Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gravitational Collapse of Self-Similar Perfect Fluid in 2+1 Gravity   Perfect fluid with kinematic self-similarity is studied in 2+1 dimensional
spacetimes with circular symmetry, and various exact solutions to the Einstein
field equations are given. In particular, these include all the solutions of
dust and stiff perfect fluid with self-similarity of the first kind, and all
the solutions of perfect fluid with a linear equation of state and
self-similarity of the zeroth or second kind. It is found that some of these
solutions represent gravitational collapse, and the final state of the collapse
can be either black holes or naked singularities.

30
The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
11
The categories of this paper are: hep-lat
18
The categories of this paper are: econ
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics
33
The categories of this paper are: plasm-ph
1
The categories of this paper are: cs
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
32
The categories of this paper are: atom-ph
0
The categories of this paper are: hep-ph
17
The categories of this paper are: q-fin
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio