Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Oppositely Directional Coupler: Example of the Forward Backward Waves
  Interaction in the Metamaterials
4
The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
18
The categories of this paper are: econ
3
The categories of this paper are: physics
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an
36
The categories of this paper are: cmp-lg
1
The categories of this paper are: cs
11
The categories of this paper are: hep-lat
27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess
2
The categories of this paper are: math
15
The categories of this paper are: nucl-ex
28
The categories of this paper are: alg-geom
5
The categories of this paper are: gr-qc