Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Double layer from least action principle   We derived the equations for the double layers in Quadratic Gravity, using
solely the least action principle. The advantage of our approach is that, in
the process of calculation, the $\delta'$-function does not appear at all, and
the $\delta$-functions appear for a moment and are mutually canceled prior to
integration. We revealed the peculiar structure of the obtained equations,
namely, that the surface energy-momentum tensor of the matter fields
(constituents of the thin shells) does not play a role in the determination of
the trajectory of the double layer. Also, we suggested that the space-like
double layers may provide us with the adequate description of the creation of
the universe from the black hole singularity. The related topics, including the
Gauss-Bonnet term and $F(R)$-theories, are shortly discussed.

31
The categories of this paper are: ao-sci
3
The categories of this paper are: physics
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
30
The categories of this paper are: q-alg
11
The categories of this paper are: hep-lat
34
The categories of this paper are: chem-ph
1
The categories of this paper are: cs
20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
32
The categories of this paper are: atom-ph
26
The categories of this paper are: bayes-an
10
The categories of this paper are: q-bio
33
The categories of this paper are: plasm-ph
28
The categories of this paper are: alg-geom
16
The categories of this paper are: stat