Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Metro-Line Crossing Minimization: Hardness, Approximations, and
  Tractable Cases
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
16
The categories of this paper are: stat
5
The categories of this paper are: gr-qc
7
The categories of this paper are: hep-th
4
The categories of this paper are: cond-mat
15
The categories of this paper are: nucl-ex
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
28
The categories of this paper are: alg-geom