Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Modified Gravity Models Admitting Second Order Equations of Motion   The aim of this paper is to find higher order geometrical corrections to the
Einstein-Hilbert action that can lead to only second order equations of motion.
The metric formalism is used, and static spherically symmetric and
Friedmann-Lema\^itre space-times are considered, in four dimensions. The
FKWC-basis are introduced in order to consider all the possible invariant
scalars, and both polynomial and non-polynomial gravities are investigated.

16
The categories of this paper are: stat
22
The categories of this paper are: chao-dyn
37
The categories of this paper are: supr-con
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
0
The categories of this paper are: hep-ph
14
The categories of this paper are: math-ph
26
The categories of this paper are: bayes-an
1
The categories of this paper are: cs
21
The categories of this paper are: adap-org
10
The categories of this paper are: q-bio
4
The categories of this paper are: cond-mat