Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Curvature scalar instability in f(R) gravity   An instability in the presence of matter in theories of gravity which include
a 1/R correction in the gravitational action has been found by Dolgov and
Kawasaki. In the present paper this instability is discussed for f(R) gravity
in general. We focus on the Palatini formalism of the theory and it is shown
that no such instability occurs in this version of f(R) gravity. The reasons
for the appearance of the instability in the metric but not in the Palatini
formalism are fully investigated.

2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
8
The categories of this paper are: hep-ex
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
21
The categories of this paper are: adap-org