Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Perfect-fluid, generalised Robertson-Walker space-times, and Gray's
  decomposition   We give new necessary and sufficient conditions on the Weyl tensor for
generalized Robertson-Walker (GRW) space-times to be perfect-fluid space-times.
For GRW space-times, we determine the form of the Ricci tensor in all the
O(n)-invariant subspaces provided by Gray's decomposition of the gradient of
the Ricci tensor. In all but one, the Ricci tensor is Einstein or has the form
of perfect fluid. We discuss the corresponding equations of state that result
from the Einstein equation in dimension 4, where perfect-fluid GRW space-times
are Robertson-Walker.

16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
23
The categories of this paper are: solv-int
11
The categories of this paper are: hep-lat
26
The categories of this paper are: bayes-an
20
The categories of this paper are: acc-phys
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
30
The categories of this paper are: q-alg
6
The categories of this paper are: astro-ph
0
The categories of this paper are: hep-ph