Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spontaneous and Induced Radiation by Relativistic Particles in Natural
  and Photonic Crystals. Crystal X-ray Lasers and Volume Free Electron Lasers
  (VFEL)   The mechanisms of spontaneous and induced radiation produced by relativistic
particles passing through natural and photonic crystals are reviewed. The
theory of Volume Free Electron Lasers based on spontaneous radiation in natural
and photonic crystals is presented.

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The categories of this paper are: cs
32
The categories of this paper are: atom-ph
26
The categories of this paper are: bayes-an
19
The categories of this paper are: eess
15
The categories of this paper are: nucl-ex
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
0
The categories of this paper are: hep-ph
29
The categories of this paper are: funct-an
35
The categories of this paper are: mtrl-th
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The categories of this paper are: q-bio
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The categories of this paper are: math
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The categories of this paper are: gr-qc
17
The categories of this paper are: q-fin
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The categories of this paper are: cond-mat
14
The categories of this paper are: math-ph