Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Boundary Conditions for the Gravitational Field   A review of the treatment of boundaries in general relativity is presented
with the emphasis on application to the formulations of Einstein's equations
used in numerical relativity. At present, it is known how to treat boundaries
in the harmonic formulation of Einstein's equations and a tetrad formulation of
the Einstein-Bianchi system. However, a universal approach valid for other
formulations is not in hand. In particular, there is no satisfactory boundary
theory for the 3+1 formulations which have been highly successful in binary
black hole simulation. I discuss the underlying problems that make the
initial-boundary value problem much more complicated than the Cauchy problem. I
review the progress that has been made and the important open questions that
remain.

14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
27
The categories of this paper are: comp-gas
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
36
The categories of this paper are: cmp-lg
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
8
The categories of this paper are: hep-ex
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph
6
The categories of this paper are: astro-ph
28
The categories of this paper are: alg-geom
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg