Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Model of the Influence of an External Magnetic Field on the Gain of
  Terahertz Radiation from Semiconductor Superlattices
7
The categories of this paper are: hep-th
12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
19
The categories of this paper are: eess
2
The categories of this paper are: math
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
36
The categories of this paper are: cmp-lg
27
The categories of this paper are: comp-gas
3
The categories of this paper are: physics
30
The categories of this paper are: q-alg
17
The categories of this paper are: q-fin
21
The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex