Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: To cross or not to cross: collective swimming of Escherichia coli under
  two-dimensional confinement
21
The categories of this paper are: adap-org
17
The categories of this paper are: q-fin
35
The categories of this paper are: mtrl-th
26
The categories of this paper are: bayes-an
25
The categories of this paper are: patt-sol
8
The categories of this paper are: hep-ex
32
The categories of this paper are: atom-ph
6
The categories of this paper are: astro-ph
2
The categories of this paper are: math
9
The categories of this paper are: nlin
14
The categories of this paper are: math-ph
7
The categories of this paper are: hep-th
24
The categories of this paper are: dg-ga
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
18
The categories of this paper are: econ