Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Comment on "Pentadiamond: A Hard Carbon Allotrope of a Pentagonal
  Network of sp2 and sp3 atoms"   In a recent Letter [1] Y. Fujii et al. proposed a new carbon allotrope and
claimed its extremely high Young's and shear moduli of 1691 and 1113 GPa,
respectively, surpassing those of diamond by 1.5 and 2 times! The authors also
claimed unique negative averaged Poisson's ratio of -0.24, bulk modulus of 381
GPa and record value of the sound speed 28.7 km/s for this carbon modification.
Here we show by two independent computations that these values were obtained as
a result of errors. The new hypothetical allotrope has in fact quite moderate
elastic moduli, unremarkable for a carbon structure with similar density: bulk
modulus of 250 GPa, shear modulus of 170 GPa,Young's modulus of 417 GPa,
positive Poisson's ratio of 0.22.

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The categories of this paper are: nlin
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The categories of this paper are: cmp-lg
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The categories of this paper are: eess
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The categories of this paper are: q-alg
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The categories of this paper are: mtrl-th
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: comp-gas
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The categories of this paper are: econ
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The categories of this paper are: physics
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The categories of this paper are: astro-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-th