Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spherically Symmetric Perturbation of Ultrarelativistic Fluid in
  Homogeneous and Isotropic Universe   A solution of the linearized Einstein's equations for a spherically symmetric
perturbation of the ultrarelativistic fluid in the homogeneous and isotropic
universe is obtained. Conditions on the boundary of the perturbation are
discussed. The examples of particle-like and wave-like solutions are given.

18
The categories of this paper are: econ
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph
2
The categories of this paper are: math
3
The categories of this paper are: physics
17
The categories of this paper are: q-fin
25
The categories of this paper are: patt-sol
22
The categories of this paper are: chao-dyn
4
The categories of this paper are: cond-mat
30
The categories of this paper are: q-alg
7
The categories of this paper are: hep-th
8
The categories of this paper are: hep-ex
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph