Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Pin Groups in General Relativity   There are eight possible Pin groups that can be used to describe the
transformation behaviour of fermions under parity and time reversal. We show
that only two of these are compatible with general relativity, in the sense
that the configuration space of fermions coupled to gravity transforms
appropriately under the space-time diffeomorphism group.

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The categories of this paper are: physics
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The categories of this paper are: solv-int
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The categories of this paper are: stat
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The categories of this paper are: funct-an
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The categories of this paper are: hep-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: q-alg
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The categories of this paper are: acc-phys
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The categories of this paper are: dg-ga
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The categories of this paper are: eess
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The categories of this paper are: econ
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The categories of this paper are: mtrl-th
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The categories of this paper are: astro-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: q-fin