Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Equilibrium hydrostatic equation and Newtonian limit of the singular
  f(R) gravity   We derive the equilibrium hydrostatic equation of a spherical star for any
gravitational Lagrangian density of the form $L=\sqrt{-g}f(R)$. The Palatini
variational principle for the Helmholtz Lagrangian in the Einstein gauge is
used to obtain the field equations in this gauge. The equilibrium hydrostatic
equation is obtained and is used to study the Newtonian limit for
$f(R)=R-\frac{a^{2}}{3R}$. The same procedure is carried out for the more
generally case $f(R)=R-\frac{1}{n+2}\frac{a^{n+1}}{R^{n}}$ giving a good
Newtonian limit.

27
The categories of this paper are: comp-gas
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess
17
The categories of this paper are: q-fin
3
The categories of this paper are: physics
30
The categories of this paper are: q-alg
20
The categories of this paper are: acc-phys