Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Apparent Horizons in Vacuum Robinson-Trautman Spacetimes   Vacuum asymptotically flat Robinson-Trautman spacetimes are a well known
class of spacetimes exhibiting outgoing gravitational radiation. In this paper
we describe a method of locating the past apparent horizon in these spacetimes,
and discuss the properties of the horizon. We show that the past apparent
horizon is non-timelike, and that its surface area is a decreasing function of
the retarded time. A numerical simulation of the apparent horizon is also
discussed.

27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
23
The categories of this paper are: solv-int
30
The categories of this paper are: q-alg
4
The categories of this paper are: cond-mat
22
The categories of this paper are: chao-dyn
20
The categories of this paper are: acc-phys
11
The categories of this paper are: hep-lat
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
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The categories of this paper are: adap-org
0
The categories of this paper are: hep-ph
17
The categories of this paper are: q-fin
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The categories of this paper are: bayes-an
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The categories of this paper are: funct-an
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The categories of this paper are: eess