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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.Comment: 4 sign mistakes due to a change of conventions are corrected in
section 2, none of the conclusions are affected, takes precedence over
published version, including corrigendu