are: hep-lat
15
The categories of this paper are: nucl-ex
33
The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
7
The categories of this paper are: hep-th