Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A no-ghost theorem for the bosonic Nappi-Witten string   We prove a no-ghost theorem for a bosonic string propagating in Nappi-Witten
spacetime. This is achieved in two steps. We first demonstrate unitarity for a
class of NW/U(1) modules: the norm of any state which is primary with respect
to a chosen timelike U(1) is non-negative. We then show that physical states -
states satisfying the Virasoro constraints - in a class of modules of an
affinisation of the Nappi-Witten algebra are contained in the NW/U(1) modules.
Similar to the case of strings on $AdS_3$, in order to saturate the spectrum
obtained in light-cone quantization we are led to include modules with energy
not bounded from below, which are related to modules with energy bounded from
below by spectral flow automorphisms.

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The categories of this paper are: acc-phys
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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: q-bio
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ex
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The categories of this paper are: cmp-lg
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The categories of this paper are: chao-dyn
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The categories of this paper are: nlin
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The categories of this paper are: adap-org
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The categories of this paper are: patt-sol
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The categories of this paper are: math
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The categories of this paper are: cs
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The categories of this paper are: hep-lat
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The categories of this paper are: alg-geom