Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Classical central extension for asymptotic symmetries at null infinity
  in three spacetime dimensions   The symmetry algebra of asymptotically flat spacetimes at null infinity in
three dimensions is the semi-direct sum of the infinitesimal diffeomorphisms on
the circle with an abelian ideal of supertranslations. The associated charge
algebra is shown to admit a non trivial classical central extension of Virasoro
type closely related to that of the anti-de Sitter case.

6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
26
The categories of this paper are: bayes-an
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph
31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
21
The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
11
The categories of this paper are: hep-lat