Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ghosts in pure and hybrid formalisms of gravity theories: a unified
  analysis   In the first order formalism of gravitational theories, the spacetime
connection is considered as an independent variable to vary together with the
metric. However, the metric still generates its Levi-Civita connection that
turns out to determine the geodesics of matter. Recently, "hybrid" gravity
theories have been introduced by constructing actions involving both the
independent Palatini connection and the metric Levi-Civita connection. In this
study a method is developed to analyse the field content of such theories, in
particular to determine whether the propagating degrees of freedom are ghosts
or tachyons. New types of second, fourth and sixth order derivative gravity
theories are investigated and the so called f(X) theories are singled out as a
viable class of "hybrid" extensions of General Relativity.

1
The categories of this paper are: cs
9
The categories of this paper are: nlin
22
The categories of this paper are: chao-dyn
26
The categories of this paper are: bayes-an
15
The categories of this paper are: nucl-ex
21
The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
13
The categories of this paper are: quant-ph
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
6
The categories of this paper are: astro-ph
4
The categories of this paper are: cond-mat
23
The categories of this paper are: solv-int
29
The categories of this paper are: funct-an
24
The categories of this paper are: dg-ga