Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Density perturbations in general modified gravitational theories   We derive the equations of linear cosmological perturbations for the general
Lagrangian density $f (R,\phi, X)/2+L_c$, where $R$ is a Ricci scalar, $\phi$
is a scalar field, and $X=-(\nabla \phi)^2/2$ is a field kinetic energy. We
take into account a nonlinear self-interaction term $L_c$ recently studied in
the context of "Galileon" cosmology, which keeps the field equations at second
order. Taking into account a scalar-field mass explicitly, the equations of
matter density perturbations and gravitational potentials are obtained under a
quasi-static approximation on sub-horizon scales. We also derive conditions for
the avoidance of ghosts and Laplacian instabilities associated with propagation
speeds. Our analysis includes most of modified gravity models of dark energy
proposed in literature and thus it is convenient to test the viability of such
models from both theoretical and observational points of view.

25
The categories of this paper are: patt-sol
31
The categories of this paper are: ao-sci
13
The categories of this paper are: quant-ph
35
The categories of this paper are: mtrl-th
36
The categories of this paper are: cmp-lg
22
The categories of this paper are: chao-dyn
19
The categories of this paper are: eess
15
The categories of this paper are: nucl-ex
11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
26
The categories of this paper are: bayes-an
9
The categories of this paper are: nlin
1
The categories of this paper are: cs
17
The categories of this paper are: q-fin
32
The categories of this paper are: atom-ph