Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cosmological perfect fluids in higher-order gravity   We prove that in Robertson-Walker space-times (and in generalized
Robertson-Walker spacetimes of dimension greater than 3 with divergence-free
Weyl tensor) all higher-order gravitational corrections of the Hilbert-Einstein
Lagrangian density $F(R,\square R, ... , \square^k R)$ have the form of perfect
fluids in the field equations. This statement definitively allows to deal with
dark energy fluids as curvature effects.

37
The categories of this paper are: supr-con
13
The categories of this paper are: quant-ph
27
The categories of this paper are: comp-gas
4
The categories of this paper are: cond-mat
23
The categories of this paper are: solv-int
34
The categories of this paper are: chem-ph
2
The categories of this paper are: math
10
The categories of this paper are: q-bio
8
The categories of this paper are: hep-ex
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
3
The categories of this paper are: physics
14
The categories of this paper are: math-ph