Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Shear-free ray congruences on curved space-times
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
5
The categories of this paper are: gr-qc
17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat
27
The categories of this paper are: comp-gas
24
The categories of this paper are: dg-ga
34
The categories of this paper are: chem-ph
7
The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph
13
The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
9
The categories of this paper are: nlin