Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The growth factor of matter perturbations in an f(R) gravity   The growth of matter perturbations in the $f(R)$ model proposed by
Starobinsky is studied in this paper. Three different parametric forms of the
growth index are considered respectively and constraints on the model are
obtained at both the $1\sigma$ and $2\sigma$ confidence levels, by using the
current observational data for the growth factor. It is found, for all the
three parametric forms of the growth index examined, that the Starobinsky model
is consistent with the observations only at the $2\sigma$ confidence level.

7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
31
The categories of this paper are: ao-sci
33
The categories of this paper are: plasm-ph
3
The categories of this paper are: physics
19
The categories of this paper are: eess
25
The categories of this paper are: patt-sol
34
The categories of this paper are: chem-ph
30
The categories of this paper are: q-alg
15
The categories of this paper are: nucl-ex
29
The categories of this paper are: funct-an
32
The categories of this paper are: atom-ph
35
The categories of this paper are: mtrl-th
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
1
The categories of this paper are: cs