Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Bilinear estimates associated to the Schr\"odinger equation with a
  nonelliptic principal part   We discuss bilinear estimates of tempered distributions in the Fourier
restriction spaces for the two-dimensional Sch\"odinger equation whose
principal part is the d'Alembertian. We prove that the bilinear estimates hold
if and only if the tempered distributions are functions.

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The categories of this paper are: gr-qc
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The categories of this paper are: q-alg
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The categories of this paper are: solv-int
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The categories of this paper are: hep-ex
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The categories of this paper are: mtrl-th
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The categories of this paper are: physics
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The categories of this paper are: bayes-an
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The categories of this paper are: dg-ga
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The categories of this paper are: econ
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The categories of this paper are: cs
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The categories of this paper are: alg-geom
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The categories of this paper are: hep-lat
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The categories of this paper are: quant-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: math-ph
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The categories of this paper are: atom-ph