Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Neutral perfect fluids and charged thin shells with electromagnetic mass
  in general relativity   We build extended sources for the Reissner-Nordstr\"{o}m metric. Our models
describe a neutral perfect fluid core bounded by a charged thin shell, and
feature everywhere positive rest mass density and everywhere non-negative
active gravitational mass, as well as classical electron radius and
electromagnetic total mass. We contrast our results with previously discussed
models featuring similar properties at the expense of including anisotropic
pressures within the fluid. Our charged thin shells are restricted by the 2D
texture equation of state which causes the continuity of the active
gravitational mass, in spite of the singularity of the energy-momentum tensor.
We mention possible extensions of this study suggested by modified active mass
formulae proposed in the literature.

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The categories of this paper are: mtrl-th
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The categories of this paper are: chem-ph
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The categories of this paper are: supr-con
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The categories of this paper are: ao-sci
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The categories of this paper are: adap-org
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The categories of this paper are: math-ph
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The categories of this paper are: stat
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The categories of this paper are: hep-lat
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The categories of this paper are: funct-an
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-ex
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The categories of this paper are: patt-sol
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The categories of this paper are: nucl-th
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The categories of this paper are: comp-gas
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The categories of this paper are: eess
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The categories of this paper are: acc-phys