Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Effective electromagnetic interaction in a dense fermionic medium in
  QED_{2+1}   The effective Lagrangian of arbitrary varying in space electromagnetic field
in a dense medium is derived. It has been used for investigation of interaction
between charged fermions in the medium. It is shown the possibility for the
formation of metastable electron bound states in the medium when external
magnetic field is applied.

9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
15
The categories of this paper are: nucl-ex
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
35
The categories of this paper are: mtrl-th
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
1
The categories of this paper are: cs
16
The categories of this paper are: stat