Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Integer quantum Hall effect and Hofstadter's butterfly spectra in
  three-dimensional metals in external periodic modulations
9
The categories of this paper are: nlin
22
The categories of this paper are: chao-dyn
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
3
The categories of this paper are: physics
1
The categories of this paper are: cs
12
The categories of this paper are: nucl-th
11
The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
17
The categories of this paper are: q-fin
13
The categories of this paper are: quant-ph
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph