Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Null surfaces formulation in 3D   The Null Surface Formulation of General Relativity is developed for 2+1
dimensional gravity. The geometrical meaning of the metricity condition is
analyzed and two approaches to the derivation of the field equations are
presented. One method makes explicit use of the conformal factor while the
other only uses conformal information. The resulting set of equations contain
the same geometrical meaning as the 4-D formulation without the technical
complexities of the higher dimensional analog. A canonical family of null
surfaces in this formulation, the light cone cuts of null infinity, are
constructed on asymptotically flat space times and some of their kinematical
aspects discussed. A particular example, which nevertheless contains most of
the generic features is explicitly constructed and analyzed, revealing the
behavior predicted in the full theory.

28
The categories of this paper are: alg-geom
30
The categories of this paper are: q-alg
20
The categories of this paper are: acc-phys
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int
21
The categories of this paper are: adap-org
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
15
The categories of this paper are: nucl-ex
37
The categories of this paper are: supr-con
9
The categories of this paper are: nlin
31
The categories of this paper are: ao-sci