Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Self-similar perfect fluids   Space-times admitting an $r$-parameter Lie group of homotheties are studied
for $r > 2$ devoting a special attention to those representing perfect fluid
solutions to Einstein's field equations.

30
The categories of this paper are: q-alg
11
The categories of this paper are: hep-lat
37
The categories of this paper are: supr-con
36
The categories of this paper are: cmp-lg
21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
9
The categories of this paper are: nlin
26
The categories of this paper are: bayes-an
19
The categories of this paper are: eess
3
The categories of this paper are: physics
6
The categories of this paper are: astro-ph
28
The categories of this paper are: alg-geom
15
The categories of this paper are: nucl-ex