Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Least action principle and gravitational double layer   The higher derivative gravitational theories exhibit new phenomena absent in
General Relativity. One of them is the possible formation of the so called
double layer which is the pure gravitational phenomenon and can be interpreted,
in a sense, as the gravitational shock wave. In this paper we show how some
very important features of the double layer equations of motion can be
extracted straight from the least action principle.

7
The categories of this paper are: hep-th
32
The categories of this paper are: atom-ph
34
The categories of this paper are: chem-ph
15
The categories of this paper are: nucl-ex
30
The categories of this paper are: q-alg
19
The categories of this paper are: eess
23
The categories of this paper are: solv-int
16
The categories of this paper are: stat
4
The categories of this paper are: cond-mat
18
The categories of this paper are: econ
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
28
The categories of this paper are: alg-geom
11
The categories of this paper are: hep-lat
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci