Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Some results on the integrability of Einstein's field equations for
  axistationary perfect fluids
30
The categories of this paper are: q-alg
13
The categories of this paper are: quant-ph
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
25
The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
3
The categories of this paper are: physics
2
The categories of this paper are: math
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
18
The categories of this paper are: econ
35
The categories of this paper are: mtrl-th
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci