Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Refractive properties of graphene in a medium-strong external magnetic
  field
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
8
The categories of this paper are: hep-ex
18
The categories of this paper are: econ
5
The categories of this paper are: gr-qc
24
The categories of this paper are: dg-ga
13
The categories of this paper are: quant-ph
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
7
The categories of this paper are: hep-th
29
The categories of this paper are: funct-an
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
34
The categories of this paper are: chem-ph
6
The categories of this paper are: astro-ph