Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A concise introduction to trapped surface formation in general
  relativity   Trapped surface formation in general relativity can be studied through a
coupled set of nonlinear equations, where various terms can be neglected, as
was proved by a rigorous mathematical analysis of Christodoulou. This paper is
devoted to a pedagogical synthesis of the mathematical formalism employed in
this analysis, i.e. the optical structure of general relativity investigated by
Christodoulou, Klainerman and other authors after them.

23
The categories of this paper are: solv-int
8
The categories of this paper are: hep-ex
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
21
The categories of this paper are: adap-org
11
The categories of this paper are: hep-lat
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
4
The categories of this paper are: cond-mat
3
The categories of this paper are: physics
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin