Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A symmetry of the Einstein-Friedmann equations for spatially flat,
  perfect fluid, universes   We report a symmetry property of the Einstein-Friedmann equations for
spatially flat Friedmann-Lema\^itre-Robertson-Walker universes filled with a
perfect fluid with any constant equation of state. The symmetry transformations
form a one-parameter Abelian group.

27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
31
The categories of this paper are: ao-sci
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
1
The categories of this paper are: cs
6
The categories of this paper are: astro-ph
0
The categories of this paper are: hep-ph
21
The categories of this paper are: adap-org
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math