Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Black shells and naked shells   We study the collapse of a thin dust shell from the point of view of the
horizon dynamics. We identify the critical surfaces at which time and space
coordinates interchange their roles and investigate their properties by using
the formalism of trapped surfaces. We show the existence of marginally outer
trapped surfaces that are associated with the presence of quasi-local horizons.
A particular shell configuration that avoids the formation of horizons is
interpreted as naked shell.

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The categories of this paper are: mtrl-th
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The categories of this paper are: q-fin
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The categories of this paper are: nucl-ex
2
The categories of this paper are: math
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The categories of this paper are: stat
3
The categories of this paper are: physics
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The categories of this paper are: acc-phys
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The categories of this paper are: atom-ph
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The categories of this paper are: solv-int
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The categories of this paper are: dg-ga
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The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
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The categories of this paper are: adap-org
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The categories of this paper are: patt-sol
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The categories of this paper are: alg-geom
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The categories of this paper are: chem-ph