Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: General-Relativistic MHD for the Numerical Construction of Dynamical
  Spacetimes   We assemble the equations of general relativistic magnetohydrodynamics (MHD)
in 3+1 form. These consist of the complete coupled set of Maxwell equations for
the electromagnetic field, Einstein's equations for the gravitational field,
and the equations of relativistic MHD for a perfectly conducting ideal gas. The
adopted form of the equations is suitable for evolving numerically a
relativistic MHD fluid in a dynamical spacetime characterized by a strong
gravitational field.

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The categories of this paper are: quant-ph
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The categories of this paper are: math
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The categories of this paper are: dg-ga
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The categories of this paper are: comp-gas
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The categories of this paper are: chao-dyn
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The categories of this paper are: stat
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The categories of this paper are: solv-int
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The categories of this paper are: acc-phys
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The categories of this paper are: math-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: cond-mat
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The categories of this paper are: q-fin
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The categories of this paper are: mtrl-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: eess
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The categories of this paper are: alg-geom