Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A unique focusing property of a parabolic mirror for neutrons in the
  gravitational field: geometric proof   An extraordinary focusing property of a parabolic mirror for ultracold
neutrons in the presence of the gravitational field was first reported by A.
Steyerl and co-authors. It was shown that all neutrons emitted from the focus
of the mirror will be reflected back upon the same focus point passing, in
between, a point of return in the gravitational field. The present note offers
a complementary geometric proof of this feature and discusses some
implications.

20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
6
The categories of this paper are: astro-ph
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
26
The categories of this paper are: bayes-an
17
The categories of this paper are: q-fin
12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
2
The categories of this paper are: math